Who are we counting?

Everything is quiet, but the sounds of clickers clicking. All eyes are looking at the sky, most through binoculars, some using a telescope. Migrating birds just keep coming, and the team of volunteers makes sure not to miss any of them. This amazing experience of migratory bird counts is one of the top experiences for bird and nature lovers, making them impactful educational and eco-tourism centres.

Beyond the unparalleled experience, these surveys have been providing the scientific and conservation community with invaluable data. Counting birds at the same place and time in different years can tell us a lot about trends in the size of bird populations and their structure (e.g., the number of adults in a population) and about changes in their phenology (e.g., migration dates). These allow for the exploration of their response to changing conditions and provide the basis to define the conservation status of bird species.

Migratory bird counts are usually situated at migratory bottlenecks, where birds are funnelled by geographic structures that narrow migratory fronts. This allows the observation of large portions of migratory populations over a relatively small area. The exact locations of migratory bird counts are usually selected by combining acquired knowledge about migratory hotspots and an understanding of the geographic conditions that create these hotspots.

As with every method, migratory bird counts have their limitations. To best understand the value of the hard work done at these sites, we need to understand which birds are counted at these stations: where do they come from, where do they go to, and what part of the population they represent. In our recently published paper, we set out to test whether we can gain better insights regarding these questions using the ever-increasing availability of high-resolution tracking data acquired by GPS trackers.

Figure 1. A Steppe Eagle (left; credit: Yael Lehnardt) and a Black Kite (right; credit: Daniel Berkowic) photographed right after being equipped with a GPS tag.

While studying Black Kites (Milvus migrans) and Steppe Eagles (Aquila nipalensis) movements using GPS transmitters, we noticed that some of our birds fly over two very famous migratory bird counts. The kites that were tagged during winter in the Middle East often migrated south using the route crossing the Batumi migratory bird count. The eagles that were tagged at their breeding sites in Kazakhstan often used the route crossed the Eilat migratory bird count on their way north.

We first examined how many of the birds we tracked passed close enough to the migratory bird counting stations for the observers to be able to count them. Then, we tested what environmental (geography and wind) and individual (age and origin) characteristics can explain the probability of a bird flying closer or further from the counting stations. We found major differences between the two sites.

Our results imply that the Batumi migratory bird count is situated in a good place for counting Black Kites. More than 45% of the kites’ tracked routes passed close enough to the counting stations to be counted, despite originating from a wide range of longitudes. Moreover, we found a strong relationship between the summer range of kites and the probability of being counted, with kites originating further east being less probable to pass over Batumi.

Figure 2. Migratory routes (black) of Black Kites on their way south during autumn migration along the Black Sea. The cyan diamond is the location of the Batumi migratory bird count.

Contrastingly, the number of Steppe Eagles’ routes that could realistically be counted in Eilat was less than 14%. In fact, almost half of the tracked routes did not originate in Africa, as the birds wintered in Asia. Thus, their northward route did not cross anywhere near Eilat. Furthermore, individual birds often shifted between wintering in Africa in one year to wintering in Asia in a different year, meaning that the counts over the years do not consistently represent the same individuals.

Even when considering only the GPS-tagged birds that did winter in Africa, only a few routes passed close enough to the Eilat counting stations to be counted. Tracked routes from Africa only passed close to Eilat under two conditions: when the eagles crossed the Gulf of Suez instead of flying along it to its northern edge, or when the wind was blowing strongly to the south.

Figure 3. Migratory routes (brown) of Steppe Eagles on their way north during spring migration along the Red Sea. The cyan diamond is the location of the Eilat migratory bird count.

What does this mean?

The interpretation of our results should be done carefully – while Batumi is a good place to count some populations of Black Kites, further research is required to best interpret the counted numbers for conservation and broader conclusions. Regarding the Steppe Eagles, our results suggest that population-wide conclusions are hard to draw from the data collected in Eilat. But, as with Batumi, further research can provide better understanding of the relationship between counted eagles in Eilat and at least parts of their population. Meanwhile, our results suggest that the recently established Galala migratory bird count in Egypt presents a better location for counting the eagles wintering in Africa.

Importantly, we so far studied the migration routes and counts of just two species at two bird count sites, but our research provides a framework that can be used for different species and many migratory bird counts around the world. GPS data of many counted bird species is readily available, and the coupling of bird counts and tracking data can yield important insights into the conservation, phenology and ecology of migratory birds.