Tracking Device Attachment Methods for American Kestrels: Backpack Versus Leg-Loop Harnesses

Abstract

The American Kestrel (Falco sparverius) is a small falcon whose population is declining across North America, but drivers of this decline have yet to be identified. Conservationists have called for greater understanding of kestrels' migratory connectivity to identify potential causes of decline that may occur during various parts of their annual cycle. For many bird species, the most common and direct way to determine migratory connectivity is by attaching a tracking device that documents their locations over time. However, American Kestrels present a challenge when attaching tracking devices due to their small size and aggressive biting capabilities. We used two attachment methods (backpack and leg-loop harnesses) commonly used for raptors on a wintering population of American Kestrels in north Texas to test whether there were differences in observed return rates the following year. Of 20 American Kestrels fit with devices, significantly more of the kestrels fit with backpack harnesses were observed the following winter than the birds fit with leg-loop harnesses. Eight of the 10 kestrels we fit with backpacks were observed to return to their winter territories compared to only two of 10 kestrels fit with leg-loops. Overall return frequency data of kestrels with both harnessing methods (50%, n = 20) was similar to the frequency we observed on marked birds without devices (52%, n = 190), though this was heavily skewed due to the high return of birds with backpack harnesses (80%). Although our sample size was limited, the disparity between the return rates of birds with the two harness types suggests that backpack harnesses are the preferred method of attaching tracking devices to American Kestrels, and also potentially to similar small raptor species.

Evaluation of tag attachment techniques for plunge‐diving terns

“…Moreover, in several raptor species, flight performance was better for leg-loop over full-body harnesses (Longarini et al 2023). Both would favour the use of leg-loops over full-body harnesses, although in other raptor species return rates were lower for birds with leg-loops compared with those with full-body harnesses (Biles et al 2022). On the other hand, placement of the load over the centre of gravity of the bird has also been suggested as the most optimal tagging strategy (Wanless et al 1989), which would favour the use of a full-body harness.…”

“…Both would favour the use of leg‐loops over full‐body harnesses, although in other raptor species return rates were lower for birds with leg‐loops compared with those with full‐body harnesses (Biles et al . 2022). On the other hand, placement of the load over the centre of gravity of the bird has also been suggested as the most optimal tagging strategy (Wanless et al .…”

The Full Annual Cycle of the American Kestrel: State of the Knowledge, Information Gaps, and Conservation Needs

“…Documenting the full annual migration cycle in American kestrels has been an extremely difficult nut to crack for several reasons, however, several papers in this issue review the challenges, offer improved approaches, and are starting to reveal exciting insights into this aspect of kestrel life history. First, Kelsey Biles and colleagues (2023) evaluate use of leg-loop versus backpack harnesses for attaching devices to track kestrel movements. They concluded that backpack harnesses were most effective and had no adverse effects on the annual return rates of wintering kestrels in Texas.…”