Can Migration Station Data Tell Us About Population Trends?

Twice a year, birds all over the world take flight to migrate hundreds or even thousands of miles back and forth between their wintering and breeding grounds. Biologists and citizen scientists take advantage of this seasonal event by stationing along known migration corridors to count the masses of passing birds. Some of these “migration stations” have been around for decades, providing a robust dataset of migratory numbers and species counts, and these data have historically been used to provide information about species’ population trends. However, there remains uncertainty regarding whether trends in migration count data are an effective indicator of population trends.

A new publication, coauthored by our partners in Kazakhstan, is examining this question by tracking migrating Steppe Eagles (pictured below left) and Black Kites (pictured below right) with GPS units near important migration stations. Together with collaborators in the Middle East and at the US Geological Survey, they tagged 23 Steppe Eagles in Kazakhstan and Israel and 114 Black Kites in southern Israel with GPS transmitters. Steppe Eagles are a focal species at a migration count site in Eilat, Israel, while Black Kites are a focal species at a site in Batumi, Georgia.

From the transmitter data, the researchers found that the Steppe Eagle migration routes differed significantly from what was described in the literature. The eagles in this study had a very broad migration corridor. Of the tracked migration routes that started in Africa, only 17.6% passed close enough to the count station in Israel such that they could have been counted there. When considering all of the tracked migration routes, the percentage drops to a mere 9%, as almost half the routes started in Asia and did not cross Israel on the way north. There was also significant variation in individual eagle migration patterns, meaning the same individuals are not likely to be counted at the station every year. Because there was so much variability in whether or not individual eagles would pass directly over the migration station, the researchers warn against using data from Eilat to make inferences about Steppe Eagle populations.

The tracking data from Black Kites, on the other hand, were consistent with previously documented migration patterns: 45% of kites tracked passed close (<3km) to the count stations. Individual variation was low, meaning that each kite took almost the same pathway year-to-year. These findings suggest that the data obtained from migration counts at the Batumi migration station could be used effectively to make inferences about trends within regional Black Kite populations.

This publication illustrates the variation in accuracy of using migration count data to make generalizations about larger population trends. The authors stress that migration count data are still an important piece of conservation, but that biologists should carefully determine how the data can best be used to make larger conclusions.