Skip navigation links

Dec. 13, 2017

Andrew Dennhardt: Estimating eagle populations

Dec. 13, 2017

Andrew Dennhardt is a doctoral student in the departments of Fisheries and Wildlife and Ecology, Evolutionary Biology and Behavior.

Have you ever looked to the autumn sky above and wondered how many of your favorite feathered friends are out there, as they migrate southward each year? If so, then you are not alone.

A common goal in ecology has been to estimate the abundance of wild populations. From basic counts to educated guesses, historically, people have tried it all. During and even before the work of Sir Ronald Fischer, the father of modern statistics, applied mathematics aided in population estimation — for humans and wild animals alike.

F. C. Lincoln helped pioneer capture-mark-recapture, or CMR. It was simple: trap a random set of animals of the same species in an area, mark those you caught with a unique tag, release them back into the wild for a period of time, return to the same area once more and randomly trap individuals of that same species again and record which previously tagged individuals, if any, came back to the area.

In brief, when you know the proportion of individuals you recaptured the second time, you can assume that you trapped the same proportion of the total population when you trapped the first time. Such information is useful to estimate population abundance.

Andrew Dernhardt holding a hawk


In a recent paper, we used modern statistical advancements in CMR to estimate abundance of golden eagles, a species of conservation concern in the United States, using an unusual approach.

In effect, we did not physically capture, mark or recapture individual animals in the wild; rather, we did so virtually in a computing environment with the help of observational data collected by some savvy citizen-scientists.

Golden eagles in eastern North America face lethal and sub-lethal threats, many of which are human caused. However, these eagles are rarely seen, broad-ranging and difficult to capture in the wild because they often avoid areas of human activity.

Despite this fact, citizen-scientists observe golden eagles frequently and regularly during their annual spring and autumn migrations in Pennsylvania. Hawk counters collect their data on golden eagles and other migrant species and archive their observations in an online database.

Now, here’s where things get really exciting, at least for me. Because historic hawk-count data included information on the timing of golden eagles migrating southward, telemetry data gave us an idea of how fast they fly while migrating in autumn and we could measure the distance between each of the monitoring sites they passed, we could then estimate how long it would take eagles to travel between pairs of sites.

Using a customized computer program, we matched records of golden eagles together, from site to site, such that we could know when observers at one site counted a bird that had probably been counted at a previous site (i.e. eagles became “captured,” “marked” and “recaptured”).

hawk flying

Most exciting for us — our methodology worked. Our population abundance models estimated that approximately 1,350 golden eagles were in our study area each autumn.

In the end, we feel that we have produced a useful framework for evaluating other migratory bird populations based on similar data and known movement behaviors.

Our proposed methodology not only builds upon the legacy of modern CMR work, but it is also far more cost-effective than physical CMR and other costly techniques such as fixed-wing aircraft surveys used in the western United States.

Most importantly, such an achievement was only made possible by the tireless work of numerous dedicated citizen-scientists, whose standardized and centrally managed data can provide wildlife researchers and managers with quality information useful in conservation decisions. To our friends at the Hawk Migration Association of North America, for all of their data collection and management year-in and year-out, our team is abundantly grateful — pun intended!