Mitigating Acute Climate Change Threats to Reintroduced Migratory Northern Bald Ibises

Abstract

For the past 20 years, reintroduction efforts have been underway to re-establish a migratory population of Northern Bald Ibises (Geronticus eremita) in Central Europe, which now consists of more than 250 birds. They breed both north and south of the Alps and migrate to a common wintering ground in Tuscany. Recently, the start of autumn migration has been increasingly delayed, which correlates with extended warm periods in autumn. Later in the year, however, the birds no longer find sufficient thermals to cross the Alps and remain in the northern Alpine foothills. In order to save their lives, we had to capture the affected birds before the onset of winter, which is not a sustainable solution. A new approach to solving the problem is the establishment of a second migration route to a wintering area in Andalusia, Spain, connecting our population with a sedentary population there. The new migration route bypasses mountain barriers and also allows the birds to reach the wintering grounds later in the year. The modelling of a pan-European population will provide the birds with high ecological and spatial flexibility. Our project exemplifies the consequences of advancing global warming for animal populations and the associated challenges for conservation projects.

1. Introduction

Conservation translocation has been developing into a well-established and diverse discipline of animal and plant conservation [1]. Due to the increasing anthropogenic influence on all habitats, translocations of endangered species or reintroductions of extinct species in the wild are increasingly becoming the last remaining options to prevent the extirpation of a species [2]. The progressively clear influence of climate change is intensifying and accelerating this development [3]. Therefore, the need for creative and innovative methods in species protection is becoming ever greater and more urgent [4].

Historically, the Northern Bald Ibis (Geronticus eremita; Figure 1) inhabited extensive areas in the Middle East, Northern Africa and Europe. The species was migratory all over its historic range, with breeding areas in Northern Africa, the Middle East and Europe and with different wintering sites, especially along the African west coast down to Mauritania and Senegal and along the east coast down to Eritrea and Ethiopia [5,6,7]. Historical records from the European population prove that the birds have left their breeding grounds over the winter, but the historic wintering sites remain unknown [8,9].

Human influence and climate change caused the extirpation of the Egyptian population by the end of the third millennium BC and the European population in the early 17th century [10]. Subsequently, the species went extinct in most of the remaining regions till the early 21st century, except for a small population with two colonies on the Moroccan Atlantic coast, which, however, have transitioned to a sedentary lifestyle [11,12,13]. The Northern Bald Ibis, with its species-typical migratory lifestyle, has completely disappeared throughout its entire former distribution area. Due to the massive population decline, the species was listed as critically endangered on the IUCN Red List for 24 years before being downlisted to endangered in 2018, due to successful conservation efforts in Morocco and elsewhere.

Since the early 1950s, approximately 100 juveniles have been imported from former breeding colonies in the Moroccan Atlas to European zoological gardens. These individuals served as founders of the captive population, which grew over time to several thousand individuals [14]. Thus, the Northern Bald Ibis is an outstanding example of the increasingly important role of zoological gardens in species conservation [15]. Well-managed zoo populations provide the opportunity to conduct research and ultimately transfer individuals back into the wild. In 2018, a comprehensive study on Northern Bald Ibis genetics was published indicating that despite the relatively small number of founding individuals the European zoo population is genetically highly structured and, thus, forms a good basis for translocation measures [16].

As a migratory species, the Northern Bald Ibis exhibits genetically determined migratory restlessness [17], strong navigation skills and persistent, energy-efficient flight techniques [18,19,20,21,22]. These traits persist even after generations of captivity in zoos, where they cannot perform migratory behaviour. However, juveniles rely on experienced conspecifics during their first autumn migration to reach a suitable wintering region. Therefore, the spatial orientation of migration and its destination are primarily socially learned traditions passed down through generations [10]. However, the historic migratory traditions have been lost with the extirpation of all wild migratory populations.

Since 2002, a European conservation translocation project, co-funded by the European Union’s LIFE programme, aims at re-establishing a self-sustaining migratory Northern Bald Ibis population in Europe. This endeavour marks the world’s inaugural attempt to reintroduce a migratory bird species that had gone continentally extinct.

However, a recent trend towards a progressively delayed onset of autumn migration has emerged. This shift towards an ever-later migration has no noticeable negative effects on birds breeding south of the Alps; they migrate later in the year but reach the common wintering area in Tuscany safely. However, it does cause increasing problems for birds breeding north of the Alps, where an ever-larger number fails to cross the Alps during the autumn migration despite repeated attempts. Ultimately, they remain north of the Alps, which poses a threatening situation since these birds hardly survive the winter in this region.

In view of this development, we were required to take emergency measures in order to avoid serious losses. This involves capturing the birds remaining in the north in time before the first onset of winter, transferring them with cars across the Alps, and releasing them at the southern foothills. It saves their lives, but it is clear that this elaborate management cannot be a sustainable strategy. Therefore, we needed to come up with strategies to enhance the survival prospects for the reintroduced Northern Bald Ibis population and mitigate the risks posed by progressive climate change.

In this paper, we present data from two decades of reintroduction efforts for a migratory Northern Bald Ibis population in Europe. We discuss the growing impact of climate change on this population and outline the mitigation strategies we have started to implement to counter its adverse effects.

2. Materials and Methods

Project implementation

The reintroduction of the Northern Bald Ibis started in 2002 with a feasibility study according to the IUCN guidelines for reintroductions and other conservation translocations [23]. From 2014 to 2019, the first European LIFE project was successfully implemented (LIFE+12 BIO/AT/000143—LIFE Northern Bald Ibis) and in 2022, a second LIFE project started with a duration of seven years (LIFE20 NAT/AT/000049—LIFE NBI; www.waldrapp.eu (accessed on 20 November 2024)). It is implemented by ten European partners under the leadership of Zoo Vienna (www.zoovienna.at (accessed on 20 November 2024)). The private company Waldrappteam Conservation & Research (www.waldrappteam.at (accessed on 20 November 2024)) is in charge of the overall management and implements the human-led migrations.

Human-led migration

Human-led migration is the main translocation method used. Its objective is to establish a new social tradition using founder birds from zoo origin (F0) [24,25]. The method involves two human foster parents hand-raising up to 36 chicks per season. Chicks are collected two to eight days after hatching. The sex of all birds is genetically determined. We practice socially involved hand-raising [26], where the foster parents are engaged in intensive interactions with the birds without the use of disguises or dummies. This ensures a close social bond between the foster parent and each of the birds, which is a crucial requirement for human-led migration. Contact of the birds with any other human is strictly prohibited to avoid general habituation to humans. Birds raised in this manner establish a long-lasting close relationship with the foster parents while remaining shy and reserved towards other humans.

After fledging, the birds undergo a comprehensive step-by-step training programme designed to teach them to follow double-seated microlight aircraft, called paraplanes (Figure 2), with a foster parent on the back seat. These aircraft are equipped with a large parachute, allowing them to fly at speeds of 40–45 km/h, matching the birds’ flight speed.

In early August, the birds enter a state of migratory readiness, reaching their maximum body weight and increasing the level of corticosterone, the main regulatory hormone for bird migration [17,27,28]. Soon after, the human-led migration begins, dependent on appropriate weather conditions. The journey is divided into flight stages with intermediate stopover days. During stopovers, the birds stay in a spacious aviary, cared for and fed by their foster parents. The length of the stopovers mainly depends on the weather conditions and ranges from a mere night stay to extended stays of a week or more.

During flight training and migration, the birds are fed individually by hand by their foster parents. The natural diet consists primarily of worms and larvae, which the gregarious birds dig out of the soil in meadows, pastures and semi-arid landscapes [5,13,29]. During human care, the food consists of equal parts minced beef hearts, rats and day-old chicks, plus insects (depending on availability), lime from ground snail shells and a vitamin supplement. The birds are fed ad libitum three times a day on stopover days and twice a day on migration days; the amount of food per bird is about 250–300 g per day. Individual feeding until the end of the migration serves to maintain the close social bond between the birds and their foster parents.

Upon arrival at the wintering site, the birds remain in an aviary for a few weeks to acclimate before they are released. During this time, they are accustomed to feeding on their own.

The birds typically return to their breeding area by their third year when they reach sexual maturity. However, some of the subadults make explorative flights for a part of the route or even return to their breeding area.

Biologging and monitoring

Since 2016, the majority of birds have been equipped with solar-powered commercial GPS devices [30]. Due to significant aerodynamic and health issues caused by the fixation of the devices on the upper back via a wing-loop harness [31,32], all tags are now positioned on the lower back and fixed via a leg-loop harness made of Teflon tube. The data are automatically transferred to the open-source animal movement platform Movebank and the freely accessible app Animal Tracker (version number 4.4.0-11106).

Timing of the autumn migration

We analysed the shift in departure time for birds of the two established breeding colonies in Burghausen in Bavaria, Germany, and Kuchl in the country of Salzburg, Austria. The birds of the two colonies regularly merge after the breeding season and migrate together from the state of Salzburg in Austria over the Alps to Italy, where the birds’ wintering area is located. We chose the day on which the first group, consisting of at least two birds, crossed the Alps each year. The period analysed was from 2011, when the first birds migrated independently, to 2023. In the first few years, the data collection was based on the team’s field observations. From 2016 onwards, more than 90% of the birds were equipped with GPS transmitters and monitoring has been carried out primarily remotely since then.

Data analysis and statistics

All analyses were carried out using data from our internal database. Values are usually given as mean ± standard deviation. All statistical analyses were performed with the free statistical software R Version ist 2023.12.1+402 (https://www.R-project.org/ (accessed on 20 November 2024)). We utilised a non-parametric Wilcoxon signed rank test to assess weight changes in birds across migration stages and ran a Linear Model regarding the temporal shift in autumn migration patterns.

Ethical note

Bird care, keeping, training, and release follow well-established standards in accordance with the legal framework and under the supervision of Waldrappteam Conservation & Research experts. Translocation and management measures were implemented in the framework of European LIFE projects (LIFE+12 BIO/AT/000143—LIFE Northern Bald Ibis and LIFE20 NAT/AT/000049—LIFE NBI). National approvals were provided by the provinces of Salzburg (21302-02/239/352-2012) and Carinthia (11-JAG-s/75-2004), as well as by Baden-Württemberg (I1-7.3.3_Waldrapp), Bavaria (55.1-8646.NAT_03-10-1) and Italy (0027720-09/04/2013).

3. Results

3.1. Reintroduction of a Migratory Population

From 2004 to 2022, a total of 15 human-led migrations from different sites in the northern foothills of the Alps to the WWF Oasis Laguna di Orbetello in southern Tuscany, Italy (42.425484° N, 11.232662° E), as the common wintering site, were carried out. The duration of the migration journeys was 25 ± 10 days, with a mean of 8.5 ± 4.7 flight stages. The total distance was 927 ± 215 km with a mean of 139 ± 56 km per flight stage and a maximum distance of 360 km covered in one flight stage (2011). Small airfields were mostly chosen as stopover locations because they best met the flight requirements and usually had the necessary infrastructure for setting up the camp. The mean duration of stopovers was 2.4 ± 0.6 days. In the frame of these migrations, we released a mean of 18.5 ± 9.1 juveniles per season, and a maximum of 30 juveniles were released in one season (2017). In total, 277 juveniles were led to Tuscany and released there. The released birds are descended from nine European zoo breeding colonies, with the majority of 74% coming from Rosegg Zoo in Carinthia, Austria (https://www.rosegg.at (accessed on 20 November 2024)).

Since 2011, a steadily increasing number of wild-living birds are migrating between the wintering site in Tuscany and four breeding areas: Kuchl (country of Salzburg, AUT; 47.633790°, 13.160413°), Burghausen (Bavaria, GER; 48.156356°, 12.825322°), Ueberlingen (Baden-Wuerttemberg, GER; 47.780569°, 9.128938°) and Rosegg (Carinthia, AUT; 46.585935°, 14.021097°). Since 2011, birds have bred successfully in the wild. So far, 324 birds fledged in the wild (2011–2023), with 74 fledglings in 2023 (Figure 3). Detailed demographic data have been available since 2014. Since then, the survival rate from hatching to fledging has reached 86 ± 5 percent (2014–2023). The fecundity increased significantly from 1.60 fledglings per nest in 2012 to 2.96 fledglings per nest in 2023 (Linear Regression, R² = 0.4446, p = 0.0179). In comparison, according to published demographic data [14], the fecundity in a sedentary Northern Bald Ibis population, established in Andalusia by Proyecto Eremita, increased slightly from 1.00 fledglings per nest in the early phases of the project (2008) to 1.28 fledglings per nest in 2020 (Linear Regression, R² = 0.4033, p = 0.0265; Figure 4).

Since 2014, extensive remote monitoring by the use of GPS devices has enabled a detailed analysis of mortality. For 55 ± 17% of the losses (2014–2023), the cause of mortality could be identified. The main mortality causes are electrocution on medium-voltage power poles (36 ± 16%), collision and injuries (25 ± 20%), and illegal bird hunting in Italy (17 ± 7%). The proportion of collisions and injuries varies greatly over the years depending on stochastic events such as storms and severe weather, with an annual proportion ranging from 7% to 78%.

At the end of 2023, the population comprised 256 individuals (Figure 3), with 54% of the birds raised by human foster parents and released (founders; F0) and 46% raised by their biological parents in the wild (successors; F1–F3).

3.2. Timing of the Autumn Migration and Immediate Emergency Measures

The timing of the autumn migration was analysed over a span of 13 years, from 2011 to 2023, for birds of the breeding colonies of Burghausen in Bavaria, Germany, and Kuchl in the country of Salzburg, Austria. During these years, the onset of the autumn migration across the Alps shifted significantly by more than a month (Multiple Linear Regression, multiple R² = 0.5757, t = 3.68, p = 0.0042; Figure 5). During this period, the size of the wild population increased steadily, from 38 birds at the end of 2011 to 256 birds at the end of 2023. The mean size of the groups that annually initiated the autumn migration across the Alps was 6.46 ± 7.41 individuals. The size of the group which initiated the autumn migration varied considerably, with a slightly increasing group size over the years (Linear Regression, slope = 1.476 ± 0.50, R² = 0.4661, p = 0.0144). Overall, the groups which initiated the autumn migration consisted of 48% adult and subadult experienced migrants and 52% juveniles. The subadult and adult birds in these groups were 48% females and 52% males.

As the number of birds lingering on the northern edge of the Alps at the onset of winter increased, we were compelled to implement an adaptive emergency measure. This involves capturing the birds remaining in the north in time before the first onset of winter. The capture process involves the use of a remote-controlled feeder and an enclosure with a drop gate (Figure 6). The birds are immediately placed in individual boxes and transferred in a car across the Alps. On the southern edge of the Alps, they are released. Most of the released birds immediately fly further south on a direct course toward the wintering area. In this way, a total of 211 birds were rescued during the years 2017 to 2023, with 40 birds alone in 2023.

3.3. Initial Human-Led Migration to Andalusia

On 21 August 2023, a human-led migration with 35 hand-raised and trained Northern Bald Ibises started from the airfield in Binningen near the breeding colony in Ueberlingen at Lake Constance. The journey of 2.319 kilometres was covered in 19 flight stages with an average stage length of 122 kilometres and a total duration of 43 days (Figure 7). The mean duration of stopovers was 1.3 ± 1.5 days. Three birds were lost during flight stages in Spain. On 2 October, the team reached the final landing site in Barbate, Andalusia, with the remaining 32 birds. After habituation in a large aviary, the birds were finally released in early December.

Before and during the entire journey, the birds were weighed every morning. They were accustomed to this procedure. During the journey, the body weight of the birds decreased significantly from 1351 ± 110 g before the onset of the migration to 1318 ± 108 g at arrival in Andalusia (Wilcoxon signed rank test; N = 31, U = 428, p = 0.00043). However, this decrease was marginal (4%) and in general, the birds showed no impairment due to the long duration and distance of the migration.

4. Discussion

4.1. The Northern Bald Ibis Reintroduction Project

In the frame of the LIFE project, migratory Northern Bald Ibises returned to their historic breeding sites in Europe. This was the first attempt to reintroduce a continentally extinct migratory bird species. At the end of 2013, the population consisted of 256 individuals, of which 46% belonged to successor generations (F1–F3). These birds grew up in the wild and naturally acquired knowledge about the migration route and the position of the wintering site from their conspecifics. This proves that the migration tradition established within the framework of the project is preserved across generations. In addition, a number of studies have shown that the birds of this released population display complex and efficient migratory behaviour in terms of energetics [33,34], flight technique [18,20,35] and navigation [19], consistent with the migratory behaviour of other species. Through a population viability analysis, the threshold for self-sustainability is set at a total population size of 314 individuals [36]. The authors anticipate achieving this threshold by the period of 2026/2027.

The migratory behaviour enables the Northern Bald Ibises to use the northern Alpine foothills as breeding grounds. This region is known as the centre of the historical distribution of this species in Europe [9,10] and it is still a region with outstanding reproductive successes. This is represented by the actual mean fecundity of 2.96 fledglings per nest, which is far above the rate of other populations [36]. A recent study, integrating GPS data of the birds and Earth observation data, revealed a surplus of suitable foraging habitats in the northern Alpine foothills, which indicates ample potential for colony expansion [30]. Thus, the northern Alpine foothills are crucial for the long-term viability of a European population, serving as an essential breeding ground. However, successful migration to more southerly regions is essential for birds breeding north of the Alps, since this species can hardly survive the winter in this region.

4.2. Impact of Climate Change

In recent years, the wild-living Northern Bald Ibises have shown significant changes in their migration behaviour in response to climate change. In particular, the increasingly pronounced and longer warm periods in autumn cause a shift in the birds’ departure from their breeding grounds from September to November. This change in the birds’ timing aligns with general evidence for the extensive and diverse effects of climate change on the migratory behaviour of various bird species [37,38,39,40,41].

The progressively failed attempts of birds to cross the Alps later in the year seem to be related to their flight technique. In alpine areas, Northern Bald Ibises soar in thermal updrafts to reach sufficient flight altitudes [18]. GPS data of unsuccessful attempts indicate that when approaching mountain slopes, the birds search for thermals and soar, but with a low climbing rate, they finally fail to reach the required altitudes. These observations are in accordance with the fact that thermal activity in alpine regions gradually decreases in autumn and early winter due to the changed inclination of the sun and the accumulation of cold air in the valleys. This behaviour of wild-living birds during migration also corresponds to experiences during human-led migration. In Alpine regions, birds and accompanying aircraft use thermals to gain altitude up to 3000 m above sea level. Attempts to lead the birds over Alpine passes without suitable thermals mostly fail.

With a western detour, the Northern Bald Ibises could overcome the barrier of the Alps to reach the wintering area in Tuscany. However, there is evidence for this detour for only one adult bird. Instead of a detour, Northern Bald Ibises preferably navigate on the shortest direct route to their wintering area. They only deviate from this path when facing barriers, but once successfully passed, they again continue along the shortest line. This direttissima navigation results in the birds approaching the Alps in the same region each year and encountering the same barriers. The birds’ lack of adaptive adjustments to the behaviour so far is consistent with the outcome of meta-analyses indicating that the adaptive responses to ongoing climate change are insufficient and may already be threatening the persistence of species [3,42,43].

A global meta-analysis revealed that larger-bodied bird species are more vulnerable to declines in offspring production and nest success due to climate change-induced temperature increase [44]. This effect is not evident in the migratory Northern Bald Ibis population. In fact, fecundity was found to increase significantly, reaching a value of almost three fledged chicks per nest, which is extraordinarily high [36]. Also, in a sedentary Andalusian Northern Bald Ibis population established by Proyecto Eremita [14], fecundity was found to increase, albeit at a significantly lower level and with a lower gradient (Figure 4). We attribute the observed increase in fecundity in both populations to their status as founder populations, characterised by a steadily growing number of experienced breeding birds and the accumulation of both individual and social breeding knowledge. The steeper gradient and generally higher fecundity in the migratory population compared to the sedentary Andalusian population likely reflect differences in breeding habitat quality, as indicated by a study on the suitability of foraging habitats in the Northern Alpine Foothills [30]. However, further investigations are needed in this area.

4.3. Mitigation Measures Against the Effect of Climate Change

In order to mitigate the increasing impact of climate change on the Northern Bald Ibises in the breeding colonies of the northern Alpine foothills, we began establishing a second migratory tradition in 2023. It leads from the northern Alpine foothills over more than 2500 km to a wintering site on the Atlantic coast in Andalusia. This migration route, even though three times longer than the originally established route to Tuscany, offers a critical advantage for the birds as it avoids any challenging mountainous barriers, which are becoming increasingly difficult for the birds to cross due to shifting climatic conditions. This also allows the birds to reach their wintering grounds safely late in the year.

The establishment of the second migration tradition takes place in close cooperation with our Spanish partners from Proyecto Eremita who are establishing a sedentary Northern Bald Ibis colony in Andalusia [14,45]. Initiating a new wintering area requires a thorough feasibility study to assess the suitability of the habitat. In addition, the necessary infrastructure must be created, and comprehensive monitoring and management must be ensured. All this was facilitated or anticipated by the cooperation with Proyecto Eremita, which enabled us to start with first releases as early as 2023.

When selecting breeding sites, we base our decisions on historical evidence as well as current and sustainable suitability. This is not possible with regard to the wintering areas and the associated migration routes of the former European population, because we only know that the birds disappeared from the breeding grounds over winter but concrete evidence regarding routes and locations is missing [9]. However, archaeological records indicate the presence of Northern Bald Ibises along Spain’s Mediterranean coast, with evidence spanning from the late Pliocene era (2.5 million years ago) near Valencia [46] to Neanderthal settlements in Gibraltar 25,000 years ago [47]. This corresponds with a recent study on former breeding populations in France [48] where the authors concluded that the birds of these breeding colonies migrated to Spain and probably further to Africa. Thus, our migration to Andalusia probably follows ancient routes once used by Northern Bald Ibises for millions of years till this tradition ended in the Middle Ages due to the extirpation of the species in Europe.

After human-led migration, we released the juvenile birds in Andalusia, a region proven to be a suitable wintering habitat for the species, as indicated by data from the sedentary Proyecto Eremita population [14]. It seems likely that a part of the historic European population wintered or even bred in Andalusia, but the migration route probably continued across the Strait of Gibraltar towards the African west coast. Northern Bald Ibises are able to cross larger open water areas like the strait between Andalusia and Morocco in active flight. This is indicated by various recorded GPS tracks of non-stop flights up to 700 kilometres over open water by birds from our project. In addition, there are numerous observations of birds from the sedentary colony in Andalusia flying over this strait [45], where the birds seem to prefer a route that is also chosen by Eurasian spoonbills (Platalea leucorodia) and other species in active flight [49].

In 2023, a human-led migration over 2300 kilometres from southern Germany to Andalusia was performed with a group of 32 juveniles. It was the longest distance ever flown by humans with migratory birds and also the largest group of birds ever led by humans. In the only human-led migration project with another species, the Whooping Cranes (Grus americana), the US team carried out migration flights over a total of up to 1800 km with up to 16 birds [50]. In 2024, we flew another successful migration to Andalusia. It started in southeast Germany and covered a distance of around 2700 kilometres. After 51 days, the team reached the destination with 36 birds. Thus, in two consecutive years, a total of 68 birds were led to Andalusia and released there. At least another three human-led migrations from the northern foothills of the Alps to Andalusia are planned in the coming years, in order to release more than 150 birds in total. They and their descendants are expected to continue a migration tradition that has been dormant since the extirpation of the species over 400 years ago.

The objective to establish a new migration tradition to Andalusia also gained impetus from the flight of a juvenile male Northern Bald Ibis named Ingrid. During the human-led migration to Tuscany in 2022, this hand-raised bird lost contact with the group and returned to the northern Alpine foothills. From there, he headed along the Alpine Arc towards the west and thereafter strictly southwest until he reached a site near Malaga where he remained. During winter, Ingrid was captured and transferred to the nearby sedentary release population of Proyecto Eremita, which he immediately joined. But after more than a year of companionship in the sedentary colony, Ingrid set off on a remarkable flight in April 2023. For three consecutive days, the male flew almost 1000 km across Spain on a straight northeast course, with the bird’s breeding grounds in the northern Alpine foothills as the obvious destination. Unfortunately, close to the French border, the bird died due to predation.

4.4. The Human-Led Migration Method

The project’s innovative use of human-led migration, where microlight aircraft guide hand-raised ibises to their wintering grounds, has proven to be a highly effective method for the establishment of new migratory patterns. In 1988, Bill Lishman became the first man ever to use a microlight plane to fly with a group of human-imprinted Canada Geese (Branta canadensis) [24]. In the frame of a 12-year feasibility study, the method was adopted for the Northern Bald Ibis, with several challenges to overcome. In contrast to geese, this species is altricial, which shortens the period of training until the onset of migration. Its active flight speed is relatively slow at about 45 km/h, which poses a technical challenge. And the species practises different flight techniques during migration, formation flight, thermal soaring/gliding and intermittent flight [18,26,51,52,53] and thus requires experienced pilots to adapt to the variable flight behaviour of these birds.

The first two migrations to Andalusia have shown the potential of this translocation method particularly clearly. The route to Andalusia is almost three times as long as the migration route to Tuscany. In 2024, twice as many young birds were led to Andalusia as on average in previous migrations to Tuscany. The average duration of the stopovers on the route to Andalusia (1.3 days) was significantly shorter than on the route to Tuscany (2.4 days). The primary reason is that there are no significant mountain barriers to overcome on the way to Andalusia, which often require long stopovers to wait for suitable weather. This allows for a faster migration. In addition, in 2024, the birds were guided by only one instead of two ultralight aircraft for the first time. This continuous technical as well as economic and ecological optimisation is also motivated by our objective of making this translocation method available for other species protection applications. For this purpose, we also started training young pilots to fly with birds in 2024, which required a second identical aircraft. This second aircraft will also be carried along during future migrations as a backup plane, to ensure that the migration can be continued in the event of technical problems or damage to the aircraft used.

Comprehensive remote monitoring by the use of GPS devices indicates that the released birds perform species-specific migratory behaviour following the direction of human-led migration and that the tradition is passed on to the subsequent generations raised in the wild [54,55]. However, the reintroduced birds usually do not migrate along the route they were led by the ultralight aircraft, and they only occasionally use the stopovers they were shown during human-led migration. Instead, the GPS data indicate that released birds which once followed the microlight from the breeding area to the wintering area have a very flexible navigation ability, which enables them to reach their breeding or wintering grounds via any route. In addition, a remote sensing study revealed ample availability of suitable foraging areas in Central Europe [31]. This justifies the selection of stopovers during human-led migration not primarily from the perspective of potentially suitable future stopover areas for the Northern Bald Ibis, but rather the flight suitability and the availability of suitable infrastructure for the camp. However, small airfields often prove to be suitable feeding habitats for Northern Bald Ibises.

The release method also allows for the continuous collection of physiological, endocrinological and behavioural data during migration and the attachment and removal of data loggers before and after migration flights. This research potential has been used extensively for scientific research on the physiology, energetics, aerodynamics and technology of bird flight and bird migration [21,22,56].

4.5. Perspectives

This new migration path connects the European migratory population with the sedentary Spanish population and thus also two well-established European conservation projects. It aims at forming a population which covers an area of about 5000 km² extending from Andalusia to southern Germany and central Italy. This population includes migratory breeding colonies north and south of the Alps and a sedentary colony in Andalusia. Another two semi-wild sedentary colonies, one in Friuli, Italy, and the other one in Gruenau in Almtal, Upper Austria [57], also increasingly interact with the migratory colonies.

We expect that the diversity and large-scale distribution of migratory and sedentary colonies will increase the ecological flexibility, which will help the population to adapt to the constantly changing environment. The project highlights the importance and potential of translocations in light of mitigating climate change. Moreover, it is also an example of efficient cross-border cooperation between previously independent projects. Transnational cooperation is becoming increasingly important due to the ever-larger number of threatened migratory animal species. In particular, however, the project has become an example of how immediate and significant the consequences of climate change can be for species and what challenges this can pose for ongoing species conservation measures.