Age: More Than Just a Number

By Zoey Greenberg 

This month, the internationally renowned ornithological journal Ibis published a paper lead by Dr. Wouter Vansteelant, a former Hawk Mountain conservation science and leadership trainee. The featured study was conducted at Batumi Raptor Count (BRC), located in the lowlands of Georgia, between the eastern shore of the Black Sea and the Lesser Caucasus. The BRC was established in 2008 by Wouter and another former Hawk Mountain leadership trainee, Johannes Jansen, and has since become an exemplary monitoring site with a unique counting protocol and dedicated team of trained citizen scientists. This qualified crew is responsible for collecting 100% of the data, an impressive feat considering more than a million raptors fly through BRC each year, making it one of the most important raptor migration flyways in the world.

The Batumi Raptor Count spent several years polishing a counting protocol that intentionally prioritizes quality of data over quantity. Rather than recording passage of all 37 raptor species passing by BRC during their movements to and from Eurasia, counters instead focus on eight species. The “chosen ones” are selected for their consistent aggregations at the site and therefore reliable representation of the species’ populations as a whole. These eight species are the European honey buzzard, black kite, booted eagle, short-toed snake eagle, lesser spotted eagle, Montagu’s harrier, pallid harrier, and marsh harrier. A central benefit to this monitoring approach is that by focusing on a select group of species, counters can take the time and attention to reliably record the age and sex of individual birds. This provides a more holistic documentation of the migrant composition for each species. For example, counters can then determine whether or not the majority of marsh harriers are juveniles or adults. These specifics can then lead to more accurate understanding of population trends, as well as predictive conservation action.

The importance of aging migrants became clear during the 1950s at Hawk Mountain. Rachel Carson, author of the famous book Silent Spring, utilized long-term monitoring data from the Sanctuary to make the case that widely-used insecticides were harming bald eagle populations across North America, as well as jeopardizing human health. At the time, Maurice Broun was aging bald eagles as they flew over North Lookout, revealing an alarming trend of fewer juveniles. Although the number of total eagles remained relatively normal, fewer juveniles meant fewer birds breeding in the future and eventually a reduction in overall population. In other words, as adults died off, there weren’t enough younger eagles to replace them. Declines within a population can be caused by a variety of factors, including survivorship (how many birds survive per year within a population) and reproductive rates (how many birds are born per year). Once a decline is proactively detected, further investigation as to why can commence. The sooner these trends are caught, the better.