A never-before-seen picture of eagle migrations

In 1899, scientists began tracking avian migration by placing a small ring with an identifier around birds' ankles. Researchers then had to recapture the birds elsewhere, leaving the journey between those points a mystery. Today, GPS tags allow researchers to see every stage of bird migration. The tags are small, have a long-lasting battery, a solar panel and track dozens of metrics. Migration is no longer described as just a theoretical range but rather an exact map. Despite this technological advancement, the routes of many species in their respective flyways remain undescribed. 

Now, a new satellite tracking study led by a University of Utah undergraduate describes the journey of migratory raptors along the understudied East Asia-East Africa Flyway. The steppe eagle, Aquila nipalensis, a highly endangered raptor species, has been tracked before. As for the booted eagle, Hieraaetus pennatus, however, Nathan Murthy’s was the first satellite-tracking study in this flyway. 

Spectacular and dangerous journeys

The East Asia-East Africa Flyway crosses more than 60 countries and stretches from northern breeding grounds in Siberia and Eastern Russia to wintering sites in eastern and southern Africa. Many of the more than 330 bird species along the corridor have to navigate major bottlenecks in the Middle East, including large bodies of water and growing anthropogenic stressors such as persecution, falconry, loss of habitat, increased pesticide use and collision with powerlines. Wildfires, advancing spring arrival times and extreme El Niño events also pose challenges. 

“These journeys are spectacular and difficult—numbering in the thousands of kilometers,” said Murthy, whose study began in the lab run by Çağan Şekercioğlu, a Utah biology professor and renowned birder. “But we just catch glimpses. A bird perched here, a bird soaring there—we get to join them for a brief second, but that's it.” He wanted to know more: what those glimpses actually mean.

Ultra-lightweight GPS-GSM transmitters were placed on the steppe and booted eagles at the Aras River Ornithological Station in northeastern Turkey in September 2022 and 2021, respectively. When released, the eagles began their fall migrations right away. 

Visualizing flight tracks with satellite data

The booted eagle migrated 5,080.6 kilometers in 49 days, crossing the Middle East and wintering on Mount Elgon in Uganda, according to the tracking data. It then largely retraced the route, traveling 6,785.7 kilometers in 35 days to breeding grounds in Russia.

The steppe eagle, which had been rescued by authorities in Turkey from a poaching operation, journeyed 4,409.6 kilometers over 38 days that fall, diverging from the typical “loop” route via Bab al-Mandeb and instead crossing through the Suez Canal. However, the tag ceased transmitting at its wintering grounds in Sudan for unknown reasons—the tag could have fallen off, or the bird may have been killed.

Murthy analyzed the satellite data by first calculating the distance between each GPS location and the original banding point. This gave him a way to visualize the flight track and determine when the bird was in flight or stationary. To calculate the range boundaries of stopovers, non-breeding sites, and breeding grounds and better understand the types of landscapes these birds prefer, he used kernel utilization distribution, a statistical probability map showing the likelihood that an animal will be found at any given point within an area.

As expected, these migrations painted a picture that opened up more questions than answers. The data could suggest that both the steppe eagle and booted eagle utilize human-dominated habitats for their breeding and non-breeding grounds. Throughout the journeys, the birds stopped at trash dumps, agricultural areas and cities. Do human-dominated habitats play a functional role during eagle migrations? That might be a question for another researcher.