Possible Functions of Tail-Pumping by American Kestrels (Falco Sparverius)

Abstract

When perched, several species of small falcons, including American Kestrels (Falco sparverius), often pump their tails, but the possible function of this behaviour is unknown. Our objective was to use observations and experiments to examine the possible function(s) of tail-pumping by American Kestrels. Fieldwork was conducted from March 2015 to December 2015 at the Blue Grass Army Depot in Madison County, Kentucky. During observations of focal kestrels, we noted their behaviour (e.g. landing on a perch, hunting, or consuming prey), including when and how often they pumped their tails (i.e. rapid movement of the tail down, then back up to its original position). Kestrels typically tail-pumped when landing on a perch (mean = 4.1±0.2 pumps per 10 s) and consuming prey (mean = 2.4±0.2 pumps per 10 s). When hunting, kestrels tail-pumped at higher rates during the 30 s prior to attacking (mean = 1.1±0.3 pumps) than they did during the 30–60 s interval before an attack (mean = 0.3±0.1 pumps). During experiments where kestrels were presented with models of a conspecific and a predator (Cooper's Hawk, Accipiter cooperi), we found no difference in rates of tail-pumping prior to and during the presentation. These results suggest that tail-pumping by American Kestrels is not used either to communicate with conspecifics or as a predator-deterrent signal. Rather, kestrels appear to tail-pump to help maintain balance on perches when landing and consuming prey. In addition, prior to attacking prey, kestrels typically bob their heads (possibly to aid in judging distances), and tail-pumping may help them maintain stability as they head-bob and prepare to attack.