C-131/24

Provisional text OPINION OF ADVOCATE GENERAL KOKOTT delivered on 18 September 2025 (1) Case C‑131/24 Umweltorganisation VIRUS – Verein Projektwerkstatt für Umwelt und Soziales and Others (Request for a preliminary ruling from the Bundesverwaltungsgericht (Federal Administrative Court, Austria)) ( Request for a preliminary ruling – Environment – Directive 2009/147/EC – Conservation of wild birds – Prohibition on the deliberate disturbance of birds – Project for the construction of a national road liable to disturb individual specimens of certain species – Measures to improve habitats – Evidence of effectiveness – Expert opinion ) I.      Introduction 1.        Article 5(d) of the Birds Directive (2) lays down a prohibition on the deliberate disturbance of birds, in so far as such disturbance would be significant having regard to the objectives of that directive. It follows from the case-law that that prohibition also covers disturbances which are merely accepted as a possibility. (3) However, does a disturbance fall within the scope of that prohibition even in the case where its subsequent effects on the habitat of birds is compensated for by an improvement of the habitat of the species concerned in other locations? And how should the effectiveness of improvement measures be assessed before being implemented? These are the questions which the Bundesverwaltungsgericht (Federal Administrative Court, Austria) puts to the Court of Justice. II.    Legal framework A.      The Birds Directive 2.        Recitals 3, 5, 7 and 8 of the Birds Directive explain its objectives: ‘(3)      A large number of species of wild birds naturally occurring in the European territory of the Member States are declining in number, very rapidly in some cases. This decline represents a serious threat to the conservation of the natural environment, particularly because of the biological balances threatened thereby. (4)      … (5)      The conservation of the species of wild birds naturally occurring in the European territory of the Member States is necessary in order to attain the Community’s objectives regarding the improvement of living conditions and sustainable development. (6)      … (7)      Conservation is aimed at the long-term protection and management of natural resources as an integral part of the heritage of the peoples of Europe. It makes it possible to control natural resources and governs their use on the basis of the measures necessary for the maintenance and adjustment of the natural balances between species as far as is reasonably possible. (8)      The preservation, maintenance or restoration of a sufficient diversity and area of habitats is essential to the conservation of all species of birds. Certain species of birds should be the subject of special conservation measures concerning their habitats in order to ensure their survival and reproduction in their area of distribution. Such measures must also take account of migratory species and be coordinated with a view to setting up a coherent whole.’ 3.        Recital 10 of the Birds Directive requires, inter alia, that the populations of certain bird species be maintained at a ‘satisfactory level’: ‘Because of their high population level, geographical distribution and reproductive rate in the Community as a whole, certain species may be hunted, which constitutes acceptable exploitation where certain limits are established and respected, as such hunting must be compatible with maintenance of the population of these species at a satisfactory level’. 4.        Article 1 of the Birds Directive governs its scope: ‘This Directive relates to the conservation of all species of naturally occurring birds in the wild state in the European territory of the Member States to which the Treaty applies. It covers the protection, management and control of these species and lays down rules for their exploitation.’ 5.        Article 2 of the Birds Directive contains the basic obligation of the Member States in relation to the conservation of bird species: ‘Member States shall take the requisite measures to maintain the population of the species referred to in Article 1 at a level which corresponds in particular to ecological, scientific and cultural requirements, while taking account of economic and recreational requirements, or to adapt the population of these species to that level.’ 6.        Article 3 of the Birds Directive lays down general obligations in relation to the implementation of Article 2: ‘1.      In the light of the requirements referred to in Article 2, Member States shall take the requisite measures to preserve, maintain or re-establish a sufficient diversity and area of habitats for all the species of birds referred to in Article 1. 2.      The preservation, maintenance and re-establishment of biotopes and habitats shall include primarily the following measures: (a)      creation of protected areas; (b)      upkeep and management in accordance with the needs of habitats inside and outside the protected zones; (c)      re-establishment of destroyed biotopes; (d)      creation of biotopes.’ 7.        Article 5 of the Birds Directive contains prohibitions applicable irrespective of location: ‘Without prejudice to Articles 7 and 9, Member States shall take the requisite measures to establish a general system of protection for all species of birds referred to in Article 1, prohibiting in particular … (d)      deliberate disturbance of these birds particularly during the period of breeding and rearing, in so far as disturbance would be significant having regard to the objectives of this Directive’. 8.        Article 9(1) of the Birds Directive permits derogations from the prohibitions laid down in Article 5: ‘1.      Member States may derogate from the provisions of Articles 5 to 8, where there is no other satisfactory solution, for the following reasons: (a)      –      in the interests of public health and safety, –      in the interests of air safety, –      to prevent serious damage to crops, livestock, forests, fisheries and water, –      for the protection of flora and fauna; (b)      for the purposes of research and teaching, of re-population, of re-introduction and for the breeding necessary for these purposes; (c)      to permit, under strictly supervised conditions and on a selective basis, the capture, keeping or other judicious use of certain birds in small numbers.’ B.      The Habitats Directive 9.        Article 6(3) of the Habitats Directive (4) governs what is known as the ‘implications assessment’: ‘Any plan or project not directly connected with or necessary to the management of the site but likely to have a significant effect thereon, either individually or in combination with other plans or projects, shall be subject to appropriate assessment of its implications for the site in view of the site’s conservation objectives. In the light of the conclusions of the assessment of the implications for the site and subject to the provisions of paragraph 4, the competent national authorities shall agree to the plan or project only after having ascertained that it will not adversely affect the integrity of the site concerned and, if appropriate, after having obtained the opinion of the general public.’ 10.      Article 12(1) of the Habitats Directive lays down basic prohibitions for the protection of species: ‘Member States shall take the requisite measures to establish a system of strict protection for the animal species listed in Annex IV (a) in their natural range, prohibiting: (a)      all forms of deliberate capture or killing of specimens of these species in the wild; (b)      deliberate disturbance of these species, particularly during the period of breeding, rearing, hibernation and migration; (c)      deliberate destruction or taking of eggs from the wild; (d)      deterioration or destruction of breeding sites or resting places.’ 11.      Article 16(1) of the Habitats Directive contains derogations from Article 12: ‘Provided that there is no satisfactory alternative and the derogation is not detrimental to the maintenance of the populations of the species concerned at a favourable conservation status in their natural range, Member States may derogate from the provisions of Articles 12, 13, 14 and 15 (a) and (b): (a)      in the interest of protecting wild fauna and flora and conserving natural habitats; (b)      to prevent serious damage, in particular to crops, livestock, forests, fisheries and water and other types of property; (c)      in the interests of public health and public safety, or for other imperative reasons of overriding public interest, including those of a social or economic nature and beneficial consequences of primary importance for the environment; (d)      for the purpose of research and education, of repopulating and re-introducing these species and for the breedings operations necessary for these purposes, including the artificial propagation of plants; (e)      to allow, under strictly supervised conditions, on a selective basis and to a limited extent, the taking or keeping of certain specimens of the species listed in Annex IV in limited numbers specified by the competent national authorities.’ III. Facts and request for a preliminary ruling 12.      Following an environmental impact assessment, the Lower Austrian Provincial Government approved the road construction project at issue on 12 November 2019. The environmental organisation VIRUS – Verein Projektwerkstatt für Umwelt und Soziales (Association for Environmental and Social Projects) along with a number of other citizens and environmental associations brought an action against that approval before the Bundesverwaltungsgericht (Federal Administrative Court, Austria), claiming in particular that the provisions on the protection of species in the Birds Directive have been infringed. Various species, in particular forest bird species such as the middle-spotted woodpecker (Dendrocopos medius or Leiopicus medius), use sites that would be adversely affected by, among other things, the noise that would be generated by the new road. 13.      According to the opinion of a nature-conservation expert consulted by that court, however, it is to be expected that the bird species concerned will benefit from the restriction of construction times to certain months that is provided for in the project and from habitat-improvement measures on sites outside the immission area which can be formulated as conditions. 14.      A particular point of contention in the proceedings is the effectiveness of measures proposed by the project applicant for improving the forest and protecting old trees at a distance of at least 300 m from the road and cover a total area of 6.6 hectares as part of a regional action space for the benefit of the middle-spotted woodpecker and other forest bird species. 15.      According to the opinions of both experts engaged to give evidence in the proceedings, implementation of those proposed measures would not lead to a disturbance of the species concerned which might be significant having regard to the objectives of the Birds Directive. Those measures do not, however, make it possible to reduce the disturbance of individual birds in the forest specifically affected by the project. In the experts’ assessment, the proposed measures nonetheless safeguard the habitat and the conditions necessary for the species to breed. 16.      The expert stated before the referring court that he was unaware of any studies calling into question the suitability of the proposed measures for protecting the middle-spotted woodpecker. Neither did the ecology of the species offer any indications of their unsuitability for that purpose. There was therefore no sufficiently reasoned doubt as to the effectiveness of those measures. The only task remaining was to perform comprehensive and technically sound monitoring, which would not only dispel any doubts but would also be able to prove the effectiveness of the measures. When asked by the court whether there was a single reliably documented case in which the planned felling measures had had the effect of preventing a project from disturbing the middle-spotted woodpecker or even of improving the conservation status of that species in the project catchment area, the expert replied that, despite having conducted intensive research and having also exchanged experiences with colleagues, he had not found such a clear-cut study. 17.      The Bundesverwaltungsgericht (Federal Administrative Court, Austria) has therefore put the following questions to the Court of Justice: ‘(1)      Is Article 5 of Directive 2009/147/EC (Birds Directive) to be interpreted as meaning that a case of deliberate disturbance, as referred to under point (d) of that article, is not constituted if, notwithstanding the fact that individual specimens of certain species could be disturbed, any effect on the objective of Article 2 of that directive will be prevented by means of measures effectively implemented in a timely and appropriate manner? (2)      If the first question is answered in the affirmative, must all scientific doubt concerning the effectiveness of the measures be excluded, as evidenced by a well-reasoned, professional appraisal of a court-appointed expert, or must there be objective scientific literature of successful practical experiences of those measures?’ 18.      Written observations have been lodged by the environmental organisation VIRUS – Verein Projektwerkstatt für Umwelt und Soziales (Association for Environmental and Social Projects), with which the citizens’ action group ‘Nein zur Spange Wörth’ (No to the Wörth Spur Road) has aligned itself, the Lower Austrian Provincial Government, the Autobahnen- und Schnellstraßen-Finanzierungs-Aktiengesellschaft (Austrian Motorway and Expressway Financing Company) (ASFINAG), the Municipality of St. Pölten, the Republic of Austria, the Czech Republic, the Kingdom of Sweden and the European Commission. 19.      In accordance with Article 76(2) of the Rules of Procedure, the Court decided not to hold a hearing, on the ground that it has sufficient information to give a ruling in the case. 20.      However, on 19 November 2024, the Court stayed the proceedings pending the judgment of 1 August 2025, Voore Mets and Lemeks Põlva (C‑784/23, EU:C:2025:609). IV.    Legal assessment 21.      The background to the two questions is the fact that both the construction and the operation of the road at issue would disturb the middle-spotted woodpecker in particular, but also other species of forest birds. The referring court would therefore like to understand whether the prohibition on disturbance laid down in Article 5(d) of the Birds Directive precludes the project in question. 22.      In accordance with Article 5(d) of the Birds Directive, Member States must prohibit the deliberate disturbance of European bird species, particularly during the period of breeding and rearing, in so far as any such disturbance would be significant having regard to the objectives of that directive. 23.      That prohibition supplements other prohibitions laid down in Article 5(a) to (c), which relate in particular to the deliberate killing or capture of those species, as well as to the deliberate destruction of, or damage to, their nests and eggs. The Court recently held that those prohibitions apply not only to such human activities, the purpose of which is to harm birds, but also to activities which involve the acceptance of the possibility of such harm, even though they do not manifestly have such a purpose. (5) 24.      In order to prevent the prohibition on disturbance from applying, measures at a distance of at least 300 m from the road and covering a total area of 6.6 hectares as part of a regional action space for the benefit of the middle-spotted woodpecker and other forest bird species are to be put in place. These measures include the preservation of old trees which might otherwise be felled. 25.      There is much interest in the use of such measures to prevent the prohibitions laid down in Article 5 of the Birds Directive from applying. As I recently explained, the rigorous application of those prohibitions would significantly restrict human activities. (6) It is therefore doubtful whether they are compatible with the protection of property and economic freedom under Articles 16 and 17 of the Charter of Fundamental Rights. (7) If, on the other hand, the application of those prohibitions could be prevented by measures to improve habitats in other locations, as is now expressly provided for in the case of certain projects for the use of renewable energy, (8) that conflict would be diffused. 26.      As I shall explain below, however, any consideration of the measures in the present case is possible only because Article 5(d) of the Birds Directive requires that the disturbance be significant having regard to the objectives of the Birds Directive (see in this regard section A). The other prohibitions do not contain any such condition. Next, in order to answer the second question, I shall discuss how the effectiveness of such measures must be assessed (see in this regard section B). A.      Question 1 – Prohibition on disturbance and compensatory measures 27.      By the first question, the referring court wishes to ascertain whether the application of the prohibition on disturbance laid down in Article 5(d) of the Birds Directive can be prevented by measures to improve the habitat of the bird species concerned in other locations. 28.      In particular, the referring court raises the question of whether the judgment in People over Wind (9) precludes such measures being considered (see, in this regard, section 1). That judgment concerns the screening of a plan or project in order to determine the need for a formal assessment of its implications for protected sites under Article 6(3) of the Habitats Directive. The compensatory measures must, however, be understood as a component of the regulatory structure of Article 5(d) of the Birds Directive (see, in this regard, section 2). 1.      Judgment in People over Wind 29.      The judgment in People over Wind, although concerning the consideration of measures to prevent or reduce adverse effects in the field of nature conservation, proves on closer examination to have no bearing on the interpretation of the prohibition of disturbance. 30.      That case concerned the screening to determine whether a formal assessment of a project’s implications for a particular protected site must be carried out in accordance with the first sentence of Article 6(3) of the Habitats Directive. According to that provision, any plan or project not directly connected with or necessary to the management of the site but likely to have a significant effect thereon, whether individually or in combination with other plans or projects, is to be subject to appropriate assessment of its implications for the site in view of the site’s conservation objectives. 31.      According to the judgment in People over Wind, it is not appropriate for the screening to determine whether such an assessment is necessary at all to take into account measures intended to avoid or reduce the harmful effects of the plan or project on the site concerned. (10) The extent to which such measures are effective must after all be examined in the formal assessment, the nature of which is different from that of the screening and which contains more extensive guarantees. (11) 32.      The request for a preliminary ruling and various interested parties correctly note that the foregoing line of argument is not transposable to the present case not least because the prohibition on disturbance laid down in Article 5(d) of the Birds Directive, unlike the first sentence of Article 6(3) of the Habitats Directive, for the purposes of protecting sites, does not provide for a screening or even a formal assessment procedure in which the effects of a disturbance must be reviewed. 2.      Component of the regulatory structure of the prohibition on discrimination 33.      The consideration of measures to improve the habitat of the species concerned in other locations must, rather, be understood as a component of the regulatory structure of the prohibition on disturbance laid down in Article 5(d) of the Birds Directive, which precludes disturbances which are significant having regard to the objectives of that directive. On that basis, it is necessary to consider, first, measures to reduce or prevent disturbances (see in this regard section a), next, the significance of compensatory measures for maintaining satisfactory bird populations (see in this regard section b), and finally, the special situation in which compensatory measures ensure that a disturbance does not prevent the population of a species from being restored to a satisfactory level (see in this regard section c). (a)    Measures to reduce or prevent disturbances 34.      Where measures prevent a project from disturbing birds, the prohibition on disturbance laid down in Article 5(d) of the Birds Directive is not applicable because there is no disturbance. And where those measures do not prevent a disturbance but at least reduce it, it is possible that the disturbance may not be significant having regard to the objectives of the Birds Directive. The assessment of whether the prohibition on disturbance precludes a project must therefore take such measures into account. (12) 35.      Measures to prevent or reduce damage in the formal assessment provided for in the first sentence of Article 6(3) of the Habitats Directive, and addressed above, are taken into account in the same way. That assessment is intended to show whether a project adversely affects a protected site, as such. If the project comes with protective measures designed to prevent or reduce its harmful effects on the protected site concerned, those measures must of course be taken into account in the assessment. (13) 36.      Although it is possible to conceive of measures which might prevent or reduce disturbances in the case in the main proceedings, such measures play only a minor role according to the information available. The only proposal in that regard is that construction times be restricted in order to ensure that birds are not disturbed by construction activities during particularly sensitive periods. 37.      Another conceivable option might, for example, be the deployment of protective walls to reduce noise or visual disturbances from road use and the risk of birds colliding with vehicles. It might also be possible to consider restricting road use, by means, for example, of a prohibition on driving or at least a speed limit during the breeding and rearing season referred to in Article 5(d) of the Birds Directive. However, the request for a preliminary ruling contains no information on any such, more extensive measures to prevent or reduce disturbance. (b)    Compensatory measures 38.      Measures which improve the habitat of the species concerned in other locations are not, however, measures to prevent or reduce disturbance. (14) They clearly cannot either prevent a disturbance of particular birds or reduce the effects of any such disturbance on those birds, as the referring court also acknowledges. 39.      Such measures are, rather, an attempt to compensate for the effects of the disturbance. It is accepted that the noise makes part of the habitat of the species concerned less attractive or even drives them away from there entirely. In counterbalance, other localities are improved, with the result, ideally, that, overall, the compensatory measures add at least as much equally suitable habitat that can be used in the same way by the species concerned – albeit not necessarily by the specimens concerned. 40.      Since compensatory measures have no impact on the disturbance, they can prevent the prohibition from applying only if, as a result of those measures, the disturbance would not be significant having regard to the objectives of the directive, within the meaning of Article 5(d) of the Birds Directive. 41.      In accordance with recitals 3, 5, 7 and 8 and Article 2 read in conjunction with recital 10 of the Birds Directive, the objective of that directive is to maintain the populations of all European bird species at a satisfactory level or to adapt those populations to that level. (15) 42.      In accordance with Article 2 of the Birds Directive, that level must correspond in particular to ecological, scientific and cultural requirements, while taking account of economic and recreational requirements. However, the Member States are not permitted to weigh those factors against each other at will in order to determine the satisfactory level of a population. In accordance with Article 1, the main objective of that directive is, rather, to conserve all wild bird species native to the European territory of the Member States. The key criterion, therefore, much as with the conservation status of species, a core concept of the Habitats Directive, (16) is whether the species forms a viable element of the natural habitat to which it belongs. The ecological and scientific requirements of the species in question are decisive in that regard. In particular, a sufficiently large habitat must be permanently available and the distribution area should at least remain stable. Cultural, economic and recreational requirements, on the other hand, can be taken into account only after and in so far as the long-term conservation of the species has been ensured. 43.      On account of the commitment to that objective, the prohibition therefore precludes disturbances which would reduce the population of a bird species to such an extent that it is no longer at a satisfactory level, or would further reduce an unsatisfactory population level. 44.      Where such a disturbance is to be expected, Article 5(d) of the Birds Directive does not provide for compensatory measures to be taken into account. (17) Even if the population of the species is increased by the compensatory measure, it continues to be reduced by the disturbance. There is nothing in that provision to indicate that those two effects can be offset against each other. 45.      Furthermore, the effectiveness of compensatory measures in the context of the protection of species is fraught with considerable uncertainty. The second question referred for a preliminary ruling illustrates that. On account of such doubts as to the effectiveness of compensatory measures, the Court regularly refuses to take them into account when assessing adverse effects on protected sites. (18) 46.      Conversely, another argument relevant to the discussion of compensatory measures in the context of the protection of sites does not at first sight appear to be transposable to the prohibition on disturbance. 47.      In the context of the protection of sites, compensatory measures, being a precondition for exceptions to the protection of sites, are provided for in Article 6(4) of the Habitats Directive in the form of measures to ensure the coherence of the (Natura 2000) network of protected sites. There, however, the legislature combined those measures with further requirements, in particular with the consideration of alternatives and the establishment of imperative reasons of overriding public interest. Those further requirements would be circumvented if compensatory measures were taken into account within the framework Article 6(3). (19) 48.      In accordance with Article 9 of the Birds Directive, on the other hand, the derogations from the prohibition on disturbance are not conditional upon compensatory measures. There would therefore be no risk of the derogating provision being circumvented. 49.      A closer examination of the matter, however, shows that taking into account compensatory measures in the context of the prohibition on disturbance could give rise to an even more pronounced circumvention. 50.      In that regard, in accordance with Article 3 of the Birds Directive, Member States already have an obligation, irrespective of any disturbance, not only to preserve or maintain a sufficient diversity and area of habitats for all birds. Rather, they must also re-establish or recreate such habitats if these are unsatisfactory. (20) It is true that the Restoration Regulation (21) spells out those obligations and grants generous time limits for implementing the measures which it prescribes. That, however, does nothing to alter the fundamental obligation, in place for over 40 years and, in Austria’s case, since its accession, (22) which is laid down in Article 3 of the Birds Directive. 51.      If the satisfactory population level of the species concerned were indeed dependent on its habitats (the only scenario in which measures to improve habitats could have a positive impact on an unsatisfactory level), Member States would have to re-establish or recreate the necessary habitats of the species in question in any event, pursuant to Article 3 of the Birds Directive. 52.      Consequently, Member States cannot allow an unsatisfactory population level to deteriorate further solely on the ground that that deterioration is coupled with improvement measures which would still not be sufficient to achieve a satisfactory population level. 53.      Rather, the effect of compensatory measures can in principle be taken into account in the assessment of a disturbance only if it ensures that the populations of the species concerned are at a satisfactory level despite the disturbance. Whether that result rests on compensatory measures or on measures pursuant to Article 3 of the Birds Directive is then irrelevant. (c)    Potential for restoration 54.      However, as with the similarly configured prohibitions relating to the protection of species under the Habitats Directive, the requirement in relation to the aforementioned result must be qualified inasmuch as a disturbance may exceptionally be permitted even if the population level is unsatisfactory, provided that the disturbance does not prevent a satisfactory population level from being restored. 55.      In the context of the protection of species under the Habitats Directive, preservation of the favourable conservation status of the populations of the animal species concerned in their natural range is a precondition for granting the derogations from the prohibitions laid down in Article 12 of the Habitats Directive which are provided for in Article 16(1) of that directive. (23) That precondition is to be construed in the same way as the requirement, under discussion here, to ensure that the population of the disturbed bird species is satisfactory. 56.      It might be concluded from the foregoing that the derogations provided for in Article 16(1) of the Habitats Directive may be granted only if the conservation status of the species concerned is favourable. However, on an exceptional basis, the Court will allow derogations even in situations where the conservation status of the species concerned is unfavourable, provided that the derogations do not hinder the restoration of a favourable conservation status. (24) 57.      That finding concerned a situation in which the population of the species in question was in fact too small to support the assumption of a favourable conservation status, but had increased significantly in previous years. (25) It therefore seemed to be a plausible prognosis that the unfavourable conservation status of the species concerned would continue to improve despite the killing of individual specimens, so that a favourable conservation status could be restored. Where there is such a favourable prognosis, it is not necessary, for the purposes of the objective pursued by the Habitats Directive, which is to achieve a favourable conservation status, for a Member State, notwithstanding otherwise overriding interests, to have to wait until a favourable conservation status has been achieved before permitting a disturbance. For that reason, the Court has recognised that a derogation may be neutral from the point of view of the species concerned even though it would lead to the loss of individual specimens. 58.      If, however, the possibility that a favourable conservation status will be restored is sufficient in the case of a derogation from the prohibition on killing laid down in Article 12(1)(a) of the Habitats Directive, it must a fortiori be sufficient, for the purposes of the objectives pursued by the Birds Directive in connection with the prohibition on disturbances to birds laid down in Article 5(d) of the Birds Directive, that a disturbance does not hinder the restoration of a satisfactory conservation status. 59.      Indeed, disturbances have significantly fewer adverse effects than killings. At the same time, disturbances are by their nature more frequent than killings. Furthermore, the prohibitions laid down in the Birds Directive apply to all birds and therefore have a significantly broader scope than the prohibitions laid down in the Habitats Directive, which apply to a few rare species. Consequently, the prohibition on disturbance under Article 5(d) of the Birds Directive is capable of restricting human activities much more significantly than the prohibition on killing under Article 12(1(a) of the Habitats Directive. 60.      When it comes to assessing the prohibition on discrimination, compensatory measures to improve the habitat of the species concerned in other locations may therefore support the prognosis that the disturbance or at a minimum the restoration of a satisfactory population level will not be impeded. Since that is a prognosis, however, both compensatory measures which have already been implemented and measures which are yet to be implemented can be taken into account in that context. Compensatory measures cannot, however, be implemented directly at the place of the disturbance as the disturbance adversely impacts the quality of the habitat there. The compensatory measures must nevertheless be implemented close to the disturbance, as otherwise it could be expected that the level of the species concerned would, at a minimum, not be maintained or restored in that locality. 3.      Answer to the first question 61.      The answer to the first question must therefore be that a disturbance of birds which leads to a reduction of the population of the disturbed bird species is compatible with Article 5(d) of the Birds Directive only if that population, notwithstanding the disturbance, remains at a satisfactory level or if it can be assumed that the disturbance will not hinder the restoration of a satisfactory population level. When assessing that question, Member States may take into account the effects of measures to improve the habitats of the species in question in another location. B.      Question 2 – Assessment of the effectiveness of compensatory measures 62.      By the second question, the referring court wishes to ascertain whether the exclusion of any scientific doubt as to the effectiveness of measures to improve the habitat of the species concerned in other locations is adequately evidenced by a well-reasoned, professional appraisal of a court-appointed expert, or whether there must, rather, be objective scientific documentation of successful practical experiences of those measures. 63.      A closer examination shows that objective scientific documentation would require two in-depth studies. The adverse effects of the disturbance on the population of the species concerned would have to be identified. In addition, it would be necessary to determine the beneficial effects of the compensatory measures on that population. Both studies would call for a long-term comparison of the situation before and after the measure in question, particularly in forest habitats. Thus, in particular, the effect of conserving old and dead wood is naturally contingent upon the passage of time. The disruptive effect of the project will also change over time. In addition, the development of both objects under examination must be isolated from other factors, such as weather conditions, food supply or disease. 64.      Even if such studies involving similar situations existed, there would still be some doubt – as the Lower Austrian Provincial Government, the Municipality of St. Pölten and ASFINAG point out – as to the extent to which their results could actually be transposed. 65.      It is not surprising, therefore, that the expert was unable to find any study conclusively clarifying the effectiveness of the compensatory measures at issue. The studies available are of indicative value only. (26) 66.      It is therefore necessary to clarify below how the effectiveness of compensatory measures is to be assessed. In that regard, it is necessary first to consider the referring court’s premiss that any scientific doubt concerning the effectiveness of compensatory measures must be excluded (see in that regard section 1), and then the general criteria under EU law for assessing complex scientific questions (see in that regard section 2). 1.      Exclusion of scientific doubt? 67.      The referring court infers from the Court’s case-law that any reasonable scientific doubt or any scientifically reasonable doubt concerning the effectiveness of compensatory measures must be excluded. 68.      It is true that there is no explicit reference to the applicability of that criterion to the prohibition on disturbance in Article 5(d) or any other provision of the Birds Directive or in case-law. However, the Court, relying on a particularly strict reading of the precautionary principle enshrined in Article 191(2) TFEU, has interpreted various other provisions in accordance with that criterion. (27) It is therefore necessary to examine in more detail whether that restrictive criterion is also applicable to the assessment of compensatory measures in the context of the prohibition of disturbance. 69.      I shall, however, demonstrate that neither the case-law on the derogating provision contained in Article 16 of the Habitats Directive (see in that regard section a) nor the case-law on the authorisation of projects under the second sentence of Article 6(3) of that directive (see in that regard section b) nor the case-law on Article 14 of that directive (see in this regard section c) can be transposed to Article 5(d) of the Birds Directive. (a)    The case-law on Article 16 of the Habitats Directive 70.      First, one might contemplate transposing the case-law on Article 16 of the Habitats Directive. That provision provides that the preservation or restoration of a favourable conservation status of the species concerned is a precondition for derogating from the prohibitions relating to the protection of species laid down in Article 12 of that directive. (28) The Member State must refrain from granting such a derogation if, following an examination of the best available scientific data, there remains uncertainty as to whether the favourable conservation status of the populations of a species threatened with extinction can be maintained or restored notwithstanding that derogation. (29) 71.      However, is the foregoing also true for the application of the prohibition on disturbance laid down in Article 5(d) of the Birds Directive? Does that prohibition preclude a project if, following an examination of the best available scientific data, there remains uncertainty as to whether a satisfactory population level of the bird species concerned can be maintained or restored notwithstanding the compensatory measures? 72.      The precondition as to the maintenance or restoration of a favourable conservation status required under Article 16 of the Habitats Directive has a function similar to the condition in Article 5(d) of the Birds Directive requiring that the population of the birds concerned remain at a satisfactory level notwithstanding the disturbance or that it can be assumed that the disturbance will not hinder the future restoration of a satisfactory population level. According to the view taken in the present Opinion, as long as that condition is fulfilled, which will in some cases be as a result of compensatory measures, the prohibition on disturbance does not apply. 73.      However, Article 16 of the Habitats Directive is a derogation from the prohibitions laid down in Article 12 and must for that reason be interpreted restrictively. (30) The Court generally requires that approach to be taken to derogations because it fears that the rules from which derogations are permitted will otherwise be undermined. (31) That idea is consistent with the restrictive assessment criterion requiring any uncertainty to be excluded before the derogation is applied. 74.      Conversely, any adverse effect on the population of the bird species concerned or on the potential for its restoration is a precondition for the application of the prohibition on disturbance laid down in Article 5(d) of the Birds Directive. Therefore, the case-law on Article 16 cannot be transposed directly. (b)    The case-law on the second sentence of Article 6(3) of the Habitats Directive 75.      It is not only derogations, however, that the Court interprets by relying on a strict reading of the precautionary principle. The strict reading of that principle has its origin, rather, in the interpretation of the second sentence of Article 6(3) of the Habitats Directive, to which the referring court also refers. According to that provision, the competent national authorities are to agree to a plan or project only after having ascertained that it will not adversely affect the integrity of the Natura 2000 protected site concerned. 76.      The Court has held with respect to that provision that plans and projects which are capable of adversely affecting a protected site may be authorised only if there is no reasonable scientific doubt that they will not adversely affect the integrity of that site. (32) 77.      Unlike the second sentence of Article 6(3) of the Habitats Directive, however, the prohibition on disturbance laid down in Article 5(d) of the Birds Directive is not worded so as to mean that any significant effects on the objectives of the directive must be excluded before disturbances can be permitted. On the contrary, such effects must be positively identified in order for the prohibition on disturbance to be applied. (33) Consequently, the case-law on the second sentence of Article 6(3) also cannot be transposed directly. (c)    The case-law on Article 14 of the Habitats Directive 78.      The case-law on Article 14(1) of the Habitats Directive remains the closest to Article 5(d) of the Birds Directive. In accordance with Article 14(1), Member States are to take measures to ensure that the taking in the wild of specimens of species of wild fauna and flora of Community interest as well as their exploitation are compatible with their being maintained at a favourable conservation status. 79.      The Court has interpreted that provision as meaning that the Member State concerned must refrain from authorising the taking or exploitation of a species if, after examining the best scientific data available, there remains uncertainty as to whether that exploitation is compatible with the maintenance of that species at a favourable conservation status. (34) 80.      Article 14(1) of the Habitats Directive and Article 5(d) of the Birds Directive are worded along similar lines, since both provisions require a positive determination of whether the taking or exploitation of specimens of species on the one hand or the disturbance thereof on the other is compatible with a favourable conservation status or a satisfactory population level of the species concerned. 81.      However, the species protected in each case differ significantly. Article 14(1) of the Habitats Directive protects species of Community interest which, pursuant to the definition given in Article 1(g) of that directive, are in need of special protection. The prohibition on disturbance laid down in Article 5(d) of the Birds Directive, on the other hand, applies to all birds, regardless of whether they are endangered or in need of protection. 82.      What is more, the prohibition of a disturbance of birds which is merely accepted as a possibility (35) is by its nature significantly broader in scope than restrictions on the taking or exploitation of species in need of special protection. Disturbances of birds are very commonly accepted, whereas the taking or exploitation of species which are in need of protection and, for that reason, often rare will take place much less frequently and by conscious decision. 83.      Those differences justify a broad scope of application being attributed to Article 14 of the Habitats Directive, but not to Article 5(d) of the Birds Directive. (d)    Interim conclusion 84.      I conclude from the foregoing that the prohibition on disturbance laid down in Article 5(d) of the Birds Directive does not automatically apply if, following an examination of the best scientific data available, there remains uncertainty or reasonable doubt as to whether a satisfactory population level of the bird species concerned can be maintained or restored. Rather, a positive determination that the disturbance adversely affects the population level of the bird species concerned or hinders its restoration is required before that prohibition becomes applicable. 2.      EU law criteria for carrying out complex scientific assessments 85.      The complex scientific assessments accordingly required for the application of the prohibition on disturbance laid down in Article 5(d) of the Birds Directive should be carried out with regard to standard criteria. 86.      In that regard, in the absence of any EU law rules on the examination of the prohibition of discrimination, mention must be made in the first instance of the principle as to the procedural autonomy of the Member States, as the Municipality of St. Pölten submits. As that principle also applies to evidential matters, (36) it is applicable to the assessment of the effectiveness of compensatory measures. It is therefore for the Member States to lay down the rules governing how such effectiveness is to be examined. 87.      The rules laid down by the Member States must meet the two conditions of being no less favourable than those governing similar situations subject to domestic law (principle of equivalence) and of not making it impossible or excessively difficult to exercise the rights conferred by EU law (principle of effectiveness). (37) 88.      In the case of the prohibition on disturbance, the principle of effectiveness means above all that it must not be made unreasonably difficult to prove the existence of significant effects on the objectives of the Birds Directive. 89.      So far as concerns the principle of equivalence, a number of interested parties have stated that the Austrian courts regularly consult the scientifically reasoned opinions of court-appointed experts. 90.      A number of other principles of EU law must also be observed, such as the principle of good administration and the precautionary principle mentioned above. 91.      The Member States are required to observe the principle of good administration in the application of EU law not on the basis of Article 41 of the Charter (38) but as a general principle of EU law. (39) That principle requires an administrative authority, inter alia, to carry out a diligent and impartial examination of all the relevant matters in order, when it adopts a decision, to have at its disposal the most complete and reliable information possible for that purpose. (40) 92.      Furthermore, measures based on the precautionary principle – that is to say, in particular, the application of provisions of EU environmental law – generally call for a comprehensive assessment of the risks concerned based on the most reliable scientific data available and the most recent results of international research. (41) The Court correctly also requires this on a regular basis in the field of nature conservation, (42) in particular when it comes to proving that the conditions for derogating from the protective rules contained in the Birds Directive are met, (43) without, however, expressly relying on those principles. 93.      Finally, the corresponding findings must be adequately reasoned. That, too, follows from the right to good administration (44) and, moreover, in the case of environmental decisions which require public participation in accordance with Article 6 of the Aarhus Convention, (45) is laid down in Article 6(9) of that convention. However, the effective judicial review of decisions and effective legal protection likewise presuppose that the court before which the matter is brought and those seeking legal protection are given access to the reasons for the contested decision, either through the decision itself or at least on request. (46) 94.      The answer to the second question must therefore be that the complex assessments required in judicial proceedings for the purposes of applying the prohibition on disturbance laid down in Article 5(d) of the Birds Directive may be based on the reasoned opinions of experts, which must in turn be based on the most reliable scientific data available and the most recent results of international research. V.      Conclusion 95.      I therefore propose that the Court answer the request for a preliminary ruling as follows: (1)      A disturbance of birds which leads to a reduction of the population of the disturbed bird species is compatible with Article 5(d) of Directive 2009/147/EC of the European Parliament and of the Council of 30 November 2009 on the conservation of wild birds [the Birds Directive] only if that population, notwithstanding the disturbance, remains at a satisfactory level or if it can be assumed that the disturbance will not hinder the restoration of a satisfactory population level. When assessing that question, Member States may take into account the effects of measures to improve the habitats of the species in question in another location. (2)      The complex assessments required in judicial proceedings for the purposes of applying the prohibition on disturbance laid down in Article 5(d) of Directive 2009/147 may be based on the reasoned opinions of experts, which must in turn be based on the most reliable scientific data available and the most recent results of international research. 1      Original language: German. 2      Directive 2009/147/EC of the European Parliament and of the Council of 30 November 2009 on the conservation of wild birds (OJ 2010 L 20, p. 7). 3      Judgment of 1 August 2025, Voore Mets and Lemeks Põlva (C‑784/23, EU:C:2025:609, paragraph 49). 4      Council Directive 92/43/EEC of 21 May 1992 on the conservation of natural habitats and of wild fauna and flora (OJ 1992 L 206, p. 7), as amended by Council Directive 2013/17/EU of 13 May 2013 (OJ 2013 L 158, p. 193). 5      Judgment of 1 August 2025, Voore Mets and Lemeks Põlva (C‑784/23, EU:C:2025:609, paragraph 49). 6      My Opinion in Voore Mets and Lemeks Põlva (C‑784/23, EU:C:2025:67, points 39, 40 and 47). 7      My Opinion in Voore Mets and Lemeks Põlva (C‑784/23, EU:C:2025:67, points 92 to 101). 8      First, Article 6 of Council Regulation (EU) 2022/2577 of 22 December 2022 laying down a framework to accelerate the deployment of renewable energy (OJ 2022 L 335, p. 36), as amended by Council Regulation (EU) 2024/223 of 22 December 2023 (OJ L, 2024/223); subsequently, Article 15c(1), Article 15e(2) and (4) and Article 16a(5) of Directive (EU) 2018/2001 of the European Parliament and of the Council of 11 December 2018 on the promotion of the use of energy from renewable sources (OJ 2018 L 328, p. 82), as amended by Directive (EU) 2023/2413 of the European Parliament and of the Council of 18 October 2023 amending Directive (EU) 2018/2001, Regulation (EU) 2018/1999 and Directive 98/70/EC as regards the  promotion of energy from renewable sources, and repealing Council Directive (EU) 2015/652 (OJ L, 2023/2413). 9      Judgment of 12 April 2018, People Over Wind and Sweetman (C‑323/17, EU:C:2018:244). 10      Judgment of 12 April 2018, People Over Wind and Sweetman (C‑323/17, EU:C:2018:244, paragraph 40). 11      Judgment of 12 April 2018, People Over Wind and Sweetman (C‑323/17, EU:C:2018:244, paragraphs 37 to 39). 12      My understanding of the judgment of 17 April 2018, Commission v Poland (Białowieża Forest) (C‑441/17, EU:C:2018:255, paragraphs 255 and 259). 13      Judgments of 15 May 2014, Briels and Others (C‑521/12, EU:C:2014:330, paragraph 28), and of 25 July 2018, Grace and Sweetman (C‑164/17, EU:C:2018:593, paragraph 51). 14      See, to that effect, in relation to the protection of sites, the judgment of 7 November 2018, Coöperatie Mobilisation for the Environment and Others (C‑293/17 and C‑294/17, EU:C:2018:882, paragraph 125). The earlier judgment of 26 April 2017, Commission v Germany (Moorburg) (C‑142/16, EU:C:2017:301, paragraph 38), is ambiguous in that regard. 15      Judgment of 1 August 2025, Voore Mets and Lemeks Põlva (C‑784/23, EU:C:2025:609, paragraph 51). 16      See Article 1(i) of the Habitats Directive and my Opinion in Eesti Suurkiskjad (C‑629/23, EU:C:2024:1029, in particular points 43 to 46). 17      See, in the similar context of the protection of sites under Article 6(3) of the Habitats Directive, the judgment of 21 July 2016, Orleans and Others (C‑387/15 and C‑388/15, EU:C:2016:583, paragraph 57). 18      Judgments of 15 May 2014, Briels and Others (C‑521/12, EU:C:2014:330, paragraph 32); of 21 July 2016, Orleans and Others (C‑387/15 and C‑388/15, EU:C:2016:583, paragraph 55); of 26 April 2017, Commission v Germany (Moorburg) (C‑142/16, EU:C:2017:301, paragraphs 37 and 38); of 25 July 2018, Grace and Sweetman (C‑164/17, EU:C:2018:593, paragraphs 51 to 53), and of 7 November 2018, Coöperatie Mobilisation for the Environment and Others (C‑293/17 and C‑294/17, EU:C:2018:882, paragraph 126). 19      Judgments of 15 May 2014, Briels and Others (C‑521/12, EU:C:2014:330, paragraphs 33 to 38), and of 21 July 2016, Orleans and Others (C‑387/15 and C‑388/15, EU:C:2016:583, paragraphs 60 to 63). 20      See judgment of 13 June 2002, Commission v Ireland (Ptarmigan) (C‑117/00, EU:C:2002:366, paragraphs 15 to 21). 21      Regulation (EU) 2024/1991 of the European Parliament and of the Council of 24 June 2024 on nature restoration and amending Regulation (EU) 2022/869 (OJ L, 2024/1991). 22      See, to that effect, the judgment of 23 March 2006, Commission v Austria (C‑209/04, EU:C:2006:195, paragraph 43). 23      Judgment of 10 May 2007, Commission v Austria (Transposition of the Habitats Directive in the [Austrian] regions) (C‑508/04, EU:C:2007:274, paragraph 115). 24      Judgments of 14 June 2007, Commission v Finland (Wolf hunting) (C‑342/05, EU:C:2007:341, paragraph 29), and of 10 October 2019, Luonnonsuojeluyhdistys Tapiola (C‑674/17, EU:C:2019:851, paragraph 68). 25      See judgment of 14 June 2007, Commission v Finland (Wolf hunting) (C‑342/05, EU:C:2007:341, paragraph 26) and my Opinion in Commission v Finland (Wolf hunting) (C‑342/05, EU:C:2006:752, point 44). 26      See, for example, Michalek, K. G., Auer, J. A., Großberger, H., Schmalzer, A., and Winkler, H. (2001), ‘Die Einflüsse von Lebensraum, Witterung und Waldbewirtschaftung auf die Brutdichte von Bunt-und Mittelspecht (Picoides major und P. medius) im Wienerwald’, Abhandlungen und Berichte aus dem Museum Heineanum, 5, p.31 to 58, or Mollet, P., Zbinden, N., and Schmid, H. (2009), ‘Steigende Bestandszahlen bei Spechten und anderen Vogelarten dank Zunahme von Totholz?| An increase in the population of woodpeckers and other bird species thanks to an increase in the quantities of deadwood?’, Schweizerische Zeitschrift für Forstwesen, 160(11), p.334 to 340. 27      In addition to the judgments concerning the Habitats Directive which will be discussed below, see also, in connection with waste legislation, judgment of 24 October 2019, Prato Nevoso Termo Energy (C‑212/18, EU:C:2019:898, paragraph 58), and, in connection with Directive 2010/75/EU of 24 November 2010 on industrial emissions (OJ 2010 L 334, p. 17), the judgment of 9 March 2023, Sdruzhenie ‘Za Zemyata – dostap do pravosadie’ and Others (C‑375/21, EU:C:2023:173, paragraph 53). See also in this regard Sobotta, C. (2020), ‘Recent applications of the precautionary principle in the jurisprudence of the CJEU – a new yardstick in EU environmental decision making?’, ERA Forum Vol. 21, p. 723. 28      See points 55 to 57. 29      Judgment of 10 October 2019, Luonnonsuojeluyhdistys Tapiola (C‑674/17, EU:C:2019:851, paragraphs 66, 68 and 69). 30      Judgments of 20 October 2005, Commission v United Kingdom (Transposition of the Habitats Directive)  (C‑6/04, EU:C:2005:626, paragraph 111), and of 10 October 2019, Luonnonsuojeluyhdistys Tapiola (C‑674/17, EU:C:2019:851, paragraph 30). See also my Opinion in ASCEL (C‑436/22, EU:C:2024:83, point 66). 31      Judgments of 12 July 1962, Acciaierie Ferriere e Fonderie di Modena v High Authority (16/61, EU:C:1962:29, p. 610 [p. 575 of the French version]); of 14 December 1962, Commission v Luxemburg and Belgium (2/62 and 3/62, EU:C:1962:45, p. 884 [p. 830 of the French version]), and of 30 April 2024, Procura della Repubblica presso il Tribunale di Bolzano (C‑178/22, EU:C:2024:371, paragraph 48). 32      Judgments of 7 September 2004, Waddenvereniging and Vogelbeschermingsvereniging (C‑127/02, EU:C:2004:482, paragraphs 58 and 59); of 17 April 2018, Commission v Poland (Białowieża Forest) (C‑441/17, EU:C:2018:255, paragraph 117), and of 15 June 2023, Eco Advocacy (C‑721/21, EU:C:2023:477, paragraph 38). 33      See my Opinion in ASCEL (C‑436/22, EU:C:2024:83, point 67) and in PAN Europe v Commission (C‑316/24 P, EU:C:2025:422, points 117 and 128). 34      Judgments of 29 July 2024, ASCEL (C‑436/22, EU:C:2024:656, paragraph 72), and of 12 June 2025, Eesti Suurkiskjad (C‑629/23, EU:C:2025:429, paragraph 42). 35      Judgment of 1 August 2025, Voore Mets and Lemeks Põlva (C‑784/23, EU:C:2025:609, paragraph 49). 36      Judgments of 21 June 2017, W and Others (C‑621/15, EU:C:2017:484, paragraph 25), and of 6 October 2020, La Quadrature du Net and Others (C‑511/18, C‑512/18 and C‑520/18, EU:C:2020:791, paragraphs 222 and 223). 37      Judgments of 16 December 1976, Rewe-Zentralfinanz and Rewe-Zentral (33/76, EU:C:1976:188, paragraph 5); of 15 April 2008, Impact (C‑268/06, EU:C:2008:223, paragraphs 44 and 46), and of 19 December 2019, Deutsche Umwelthilfe (C‑752/18, EU:C:2019:1114, paragraph 33). 38      Judgments of 17 July 2014, YS and Others (C‑141/12 and C‑372/12, EU:C:2014:2081, paragraph 67), and of 17 December 2015, WebMindLicenses (C‑419/14, EU:C:2015:832, paragraph 83). 39      Judgments of 8 May 2014, N. (C‑604/12, EU:C:2014:302, paragraphs 49 and 50); of 14 May 2020, Agrobet CZ (C‑446/18, EU:C:2020:369, paragraph 43), and of 22 September 2022, Országos Idegenrendészeti Főigazgatóság and Others (C‑159/21, EU:C:2022:708, paragraph 35). 40      Judgments of 14 May 2020, Agrobet CZ (C‑446/18, EU:C:2020:369, paragraph 44), and of 21 October 2021, CHEP Equipment Pooling (C‑396/20, EU:C:2021:867, paragraph 48). 41      See, to that effect, the judgments of 28 January 2010, Commission v France (C‑333/08, EU:C:2010:44, paragraph 92); of 1 October 2019, Blaise and Others (C‑616/17, EU:C:2019:800, paragraph 46; see also paragraphs 93 and 94), and of 9 November 2023, Global Silicones Council and Others v Commission (C‑558/21 P, EU:C:2023:839, paragraph 66). 42      See, for example, the judgments of 7 September 2004, Waddenvereniging and Vogelbeschermingsvereniging (C‑127/02, EU:C:2004:482, paragraph 54), and of 12 June 2025, Eesti Suurkiskjad (C‑629/23, EU:C:2025:429, paragraph 42). 43      Judgments of 23 April 2020, Commission v Finland (Spring hunting of male common eiders) (C‑217/19, EU:C:2020:291, paragraph 70), and of 17 March 2021, One Voice and Ligue pour la protection des oiseaux (C‑900/19, EU:C:2021:211, paragraph 30). 44      Judgments of 8 May 2019, PI (C‑230/18, EU:C:2019:383, paragraph 57); of 24 November 2020, Minister van Buitenlandse Zaken (C‑225/19 and C‑226/19, EU:C:2020:951, paragraph 34), and of 24 February 2022, SC Cridar Cons (C‑582/20, EU:C:2022:114, paragraph 54). 45      1998 Convention on access to information, public participation in decision-making and access to justice in environmental matters (OJ 2005 L 124, p. 4), adopted by Council Decision 2005/370/EC of 17 February 2005 (OJ 2005 L 124, p. 1). 46      See, to that effect, the judgments of 15 October 1987, Heylens and Others (222/86, EU:C:1987:442, paragraph 15), and of 30 April 2009, Mellor (C‑75/08, EU:C:2009:279, paragraph 59). See also my Opinion in Holohan and Others (C‑461/17, EU:C:2018:649, point 65).