Reproductive Success Increases with Age in American Kestrels (Falco sparverius), Especially in Breeding Males

Age-related differences in reproductive success have been well documented in many species of birds. The experience gained through successive breeding attempts can improve survival skills, foraging abilities, access to resources through social dominance, and familiarity with each stage of reproduction. In this study we examined 669 nesting attempts by American kestrels (Falco sparverius) from a population that bred in nest boxes in northwestern New Jersey, 1995 to 2018. We tested the hypothesis that older kestrels would have greater nesting success than those in their first reproductive attempt. Clutch size, hatching rate, and the number of chicks that survived to fledging varied significantly and positively with age for both male and female breeders. These trends also were associated with the date of clutch initiation and amount parental attentiveness. Older birds initiated breeding earlier than those in their first breeding attempt, and older birds of both sexes also were more likely to be present in the vicinity of the nest site, especially during the incubation and nestling periods. The relationship between age and breeding success was much more pronounced in males than in females. This difference is consistent with the behavioral role that each sex has during a breeding attempt. After laying a clutch, the female performs most of the incubation and broods the young chicks. In contrast, the male provides food to the female prior to egg laying and throughout incubation, and to the entire family until the chicks no longer are brooded by the female. Pairs in which both adults were older had significantly greater nesting success than mixed or young pairs, and we never observed an older female paired with a male in his first reproductive attempt.