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.

The functions of tail flicking in birds: A meta-analysis

“…Concerning the balance hypothesis, the results of the available studies are contradicting. A study in Eastern phoebes by Carder and Ritchison 5 did not find significant evidence for the balance hypothesis, whereas Suich and Ritchison 4 found American kestrels to flick more after landing on a perch than while sitting on a perch. Finally, Jackson and Elgar 6 found willie wagtails to not flick more frequently while being on the ground than while sitting on a perch.…”

“…Another explanation could be that studies greatly differed in how they measured tail flicking rate. Some reported results were from experimental studies, 4,16,24 whereas other results were purely observational without any manipulative treatment. 7,13,17,22 Therefore, future studies should focus more on possible differences in flicking behavior between males and females.…”

“…American kestrels ( Falco sparverius ), for example, flick more frequently when landing on a perch. 4 Carder and Ritchison 5 in contrast found no relationship between perch stability and tail flicking in Eastern phoebes ( Sayornis phoebe ). Moreover, Jackson and Elgar 6 found willie wagtails ( Rhipidura leucophrys ) to flick more while being on the ground than while sitting on perch.…”

“…10 American kestrels show no change in flicking rate while hearing conspecific calls. 4 This indicates that flicking might not be used as a form of social communication. However, there is evidence in favor of the communication hypothesis from a study with American crows ( Corvus brachyrhynchos ) as crows increased tail flicking after hearing conspecific calls.…”

Functions of avian elongated tails, with suggestions for future studies

“…Although ETs may be negatively related to birds' static balance, longer tails may be beneficial to birds' balancing when they forage on unstable substrates (Short 1970). Also, it is possible that tails provide most effective balancing support through movement (Suich & Ritchison 2018). Overall, we still need to improve our understanding of the perch balancing function through manipulations of tail length and possibly manipulations of substrates on which birds forage.…”