Sneaking out: a new study shows that female falcons scout future homes

Twelve female American Kestrels (Falco sparverius) demonstrated surprising movement decisions in a recent study published in the Journal of Raptor Research. Breeding females ventured away from their nest on trips called “forays” to scout future territories, a behavior previously undocumented in American Kestrels. These findings call attention to the flexibility of movement strategies for a species that has undergone an alarming decline in recent decades.

American Kestrels are North America’s smallest falcon. Charismatic and colorful, their decline has sparked studies into breeding success and the efficacy of kestrel nest boxes. However, in their paper, “High-resolution GPS Tracking of American Kestrels Reveals Breeding and Post-breeding Ranging Behavior in Northern Virginia, USA,” lead author Joseph Kolowski of the Smithsonian National Zoo and Conservation Biology Institute and his team emphasize the dearth of information on kestrel activities that fall outside of the breeding season. Targeted research on what kestrels do after they raise their young, termed the “post-breeding” stage, could help establish a more holistic understanding of the species’ full annual cycle and shed light on overlooked habitat types the birds might depend upon.

Between May 21 and August 31 of 2021, researchers obtained detailed movement data on 12 adult female kestrels outfitted with GPS transmitters in northern Virginia. The objective of the study was to document the size of home ranges, which are areas in which the birds fulfill their basic life processes, including nesting and raising young. The team also hoped to better understand their potential forays, described as “movements outside of established home ranges.”

All 12 females engaged in forays, and, notably, their average foray distances (relative to home range size) are some of the largest recorded for any wildlife species. Furthermore, the study documented the first robust estimates of home range size for this species in the eastern United States. They found that these tracked females maintained smaller home ranges than previously reported for the species and speculate that females use smaller areas than males, who do most of the hunting during the breeding season. Future studies on male movements will help illuminate this hunch.

Between the 12 females, 128 foray flights were documented over the course of the study. That’s a lot of scouting. Here’s the kicker: the females who forayed most frequently were also the ones who moved out of their nesting home range and into the prospected territories following breeding. In other words, these females raised their young in what may have been subpar habitat and later retired to better habitat, essentially holding multiple summer homes. Lead author Kolowski says “We were surprised by the long-distance movements documented in nearly all of our females, many of which were during the period when these females had nestlings to care for. We were able to show that at least some of these movements were exploratory in nature.”

One outlier female traveled 80 miles away from her home range. The reasons for this fly-about are unknown, but future tracking studies will hopefully unveil the motivations behind such unexpected movements.

The research team is excited to study the movement ecology of male kestrels and link movement data with field habitat selection. “We are very interested to document how the kestrels use the different agricultural field types in our region, how this use changes with seasonal vegetation change, and how kestrels respond to harvest events,” says Kolowski. He emphasizes that such information will inform land management recommendations that could directly benefit American Kestrels across the region. Regarding local enthusiasm for the kestrel project, Kolowski says landowners have been generously supportive of the work. “Despite the challenges it faces, the kestrel is certainly not without its champions on the landscape,” he says.

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Paper

Kolowski et al. (2023). High-resolution GPS Tracking of American Kestrels Reveals Breeding and Post-breeding Ranging Behavior in Northern Virginia, USA. Journal of Raptor Research, 57(4): 1-19.

DOI: 10.3356/JRR-22-106

Notes to Editor:

1. The Journal of Raptor Research (JRR) is an international scientific journal dedicated entirely to the dissemination of information about birds of prey. Established in 1967, JRR has published peer-reviewed research on raptor ecology, behavior, life history, conservation, and techniques. JRR is available quarterly to members in electronic and paper format.

2. The Raptor Research Foundation (RRF) is the world’s largest professional society for raptor researchers and conservationists. Founded in 1966 as a non-profit organization, our primary goal is the accumulation and dissemination of scientific information about raptors. The Foundation organizes annual scientific conferences and provides competitive grants & awards for student researchers & conservationists. The Foundation also provides support & networking opportunities for students & early career raptor researchers.