Hazards to Wild Birds Associated with Anthropogenic Structures and Human Activities—Results of a Long-Term Study in an Urbanised Area of the Alps

Simple Summary

We reviewed the wild-bird database of the Innsbruck Alpenzoo which also functions as a regional wildlife rescue and rehabilitation centre in the densely populated Inn Valley around Innsbruck, Austria. We analyse the seasonal, species- and age specific causes of admissions and explore long-term trends of the problems wild birds face in heavily urbanised landscapes. This is possible because the database covers 33 years from 1988 until 2020. Overall, our data indicate that basically all bird groups and species are at least occasionally affected by human activities and structures, and that this kind of threat to wild birds is still an underestimated conservation problem. At the same time the results of the study demonstrate the value of databases from rescue institutions for a better understanding of threats to avian wildlife.

Abstract

We analyse data from a rescue database collected at the Innsbruck Alpenzoo (Tyrol, Austria). The sample covers 33 years (1988–2020), and more than 5250 wild birds from 145 species originating from Innsbruck and the surrounding Inn Valley, one of the most densely populated areas in Europe. Both, the total number of birds as well as the number of bird species yearly admitted have increased since 1988. Orphaned nestlings and victims of glass collisions were the most common reasons for admission and responsible for the increase. Species’ susceptibility to accidental causes increased with regional abundance and degree of urbanisation. More urbanised species are characterised by a high proportion of nestlings and juveniles in the sample. The seasonal patterns of deliveries in these species show a peak in the late breeding season, and young birds are particularly susceptible to glass collisions and cat attacks. The species list also includes regionally rare wetland, upland and forest breeders and foreign migrants. Such species show a high proportion of admissions in autumn and collisions with windows play a greater role for short-distance migrants. Our data also suggest that small birds (<15 g body mass) are more likely to collide with glass panes than larger species. In conclusion, our data suggest that basically all bird groups and species are at least occasionally affected by human structures and activities in urbanised landscapes but support the notion that juveniles and migrants are more prone for accidents due to the lack of experience with anthropogenic structures in new areas.

1. Introduction

Urbanisation and suburbanisation are progressive in all parts of the world [1] and are accompanied by severe habitat changes and loss of natural habitats for wild animals [2,3]. Therefore, the importance of cities and other human settlements as replacement habitats, is increasing. This applies in particular to many mobile and adaptable animals such as birds [4,5].

On the other hand, the increasing contact of wild birds (as migrants as well as residents) with heavily transformed landscapes also increases the risk of accidents at structures that are unfamiliar to wild birds or of being impaired by human activities. These risks are considerable, and the sources of danger are manifold (overviews with further literature e.g., [2,4,5,6,7,8]). In addition to the wiring of the open landscape [9,10] and two (relatively new) threats, namely solar parks and wind farms (e.g., [11,12]), two causes of accidents have received particular attention in scientific literature and in public perception: collisions with glass bodies, and attacks by domestic cats. For both aspects, there are many, sometimes dramatic, assessments at global to regional scales [13,14,15,16,17,18]. Accordingly, a wealth of recommendations and guidelines for mitigating the problems have been published [19,20,21]. Other causes of accidents are less well accounted for and, in general, many studies and reports focus on the dimension of fatal accidents and the assessment of the number of unreported cases [22,23]. Much less is known about the seasonality and species-specific dimensions of these problems, and about the fate and survival chances of birds that are “only” injured by anthropogenic structures or weakened by human disturbance or other environmental influences (though see [8,24,25] with further references).

Wildlife rescue and rehabilitation centres are common in many countries resulting in the rescue, care, and release of maybe millions of animals every year [8,26,27]. In many cases, however, these centres are run by volunteers, or animal care people. These people often lack time, money, motivation and scientific experience to invest in a standardised recording and analysing the wealth of data they dispose of. Sound published studies based on information in rescue databases thus are still relatively rare, and despite a growing recognition of the utility of wildlife rescue and rehabilitation data, it remains a largely underutilised source of information [25]. In addition, as Cope et al. have shown [8], causes for bird rescues and chances for their rehabilitation also vary depending on study locations. While, for instance, studies examining causes for bird hazards are more commonly published for North American, Australian and South-African species (often with a focus on raptors, oiled seabirds or bush fire victims) such overviews are still limited in Europe or also mainly focus on raptors [10,28,29,30,31,32,33], or only single other species [34]. For densely populated central European study areas—to our knowledge—published data with a focus on small birds are nearly lacking (but see a popular overview in [27]) or restricted to bird-window hazards (overview [18]), or to bird-health problems caused by pathogens and diseases [35,36].

Here we present long-term data from a rescue database supervised and organized by professional ornithologists at the Alpenzoo Innsbruck, Austria, one of the few zoological gardens that also takes care of orphaned or stranded juveniles, disoriented, weakened or injured birds collected by the public in the wild and delivered to the zoo [37,38]. This data sampling is valuable not only from a regional faunistic perspective [38]. It also provides deeper insights into the long-term development of problems faced by birds in the modern cultural landscape as our sample originates mainly from urban and settlement areas in the Tyrolean Inn-valley, one of the most densely populated areas in Europe.

2. Materials and Methods

2.1. Study Area and Origin of Rescued Birds

The federal Austrian state of Tyrol (12,648 km²) lies at the heart of the Alps and is dominated by mountainous terrain. The Innsbruck Alpenzoo is located on a southern slope on the northern outskirts of Innsbruck, the Tyrolean capital with around 132,000 (2024) inhabitants [39] (city centre at 575 m above sea level, built-up area between 565–740 m asl). Innsbruck lies in the centre of the densely populated Inn Valley (see discussion). Although people’s motivation to bring rescued birds to the zoo apparently also depends on the size and conspicuousness of the bird, most individuals came from an area nearby. While the median/mean distance to the Alpenzoo was 3/16 km for small birds (<50 g) and 5/20 km for medium-sized species (50–700 g), even larger conspicuous species were collected only 15/31 km away from Innsbruck Zoo (for details see [38]).

2.2. Bird Admittance Procedure and Data Stock

As regional visitors can drop off young, weak or injured birds, the Alpenzoo has seized this opportunity to collect data about such birds via a standardised admittance procedure. The data sets include the circumstances of discovery (e.g., date and place of rescue, causes of accidents), the age and sex of birds, the general condition (health state) and the type and severity of injuries sustained by the birds [37,38]. As records were subsequently also kept of failures and successes of care measures (death or recovery) and—in the case of success—the duration until release from care, conclusions can be drawn, for example, on the age- and species-specific chances of survival depending on the type of injury or cause of the accident (Böhm et al. in prep.). However, here we focus on hazard risks and causes of accidents.

Over a period of 33 years (1988–2020) the Alpenzoo has collected information about 5379 bird individuals, not regarding a few birds without a clear species identification. These birds belong to 163 species, from which 145 species (excluding Feral Pigeon Columba livia var. domestica) and 5257 individuals were wild birds found within a maximum distance of 150 km, but mostly close around Innsbruck (see above,

Table 1. Number of rescued birds and bird species delivered to Innsbruck Alpenzoo since 1988 and sample sizes useable for analyses of different aspects.

Categories/Causes for Admission	n Individuals	n Species
Total rescued/admitted birds *1	5379	163
Wild birds from the study area *2	5257	145
– period I: 1988–1999	1365	110
– period II: 2000–2010	1511	107
– period III: 2011–2020	2358	106
– winter (December–February)	501	67
– spring (March–May)	1165	95
– summer (June–August)	2640	98
– autumn (September–November)	923	111
– cause of admission unknown/not registered *3	2205	126
– orphaned or stranded nestlings *4	1638	65
– “weak” birds *5	335	64
– other specific causes of admittance *6	1079	106
Total number of specific causes analysed	3052	123

^*1 including data sets of 44 nestling birds of questionable species affiliation, Feral Pigeons and allochthonous captive escapes. ^*2 excluding local nestlings of questionable species affiliation, Feral Pigeons, allochthonous captive escapes and specimens from abroad ^*3 including 889 adult, 421 juvenile and 895 birds not assigned to an age group; data only included in analyses of long time- and seasonal patterns ^*4 not included in some accident analyses ^*5 emaciated, exhausted birds able to fly including fledglings (reasons for weakness partly unknown—no evident injuries or causes for weakness) ^*6 injured or impaired victims of attacks of cats and other raptors or of collisions and other accidents (only adults and juveniles able to fly).

and Table S1). The wild bird list includes species from 21 orders and 52 families (see [38]). Overall, 94 out of these 145 species are regular breeding birds in North Tyrol, 18 have been breeding locally within the last 50 years, and 33 occur only as migrants, including some rare guests.

The data collection was supervised and controlled by professional scientists employed by the Alpenzoo (1988–1994 by the renowned ethologist and ornithologist Ellen Thaler ✞ 2024 [40]), and from then on by curator Dirk Ullrich and one of us (CB).

The finder of the bird was asked to provide a detailed description of the location and conditions under which the bird was found. All admitted birds were thoroughly examined by both the bird keepers and the vets for injuries, broken bones and conspicuous wounds. In addition, the behaviour and type of injuries usually provide good indications of the causes of the accident: Surviving glass victims show clear, often unilateral paralysis of the legs and/or wings. They frequently squint their eyes, often unilaterally, and appear apathetic. Cat victims also often appear apathetic but have obvious injuries that were detected during the initial examination. However, it cannot be ruled out that some glass victims were later seized by cats and counted as cat victims. Very emaciated individuals without any injuries were classified as “weak”. If no obvious injuries could be found, X-rays did not provide any information and there were no signs of poisoning, the bird was also classified as “weak”.

2.3. Study Limitations

Unfortunately, the individual data records are often not complete and there are regrettable information gaps in the entry lists (Table 1), as sometimes other zoo staff were also entrusted with taking over the birds but mainly because the information provided by the people who handed over the birds was of varying quality and completeness.

Therefore, we decided to categorise many cases as ‘causes of admission unknown’ (see Table 1), even when there were good indications of specific causes. Thus, the sample size that can be analysed, varies for individual aspects or species (Table 1). In total, only 25 of the 145 species have more than 10 data with exact information on the specific causes of admission (excluding nestling data). The structure of the datasets also makes statistical processing complicated. Not only we had to omit many incomplete datasets for several specific statistical analyses that are useful and informative from other points of view and general aspects, and also data for species with only a few admissions are not sufficient for statistical treatment. Therefore, we have refrained from complex modelling of the data and have mostly relied on simple pairwise comparisons and tests. Problems relate not only to the completeness of the data, but also to the registration itself. Particularly in the case of Blackbirds (Turdus merula), Common Swifts (Apus apus) and Carrion Crows (Corvus corone) and House Sparrows (Passer domesticus), the four species most frequently delivered, the number of specimens handed over to the zoo is probably even higher than our data show due to poor registration during the stressful breeding season. Feral pigeons, which are not accepted for care, are also underrepresented in the material.

2.4. Data Preparation and Statistical Analyses

The original data were checked for accuracy and consistent formatting. Records with large gaps, unclear and uncertain entries, and data belonging to captive refugee bird individuals or species brought in from outside our study area were omitted. The database was supplemented by us for this study with species-specific classifications. For each species we (1) assessed the degree of urbanisation (with a focus on the situation in Tyrol; seven-level scale), (2) estimated the regional frequency on a scale of 1 (very rare) to 10 (very common) from literature data [38,39,40] in combination with our own avifaunistic experience (see Table S1), and (3) assigned each species to one of six body size classes (using mean adult body weights from literature). For definitions and class boundaries see Table S2. Furthermore, we entered: (a) the regional status of the respective species (regular, irregular breeding bird or passage migrant, winter visitor in Tyrol—data from [41,42,43], (b) an affiliation to ecological guilds (food preference, type of nesting site, regional breeding phenology, migration type), and (c) the national (Austria) and regional (Tyrol) Red List status if applicable [44,45].

To compare the importance of single risk factors between species we calculated the proportion of a single factor in relation to all cases of a species (excluding unknown admittance causes and nestlings). Differences between the proportion and seasonal frequency of the various causes of bird admissions to the zoo and between species or ecological groups were tested using standard univariate test statistics (chi²-tests with Yates correction; Fishers exact test for small sample sizes; t-tests).

To analyse the influence of ecological traits (regional abundance, regional urbanisation and body mass as independent variables), we used (a) the total number of individuals admitted over the years and (b) the regularity of admission (expressed as the number of years with at least one admitted specimen) as dependent variables. In the respective analyses, nestlings or years in which a species was recorded only as a nestling were omitted to avoid bias due to the overrepresentation of urban and regionally common species, from which most nestling data originate.

We performed a Kruskal-Wallis one-way analysis of variance by ranks to test for pairwise differences because normality tests (Shapiro-Wilk) failed. To avoid spurious correlations when multiple pairwise testing was performed, we used adjusted p-values by applying the FDR method which not only reduces false positives but also minimises false negatives [46]. Correlations between the variables across all classes were also tested with simple linear models and Pearson product moment correlations. Data were arranged and pre-analysed with Excel (vs. 16 for Windows Office 365, Microsoft). For graphics and statistical analyses, the respective tools in SigmaPlot 12.0. (Systat) or Excel were used.

3. Results

3.1. Causes and Development of Admissions over 33 Years

Since 1988 at least 5257 either injured, weakened or disorientated wild birds from the study area have been handed in to the Innsbruck Alpenzoo. These birds belong to 145 bird species but are very unevenly distributed across the species. 26 species (18%) are represented by only a single individual, while a further 49 species are represented by fewer than 10 birds. In contrast, the 15 most frequently admitted species already represent two-thirds (66.7%) of all individuals (cf. Table S1). The total number of birds and bird species brought in has fluctuated from year to year but have shown upward trends since 1988 (Table 1, Figure 1). Bird deliveries have increased from the last decades of the 20th century (1988–1999: average 114 ± 53.4 birds/year) to 137 ± 38.5 birds/year between 2000 and 2010 and up to 236 ± 38.4 birds/year in the last decade (2011–2020). The differences between the time periods are significant (p < 0.001; two-tailed t-tests, t = 7.37 period I vs. II, and t = 11.91 period II vs. III). The total number of species recorded varied only slightly between the 10- to 12-year long periods between 106 to 110 (Table 1), but on average more species were also recorded per year since 1988 (1988–1999: 25 species/year; 2000–2010: 44 species; 2011–2020: 54 species).

Four reasons dominate across all years and time periods (Figure 1): (1) ‘orphaning’ of nestlings, including those dependent birds rescued in the course of nest destruction by human activities, e.g., tree felling, removal of hedges, building renovations (53.7% of 3052 documented cases), (2) glass collision victims (20.8%), (3) “weakness” of birds delivered (11%), and (4) victims of cat attacks (8.6%). All other reasons together account for less than 5% of all cases, including only 2.3% of victims of car accidents, which have proportionally decreased over time (from 4.9% to 2.9% to 0.8% share of all reported cases from period I to period III). The same applies to the proportion of ‘weak’ birds which were handed over to the zoo. In contrast, the other three main reasons for rescues showed upward trends from the beginning to the end of the records, both in terms of the number of birds and the number of bird species involved (Figure 2).

3.2. General Seasonal Aspects

Half (50.5%) of all deliveries (including those with an unknown background—Table 1) take place between June and August, and only 9.6% in the actual winter months (December to February). However, the differences between the seasons are far less pronounced in terms of the number of species. The number of species delivered in spring was almost the same as in summer, although the number of delivered birds in summer was about 2.3 times higher than in spring, and the number of species in autumn was even higher than in summer, although the number of birds delivered in this season was almost three times lower than in summer (see Table 1 and graphs in [36]).

Depending on the causes there are strong differences in the seasonal distribution of admissions (Figure 3). Not surprisingly, nestlings are only admitted during the breeding season, but the increase in cases towards the end of the breeding season (June, July) is remarkable. This increase is roughly mirrored in the admittance patterns of the other three main causes, as high proportions of juvenile (but already flyable) birds are typical in the late breeding season until mid-August. In particular, a high proportion of the victims of cat attacks at this time of year are juveniles (65% of all cat-victims from June to August). It is also noticeable that the number of victims of glass collisions and—to a lesser extent—the proportion of victims of cat attacks peak or reach a second peak during the autumn migration (Figure 3). For instance, less than 8% of 262 cat-victim admissions stem from December to March but at the same time a relatively high proportion of 19% admissions occurred during the late autumn.

There are also differences in the proportion of ‘accident’ causes between adult and juvenile birds. While adult birds are significantly more frequently delivered as victims of glass/window collisions (χ² = 67.0; p < 0.001), juvenile birds are more frequently handed in as victims of cat attacks (χ² = 35.3; p < 0.001) or due to “weakness” (χ² = 50.9; p < 0.001) compared to adult birds. Since for all three causes the proportions of birds delivered do not differ between age-unassigned birds and adult birds but are significantly different to juvenile birds (p < 0.001), we conclude that the data without clear age assignment (see Figure 3 and Figure 4) probably also mainly refer to adult birds.

3.3. Species-Specific Seasonal Patterns

The admission list of 145 species (see Table S1) is dominated by individuals of bird species which regionally are common breeders in cities and villages. Among the most common 25 species on the list, which together account for more than three quarters (76.5%) of all individuals in the sample, 18 species belong to this group. These 18 more urbanised species are characterised by high proportions (65.8 ± 19.5%) of nestlings (and juveniles) in their sample, and together account for 85% of all 1638 deliveries of nestlings. Accordingly, the seasonal patterns of admittances in such species show a clear peak in the (later) breeding season (May to July). As examples, Figure 4 depicts the patterns of Blackbird and Common Kestrel (Falco tinnunculus), which are even more pronounced in some other common urbanised birds with high proportions of nestlings in the admittance list (cf. data in Tables S1 and S3 for e.g., House Sparrow, Great Tit (Parus major), Coal Tit (Periparus ater), Blue Tit (Cyanistes caerulea), Black Redstart (Phoenicurus ochruros), Common Swift or Mallard (Anas platyrhynchos).

Bird species that mainly breed in the surrounding montane forests show different seasonal patterns. Such species increasingly occur in or near settlement areas after the breeding season or in winter and are usually found near feeding sites, windows (e.g., European Siskin Spinus spinus, Goldcrest Regulus regulus, Sparrowhawk Accipiter nisus) or roadsides (raptors, especially Eurasian Buzzard Buteo buteo). Thus, they are mainly delivered at the Alpenzoo from late autumn to February (see Figure 4). Similar patterns, but with a stronger emphasis on deliveries from September to November, are typical for short-distance migrants such as the European Robin (Erithacus rubecula; Figure 4: see Table S1 for further examples).

3.4. Species-Specific Differences in Risk Factors

3.4.1. Susceptibilities to Glass Collisions

Not regarding nestlings, glass collision is the dominating reason for admission in most more common species of urban songbirds, for Great spotted Woodpecker (Dendrocopus major,), and for some raptors (cf. Tables S1 and S3). In raptors, especially Sparrowhawks (84%) face a significantly higher glass collision risk than Common Kestrels (41%, χ² = 24.9; p < 0.001), Eurasian Buzzards (37.3%; χ² = 36.8, p < 0.001) and Tawny Owls (Strix aluco, 50%; χ² = 9.3, p = 0.002). There are also pronounced differences in glass collision risks for the 20 most abundant songbirds in our sample (cf. Table S3). Disproportionally high risks are obvious for European Bullfinch (Pyrrhula pyrrhula), Common Chaffinch (Fringilla coelebs), European Siskin, Song Thrush (Turdus philomelos), European Robin and Goldcrest (glass collisions accounting for >50% of all reported cases) (see Table S3 for significant differences between single species).

Data for species with only a few admissions are not sufficient for statistical treatment, but, nevertheless, it is striking that even some species that are only rarely handed in were brought to the zoo predominantly or exclusively due to one specific cause of accident. This applies especially for victims of glass collisions and holds true for most raptors and woodpecker species with few admittances (see Tables S1 and S3), as well as for some other larger migrants (e.g., European Woodcock Scolopax rusticola 81%, Common Snipe Gallinago gallinago 71%). Similar percentages are noticeable for a greater number of regionally uncommon songbirds (e.g., Garden Warbler Sylvia borin 88%; European Starling Sturnus vulgaris 83%; Willow warbler Phylloscopus trochilus, 80% or European Pied Flycatcher Ficedula hypoleuca, 70%; absolute numbers and other species see Table S1.

3.4.2. Victims of Cat Attacks

Some species seem to fall victim to cat attacks more easily (or more often) than others (cf. Tables S1 and S3). Most individuals of species which were handed in as cat victims are small (song)birds with body weights below 100 g (42 of 52 species) or even below 50 g (30 species). Nevertheless, sometimes even larger birds are attacked by cats (Carrion Crows four, European Woodcock three, Mallard two, Eurasian Hobby Falco subbuteo, Common Kestrel and Common Moorhen Gallinula chloropus one case). More common garden birds, which are more often brought to the zoo under the label “cat victim” according to our data are not equally prone to cat-attacks. Especially Blackbirds, which in absolute as well as in relative numbers are most often brought in as cat victims, seem to be in danger and are significantly more often attacked than other common songbirds like European Robin, House Sparrow or European Greenfinch Chloris chloris (p < 0.001, < 0.01, < 0.05 respectively; χ² tests). Other species that have been preyed on more than once case by cats, see Table S1.

3.4.3. “Weak” Birds

In addition to some more common songbirds, birds of prey and all aerial insect hunters are prominent in the list of birds delivered in an emaciated or exhausted state (see Tables S1 and S3). Among these species especially European Eagle Owls (Bubo bubo, 10 of 16 cases), Eurasean Hobbies (7 of 15), Common Swifts (88 of 115), Barn Swallows (Hirundo rustica, 9 of 17), House Martins (Delichon urbicum, 5 of 7) are disproportionally often brought to the zoo in an “weak” state (figures for songbirds and other raptors see also Table S1, significant differences cf. Table S3). The Common Swift, the species exhibiting the highest proportion (76.5%) of “weakness” as admittance cause has significantly higher proportions than all common urban songbirds like Carrion Crow, European Greenfinch, House Sparrow or Blackbird (p < 0.001; χ² tests).

3.4.4. Nestlings

As already mentioned, 85% of all 1638 nestling deliveries at the Innsbruck Alpenzoo are accounted for by a few urbanised species (e.g., in Figure 3). But even among the species with the highest proportions in this respect, there are significant differences (cf. Table S3). Especially Black Redstarts are almost exclusively admitted as nestlings (92.5% of 53 recordings). This proportion is significantly higher than that of other common building breeders and of other garden birds (cf. Table S3). Overall, common urban songbird species (as well as Common Swifts) that prefer to breed in and on buildings are delivered to the zoo as nestlings in more than 50% of all cases. The same applies to three larger bird species that also breed (in urban areas) exclusively or in increasing numbers on or near buildings (proportion of nestling admittances: Common Kestrel 54%, Mallard 92%, Goosander Mergus merganser, 91%).

3.4.5. Various Other Admittance Causes

Among the 138 other admission causes, 71 (51.5%) refer to victims of car collisions. These cases involve 21 species but mainly affect Eurasian Buzzard (39 cases) and—to a lesser extent—other raptors: Sparrowhawk five, Common Kestrel and Carrion Crow four, and single cases of Red Kite (Milvus milvus), Marsh Harrier (Circus aeruginosus), European Eagle Owl, Tawny Owl.

Half of the other 67 recorded accidents that led to the admission of birds at the zoo were due to attacks by predators other than cats (i.e., dogs, foxes, birds of prey, but in most cases by Carrion Crows or Ravens). Although the other half of causes tend to be anecdotal in nature and usually refer to only a few individuals of a few species, they additionally exemplify the variety of threats that wild birds face in the cultivated landscape. For instance, 27 cases of collisions with various artificial structures (fences, electric fences, barbed wires, power lines, nets, and even house walls, including one Capercaillie (Tetrao urogallus) and one Mute Swan (Cygnus olor) are reported. Other cases include a Barn Swallow trapped in a sticky fly trap in a stable, Tawny Owls, Goshawks and Common Swifts trapped in cellar shafts and a Wallcreeper (Tichodroma muraria) found unconscious after an avalanche blast.

3.5. Ecological Backgrounds for Differences in Accident Probabilities

The total number of individuals recorded during the study period and the number of years with at least one bird delivered, differ significantly between species and species groups. These dependent variables are significantly correlated with the regional abundance as well as with the degree of urbanisation of a species used here as independent variables (p < 0.001; Figure 5, Table S1; see also Table S2 for definitions and classifications). The regularity of admissions was more strongly related to the two independent variables than the total number of individuals delivered to the zoo (Figure 5 left vs. right column), and the degree of urbanisation was a slightly better predictor of the number and regularity of admissions than the regional abundance of a species (Figure 5, top vs. middle row), although it should be stated that both “independent” variables are clearly correlated (r = 0.687; p < 0.001).

Other species-specific attributes, such as the mean adult body weight of a species (see Figure 5, bottom row), membership of a particular ecological guild (food preferences, foraging locations) or migratory guild (short-distance vs. long-distance migrants, resident birds, winter guests) showed no clear correlation with the probability of a species being involved in an accident other than glass collision (see below) or being affected by human activities in our study area. Apart from the higher proportion of building breeders among nestlings (see above), there is also no evidence that nest site preferences (cavity breeding vs. open tree, hedge or undergrowth breeding) have a considerable influence on the frequency with which a species is brought by the public to the Innsbruck Alpenzoo.

Glass collisions accounted for a significantly higher proportion (52% vs. 36%; χ² = 19.1; p < 0.001) of all accidents (excluding nestling admissions) in 28 species with a low degree of urbanisation (index values 1–3) than in 30 urbanised species (index values 5–7). Similar differences (59% vs. 41% share of glass collisions; χ² = 35.4; p < 0.001) also exist between regionally only moderately common to relatively rare species (regional abundance indices 1–5) and regionally more common birds (indices 6–10). There are also differences with respect to migration strategies. Overall, the proportions of glass collision victims in 61 species of short distance migrants (including partly migrating and winter invasion species) are significantly higher than for 30 resident species (58% vs. 39%; χ² = 10.5, p = 0.001), and was also higher in short distance- compared to long distance migrants (46 species, with 28% glass collision victims; χ² =36.7; p < 0.001). In addition, very small songbirds (body mass < 15 g; 21 species) appear to be more prone to glass collisions on average than heavier songbirds (15–50 g; 38 species; 63% vs. 38% glass casualties; χ² = 17.5; p < 0.001) and also than larger birds (>300 g; 40 species; 43%; χ² = 11.7; p < 0. 001).

4. Discussion

4.1. Data Stock and Origin of Rescued Birds

Studies reporting on a wide range of rescued animals and studying patterns and causes affecting wild birds over an extended period and thereby considering long-term trends, are still scarce and most studies cover only a few years and only a few dozen species [8,25,26]. According to a review of research papers analysing rescue databases [26] such studies averaged 8.9 years in duration (maximum 17 yrs.) and included on average only 26.9 species and 4163 individuals but including not only birds but also reptiles, mammals and amphibians (e.g., [47,48]). On the contrary, we here analyse data on 145 wild bird species alone and about 5257 bird individuals collected over a period of 33 years which enables a more precise analysis of the seasonality, species-specific dimensions and long-term trends of the problems wild birds face. What is more, the area from which rescued animals come from in many studies is either very large, spatially unclear delimited or not specifically accounted for. This makes it difficult to assess the influence of landscape features on the dimension and causes of bird hazards. In our study the majority of birds was rescued in the central Tyrolean Inn valley and nearby terrasses and most data origin in the city of Innsbruck and suburban areas around the capital (see also [38]). The Inn Valley around Innsbruck can be considered one of the most densely populated areas in Europe. In a section of the valley floor about 25 km upstream and downstream of the city centre, around 265,000 people live in an area of only 125 km² (including farmland, forest edges, river, traffic areas and some foothills). This population density of around 2150 people/km² is—for comparison—four times higher than that of the densely populated Netherlands or San Marino, 8.5 times higher than that of Luxembourg and almost 20 times higher than the average for Austria [49]. Our data therefore mainly reflect seasonal patterns and problems faced by birds in urbanised landscapes heavily influenced by human structures and activities.

4.2. Spectrum and Systematics of Affected Species

According to regional faunal lists [41,42,50] nearly three quarters of the local breeding bird species and about the half of all species recorded since 1970 in North Tyrol have been brought to the zoo with at least one individual. In addition, of the breeding species, 37 are listed in the regional Tyrolean Red List and 24 in the national Austrian Red List [44,45]. In our opinion, this not only indicates that basically all bird groups and species are at least occasionally affected by human activities and structures, but also that these threats to wild birds are still underestimated conservation problems compared to habitat destruction.

With 23 families, 70 species and 3125 individuals, songbirds are the dominating order in our sample. This fits to the well know dominance of this adaptive bird order in European cities and rural settlements (e.g., [51] with further references, for local villages and the city of Innsbruck see [52,53].

4.3. Long-Term Trends in the Number of Admitted Birds and in Causes for Admission

To our knowledge, there are no published long-term trends in the admission rates of wild animals to rescue centres, at least for European birds or urbanised landscapes. Both, the total number of wild birds and the number of bird species brought to the Innsbruck Alpenzoo have shown an upward trend since 1988. This trend is mainly due to a significant increase in the number of nestlings and victims of glass collisions and cat attacks. In contrast, a certain trend towards a decrease in the number of cases in which ‘weakness’ and car accidents were cited as the reason for hospitalisation was observed. The reasons for the observed trends are diverse and may be complexly interwoven.

At first, in Tyrol the topographical conditions limit the space available for agriculture, settlements, traffic systems and industrial development, which in the last decades has increased the pressure for landscape transformation and concentration of anthropogenic structures and urbanisation in the central Inn valley and at other lower altitudes which are favourable for human needs, but also for wildlife [43,54]. Thus, in Tyrol—as in the Alps in general [55,56]—settlement areas have been among the fastest growing land use types for decades, with an extraordinary strong increase in built-up areas and buildings (e.g., an overall 80% rise in the number of buildings since 1980 [57], a trend that is likely to be even stronger in the actual study area). At the same time rural settlements in the vicinity of Innsbruck have undergone rapid transformation, with clear tendencies towards suburbanisation (transformation of farming villages into suburban-like residential areas– see [53,58]. With the increase in population and the changing population structure, building design techniques and fashions have also changed in recent decades. In addition to the remodelling and mostly less bird-friendly renovation of existing buildings [7,59], the trend towards glass is particularly noticeable in new buildings. The trend towards large glass façades and more glass panes has not only prevailed in urban areas, but also in many open spaces [20], and so the increase in the number of victims of glass collisions over the years is not surprising.

Secondly, we speculate that emotional attitudes of people towards nature and animals may have changed as well. This on the one hand is expressed by the obvious increase of the proportion of people keeping cats, dogs and other pets which is well documented in industrial countries all over the world (e.g., [13,14,60,61], and which may in part explain the increasing “cat problem”. On the other hand, it seems likely to us that as the living environment becomes less natural and thus contact with nature decreases, the public’s motivation to help injured wild animals and to bring (often only apparently) abandoned dependent young birds and nestlings to an accessible, nearby animal care centre has increased. For instance, about 39% of 3279 wild birds delivered to the Leipzig wild bird rescue centre between 2013 and 2023 refer to nestlings [27]; in our study this figure is 31% (all cases) or 54% when only cases with known background of admittance are considered.

At first glance, the decline in the number of victims of car accidents during the study period does not seem to be consistent with the increasing urbanisation in the Inn Valley, which was of course also accompanied by an increase in car traffic. However, the car accidents in our sample mainly affected birds of prey, especially Eurasian Buzzards, which are apparently injured mainly in cold and harsh winters when searching for prey at the roadside (compare e.g., [62]). As mean winter temperatures in the Inn Valley have risen significantly since the late 1980s and the length and extent of snow cover has decreased [63], it is possible that warmer winter half-years will reduce wintering problems for birds of prey at forest edges and open farmland and thus the need to search roadsides. In addition, less severe winters may also reduce the frequency with which raptors such as Sparrowhawks, Goshawks and Tawny Owls enter urbanised areas and become victims of collisions (cf. negative trends in admittances for Eurasian Buzzards, Sparrowhawks and Tawny Owls in [38]).

4.4. Seasonal Aspects and the Bird-Window Collision Problem

While a wealth of data on the seasonal patterns of bird-window collisions (BWCs) is available in the literature, comparable data for other causes are scarce (but see [62] for roadkills) or missing. At least there are some studies indicating that cats in temperate climates are more active during summer or mild weather months than during winter [64], which seems to be the case in our study area as well.

Many North American [15,65,66,67,68], but only few European studies [69] have found an increase in bird window collisions during the migration season with a focus on autumn migration. The number and proportion of BWCs also increased in our sample during this period, and more species are involved in such accidents then. The number of BWCs peaked in later autumn (from October onwards), as in our sample mainly short-distance migrants and vagrant birds from forest habitats are affected. This is also reflected in the recording patterns of individual species such as Sparrowhawk, European Woodcock, Great spotted Woodpecker, Goldcrest, European Robin, European Siskin, Song Thrush or Common Chaffinch). It is noteworthy that these species are also prominently represented in the lists of BWC victims in other urban locations in Europe (e.g., [28,70] for sparrowhawks or in general [27,69]) and should therefore be emphasised as species particularly at risk from glass collisions.

The predominance of BWCs during migration periods is obvious in our data if only adult birds are considered. However, BWCs were more frequent in juveniles than in adults in late spring and summer (67% of the 115 cases from June to August), and the number of reported BWCs in summer was similar to that in autumn (67 vs. 69 birds per month) and significantly higher than in winter (36 birds) and spring (March–May: 40 birds/month) if young birds are included. Our data thus suggest that BWCs may be an underestimated problem during the breeding season (see also [71,72]). Furthermore, our data support the hypotheses of [73] that: (a) the lack of experience with anthropogenic structures in new areas may explain that migratory birds are more prone to glass collisions than resident birds, and (b) that within a breeding population, young birds are proportionally more likely to crash into windows than adult birds. The first hypothesis is supported in our study not only by the predominance of BWCs during autumn migration, but also by the fact that the proportion of BWCs in all accidents was significantly higher in species with a low degree of urbanisation than in urbanised species. The second hypothesis is also supported by our data, although the study of Sabo et al. [73] itself found no differences between adult and juvenile birds, which in our view has little to say given the small sample size of this short-term study (compare [70,74] with young birds also overrepresented as BWCs victims).

Lack of experience in combination with lower mobility could also explain the higher proportion of cat casualties in juveniles compared to adults in our data set. Seasonal declines in reproductive performance and lower fitness and/or survival of nestlings and second-brood juveniles, mainly due to a seasonal decline in food quality and/or lower parental investment, have been observed in many bird species, and such problems may be even more pronounced in urban habitats (e.g., [75] with further references). In our opinion, this may partly explain the fact that the number of victims of cat attacks as well as of weak and/or emaciated birds in our sample is low in the first half (March to May), but increases towards the end of the breeding season, although a large proportion of the data refer to common urban bird species with early first broods (from March onwards) and thus fledglings already early in the season.

4.5. Ecological Traits and Vulnerability to Accidents Causes

The total number of individuals taken into care by the Innsbruck Alpenzoo in 33 years as well as the regularity of admission and the reasons for admission, differ between species and species groups. Rescued nestlings and victims of glass collisions have been the two most common specific reasons for admission. Due to the erratic and variable nature of the information provided to the Zoo staff by the deliverers, we are unable to analyse the influence of the specific nature of the locations where the birds were found. However, our data may provide general insights into ecological traits that can improve our knowledge of differences in group- and species-specific vulnerability to the main hazards that threaten birds in urbanised landscapes. Again, only for BWCs are there a larger number of detailed studies examining species characteristics responsible for differences in vulnerability, and most of these relate to North American birds [15,17,66,67,68,71,72,73,76]. The factors most often linked to high collision rates are the area, size and lighting of glass panes, the structure, presence and distance of glass bodies from vegetation and feeding sites, the local and regional abundance of birds and the variation in species and life history traits such as habitat preferences, migration strategies or foraging behaviour of the species concerned. In our case BWCs, in contrast to the overall patterns (see below) accounted for a significantly higher proportion of all accidents in species with a low degree of urbanisation and/or in regionally moderately common to rare species than in stronger urbanised and/or regionally more common species. In accordance to results of other studies also in the Inn valley migrant species exhibit higher proportions of glass collision than resident species albeit these species face this threat the whole year round. Klem [77] stated that window strikes are equally lethal for small and large species, but in our sample very small songbirds are overrepresented among the BWC victims compared to heavier songbirds and larger, mostly non passerine birds. It might be that this result is an artefact, but there is evidence also that fragile small species of North American birds, like hummingbirds, wood warblers and swifts, are prone to high fatality rates [67,74].

Our analysis revealed that in general (all cases, hazard causes and species combined) and also when excluding nestlings to avoid a overrepresentation of urbanised species, the regional abundance of a species as well as the degree of urbanisation, are fairly good predictors of the numbers and frequency a species is rescued in urbanised landscapes, a result which could have been expected from a general point of view (e.g., [78]). The correlation between regional abundance and urbanisation degree on the one hand indicates that flexibility to cope with the conditions of an increasingly urbanised landscape is an important prerequisite for success but on the other hand also hints to the disadvantages and manifold threats of urbanisation for wild birds (especially true e.g., for Blackbird, House Sparrow, Carrion Crow, Great Tit).

However, there are some notable deviations from the general pattern of an increase in admittances with increasing regional abundance and/or with greater regional urbanisation at the guild as well as at species level. Some regionally only moderately common or even relatively rare species and/or migratory birds that do not breed in the region and normally tend to avoid urban habitats were brought to the Alpenzoo in disproportionately large numbers and/or frequencies as expected. This holds true for most raptors, owls, and some waterfowl and wetland species but also for some songbirds (see [38] and Table S1). In addition, some urbanised bird species with comparatively small regional populations, noticeably Common Kestrel, Common Swift, Mallard and Goosander are proportionally over-represented within our data stock. These later species were delivered mainly as nestlings or as still dependent fledglings or, in the case of the Common Swift, also in emaciated conditions after prolonged periods of cold weather. Particularly noteworthy is the case of the Goosander, because it demonstrates a special problem adaptive waterbirds face when breeding at artificial structures in urban areas (see [33] for mallards). The Goosander, a regional and national Red List species [44,45], like in other places near the Alps [79], in the study area in the last decades started to use urban buildings as nest sites and to use public and private swimming pools to carry out dependent young birds that are still unable to fly. Accordingly, similar to Mallards and most other common urban bird species that prefer to breed in and on buildings, the majority of admissions in Goosander refer to “nestlings” albeit to a much higher extent than in most other well established building breeders.

5. Conclusions

Our data demonstrate that, although the ability to use buildings and their proximity for breeding, as a roost and/or as a food source is a characteristic trait of many particularly successful urban birds, this not only poses considerable new dangers for adult birds (e.g., glass collisions, attacks by pets), but in particular, for their unexperienced offspring and thus for the entire reproductive fitness of those species. In addition, our data support the hypothesis that the lack of experience with anthropogenic structures and settings in new areas may explain that migratory birds are more prone to glass collisions and other threats specific for urban areas than adult resident birds. However, the diversity of species brought to the Innsbruck Alpenzoo in the 33 years analysed shows that basically all bird groups and species are at least occasionally affected by human activities and structures and that this type of threat to wild birds is still an underestimated conservation problem. At the same time, the results of our study show the importance of raising public awareness of the problems facing wild birds and the value of rescue organisation databases (which rely on the cooperation of dedicated individuals), which we believe are still underused to gain a better understanding of the threats to birdlife.