This blog post is provided by Evan Buechley, Ron Efrat and Steffen Oppel and tells the #StoryBehindThePaper for their article “Differential survival throughout the full annual cycle of a migratory bird presents a life history trade-off“, which was recently published in the Journal of Animal Ecology.

Evan Buechley received his PhD from the University of Utah with a thesis focused on the movement ecology and conservation of the Egyptian vulture, was a postdoctoral researcher at the Smithsonian Migratory Bird Center from 2019-2020, and is currently the International Program Director for HawkWatch International. Ron Efrat is a PhD Candidate at Ben-Gurion University of the Negev, Israel, studying the behavior and survival of captive-bred and wild vultures. Steffen Oppel is a Conservation Scientist at the Royal Society for the Protection of Birds (BirdLife International) in Cambridge, UK, providing scientific support for the effective conservation of threatened species and their habitats, including the Egyptian vulture along the flyway in the frame of the Egyptian vulture New LIFE project.

Bird migration has long fascinated humanity and scientists have long puzzled over how migration evolves and how it can be maintained in natural populations. Crossing vast stretches of hostile desert or open ocean seems incredibly risky, so many birds pay for the attempt to migrate with their lives. By the same token, wintering in freezing conditions at higher latitudes would also impose a heavy toll on survival, and, on balance, the cost that migration imposes must be offset with a benefit accrued in another stage of an animal’s life cycle. But where and when migratory birds die has been extremely challenging to study – until recent technological advances have made it increasingly possible to track the movements and fates of migratory birds.

With modern remote-tracking technologies we can now follow birds movements very precisely, and therefore determine where and when they die. This allows addressing fundamental questions in ecology, such as whether it is worthwhile for migratory birds to travel vast distances over hostile terrain to reach more favorable over-wintering conditions. In a new study published this week in the Journal of Animal Ecology, a collaboration of 38 researchers, led by Evan Buechley, Steffen Oppel, and Ron Efrat, addressed this question with the example of a migratory vulture.

The Egyptian vulture is an enigmatic species found throughout primarily arid regions of southern Europe, southern Asia, and Africa, and is closely associated with humans. The species primarily scavenges for food, and has often lived in proximity to people, benefiting on excesses of human civilizations. In adult plumage, the species’ plumage is a crisp white with a bright-yellow head, making it a highly recognizable and revered member of the avifauna. Accordingly, there are depictions of Egyptian vultures in Egyptian tombs, and in parts of the Balkans the species is an important symbol of spring locally known as the ‘cuckoo’s horse’, due to its migratory nature and relatively early arrival to breeding grounds in southern Europe.

Unfortunately, in recent decades, populations of the Egyptian vulture have declined rapidly, and the species has been extirpated from large swaths of its former range. In 2007, the International Union for the Conservation of Nature (IUCN) uplisted the species from Least Concern to Endangered, reflecting global declines. The Egyptian vulture is not alone in this regard, as many other long-distance migratory bird species have experienced similarly concerning declines – making the Egyptian vulture a suitable case study to address questions about survival of migrants more generally.

Encouragingly, Egyptian vulture population declines have been met with broad concern by scientists and conservationists. There are now many research and conservation projects focused on identifying the causes of the species’ decline, mitigating known threats, and augmenting populations via captive breeding and releases.

This paper represents a large collaboration between many such interested parties (38 researchers from 12 countries!), aiming to understand when, where, and why Egyptian vultures are dying.

Since the early 2000s, scientists studying Egyptian vultures have tagged them with remote tracking devices to study their migrations, habitat use, and to better understand causes of mortality. This included transmitter deployments by researchers in Spain, France, Italy, Bulgaria, Greece, Turkey, Armenia, Russia, Israel, Bulgaria, Ethiopia, and more. In this study, we compiled and analyzed the existing satellite-tracking data from these projects, which enabled us to study the movements and eventual fates of 220 individual vultures that ranged from Portugal in the west to Iran in the East, from Russia in the north to Kenya in the south, or over roughly 70% of the species global distribution.

We first identified the fates of each tracked individual by a combination of direct observations and by reviewing patterns in the tracking data. For example, the carcasses of some individuals were recovered in the field, so we knew exactly where and how they died. In other cases, the tracking data indicated that an animal probably died, such as a transmitter signal stopping or drifting in the middle of the Mediterranean Sea, indicating that the bird probably drowned. We also accounted for the fact that for some birds we did not know exactly what happened to them. For example, when a transmitter abruptly stopped transmitting, we did not know whether the transmitter malfunctioned or whether the bird was shot or electrocuted.

After classifying the fates of each of the 220 tracked individuals, we developed a statistical model that contrasted the survival for months when the bird was migrating and those when it was stationary. This model also included differences between breeding and non-breeding grounds, among breeding populations, and the fact that survival generally increases with age as birds become more experienced.

We showed that survival was lower during months when a bird migrated compared to stationary periods, indicating that there is a direct survival cost associated with long-distance migration. However, we also found that adult birds had higher survival in non-breeding grounds in Sub-Saharan Africa  compared to breeding grounds in southern Europe, the Caucasus and the Middle East. We also found that Egyptian vultures originating from western Europe, where population trends are more positive than further east, had overall higher survival throughout the year. Surprisingly, while we found high mortality over the Mediterranean Sea, very few Egyptian vultures died in the Sahara Desert – which is widely considered as a major barrier for migratory birds. Unlike other species, Egyptian vultures are supremely adapted to live in deserts and therefore handle the desert crossing well, but fare considerably worse when they have to cross large bodies of water. Thus, what constitutes a major ecological barrier that causes the death of many migratory birds varies among species.

From an evolutionary ecology perspective, our results indicate that there is a direct survival cost associated with long-distance migration, but that this cost could be offset by the benefits of migrating to southern non-breeding grounds. These results are consistent with theoretical predictions about the evolution and maintenance of migratory systems. An intriguing question for future research would be to evaluate whether there is differential breeding success across latitudes, which could help reveal whether Egyptian vultures that migrate from Africa to higher latitudes to breed are rewarded with higher productivity.

Population declines of many long-distance migratory birds are often presumed to be a consequence of deteriorating conditions in non-breeding areas, but we found more mortalities occurred further north. Disconcertingly, we also found that roughly half of the deaths for which we could identify the mortality reason were caused by people. These deaths were primarily from electrocution or collision with energy infrastructure, direct persecution by humans (e.g. being shot or trapped), and poisoning. Addressing these human-caused threats will be essential for conserving the Egyptian vulture and many other migratory species, and the burden of addressing these threats falls on all countries along bird’s migratory flyways.

Read the paper

Read the full paper: Buechley, ER, Oppel, S, Efrat, R, et al. Differential survival throughout the full annual cycle of a migratory bird presents a life‐history trade‐off. J Anim Ecol. 2021; 00: 1– 11. https://doi.org/10.1111/1365‐2656.13449