North America's smallest falcon, the American Kestrel (Falco sparverius), has declined across the continent since the 1970s, yet the causes continue to stump raptor biologists. A new study published in the Journal of Raptor Research adds a piece to the puzzle with the discovery that in the Northeast, where declines are most alarming, fledglings demonstrate a relatively high survival rate. This paper, titled "Juvenile and Adult Survival Estimates of American Kestrels Throughout the Full Annual Cycle in Eastern North America," is the first of its kind. No other study has assessed winter survival rates for kestrels at northern latitudes, fledgling survival following nest departure for both males and females, or tracked individual kestrels through multiple seasons in migratory and residential populations. These breakthroughs may help redirect future studies to untapped arenas of inquiry.
American Kestrels are small colorful falcons native to the western hemisphere, ranging from Southeast Alaska south to Argentina. Most previous kestrel studies have focused on the birds' nesting season. Breeding birds face many challenges given the high energetic demand required to successfully raise chicks. However, not all members of a population are breeders. Young, non-breeding raptors experience high mortality rates in the first months outside of the nest, and birds of all ages face a multitude of risks during migration, as well as on their overwintering grounds. These are important threads to follow for a holistic assessment of a species-wide decline.
Lead author Dr. Mercy Melo and a team of experts from Hawk Mountain Sanctuary, The Smithsonian, and the University of Massachusetts-Amherst, followed some of these threads by fitting 202 kestrels with radio transmitter backpacks that collected data on their whereabouts for six months. They tagged 146 fledglings, all from the Northeast United States, and 56 adults from Pennsylvania and Florida, between 2021 to 2023.
Of their tagged birds, overwintering adults demonstrated a lower survival rate in Pennsylvania than in Florida, and most confirmed adult deaths across sites were human caused. Mechanisms of these mortalities included vehicle collisions and entrapment in buildings.
The most surprising finding was that during the first weeks outside of the nest, juvenile survival was much higher than expected across sites. With any bird species, juveniles are anticipated to have lower survival rates due to lack of experience in foraging and evading risks. "Our study documented very high survival rates for juveniles across the eastern states, with some sites even boasting one hundred percent survival," says Melo. Although these young birds could not be tracked once they left the nesting area, and therefore their fates following migration and overwintering are unknown, the confirmation of their success during that first stage of initial independence fills an important knowledge gap.
When wildlife species falter, the reasons can be overwhelmingly complex. Studies like this focus research efforts towards probable realms of discovery and fuel a necessary process of elimination — if fledgling survival is high at these study sites, then the next step is to ask new questions about adults. "Understanding the factors influencing the American Kestrel population decline will help kestrels but will also improve our understanding of other declining species," says Melo. She points to grassland songbirds as one such group. "We hope this article entices researchers to adopt collaborative approaches across geographical regions." Though they are pint-sized, kestrels are top predators and therefore pack an ecological punch. Their absence would be felt across the Americas, especially within open habitats where fewer and fewer species are maintaining a foothold.
