Sex-dependent frequency and consequences of natural handicaps in American Kestrels

Natural selection theory suggests that there should be fitness consequences for animals that possess morphological abnormalities such as missing or deformed appendages and other structures. As these characters are expected to be detrimental, morphological abnormalities could be considered ‘‘handicaps’’. However, little information exists about the nature, prevalence or consequences of naturally occurring handicaps in birds, even though experimental handicapping has been used in some behavioural studies. Here, we document and describe the frequency of natural handicaps in American Kestrels Falco spar erius, and investigate their influence on condition and survival. We considered three different categories of handicap that should negatively impact falcons: missing remiges and rectrices, broken remiges and rectrices, and foot handicaps. Broken feathers, followed by missing feathers, were the most common handicaps detected in kestrels and their prevalence increased throughout the breeding season. Foot handicaps were relatively rare and showed no seasonal trends. There was no effect of any handicap type on body condition or return rates of male kestrels. In contrast, females with either foot handicaps or broken feathers were in significantly poorer condition than control females, and females with broken feathers were less likely than control females to return to the study area in years following initial capture. Given that female kestrels with broken feathers have more fault bars than those without broken feathers, and fault bar formation is related to stresses in a bird’s past, we suggest that female kestrels that had broken feathers were of poorer quality than those without broken feathers or those with missing feathers. Females with broken feathers may therefore have been in poorer condition initially than other birds, or if broken feathers are indicative of poor quality, then females with broken feathers may have been less able to cope with the negative impacts of feather loss on flight performance compared to birds with missing feathers.