TRACKING DEVICE ATTACHMENT METHODS FOR AMERICAN KESTRELS: BACKPACK VERSUS LEG-LOOP HARNESSES

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.

Automated summary

The population of American Kestrels, a small falcon species, is declining across North America, and the reasons for this decline are still unknown. To understand their migratory connectivity and identify potential causes of decline, tracking devices are usually used, but attaching them to American Kestrels is challenging due to their small size and aggressive behavior. This study found that attaching tracking devices using backpack harnesses had higher return rates compared to leg-loop harnesses, suggesting that backpack harnesses are the preferred method for attaching devices to American Kestrels.