Prey Size Influences Female Competitive Dominance in Nestling American Kestrels (Falco Sparverius)

Abstract

Body size differences typically influence sibling competition among nestling birds. In many species hatching order determine nestling size rank; however, sexually dimorphic growth may confound this correlation, giving one sex a consistent competitive advantage and leading to differential survival of the sexes. We found that captive female American Kestrel (Falco sparverius) nestlings were larger than male sibling nestmates from shortly after hatchling through fledging. When food delivered by parents was small enough to be seized and swallowed immediately by a nestling, females showed clear competitive superiority over male sibling nestmates in scramble competition. Females also took larger portions than males of small, yet unmonopolizeable, items. Competitive ability and gender were unrelated when parents delivered large, unmonopolizeable prey. Choice experiments demonstrated that parent feeding preferences did not account for the females' advantage. Based on the above, we expected fledging sex ratio to become increasingly female biased as nestling mortality increased, because nestling mortality increases and prey size decreases when preferred prey (unmonopolizeable rodents) are not available. Data from a wild kestrel population in eastern Pennsylvania supported this prediction, showing a significant positive relationship between fledging brood size and representation of males in the brood, although we note an alternative explanation for this result. Female competitive dominance and its interaction with prey size may influence the evolution of sexual size dimorphism and primary sex ratio in this species and other raptors.

Roof nesting by gulls for better or worse?

“…This study outlined a prevalent success of male offspring from the third laid egg, according to recent studies (Muller et al 2005) the adaptative significance of the male bias in the last-laid egg may relate to the interference competition and the dominance of the larger sex (Bortolotti 1986;Anderson et al 1993;Velando et al 2002). Our results underline the lack of any evidence that the urban sites are doing better than natural ones due to better access to food.…”

Year-dependent sex-biased mortality in supplementary-fed Tengmalm's owl nestlings

“…Thus, despite females being larger than males (to a similar degree as in Tengmalm's owl) and requiring more food during growth (Anderson et al 1993b), their large size provided an advantage in interference competition for food items. Admittedly, unlike Anderson et al (1993a), we were unable to directly observe nestling interaction. However, Anderson et al (1993a) stated that dominance of the larger sex could be expected in sexually size-dimorphic semi-altricial nestlings (sensu Ricklefs 1972), which have well-developed motor functions, and are able to physically compete with their nest-mates.…”

Hatching asynchrony in the Eurasian kestrel Falco tinnunculus: an experimental test of the brood reduction hypothesis

“…Wiehn, P. Ilmonen, M. Pahkala & E. KorpimaÈ ki, unpublished data). Because overt pecking of siblings is rare, the proximate reason for mortality is probably larger siblings preventing the smallest receiving adequate food for survival (see also Mock 1985;Anderson et al 1993). Because older kestrel nestlings monopolized food to the detriment of younger siblings in both supplemented and non-supplemented treatments there appears to be a cost to asynchronous hatching also reported for American kestrels and other species (Wiebe 1995).…”