Female demands: some fitness implications in chickadee flocks

Abstract

Female black-capped chickadees (Parus atricapillus), besides searching for their own food, "demand" food from their mates in the spring for extra energy needed to produce and incubate eggs. This behaviour signals their readiness to copulate and it is followed by increased levels of male – male aggression, probably to prevent cuckoldry. Increased aggression owing to limited nest site availability may also be locally important. Subordinate pairs disperse and many presumably die. The extra food allows females with nest sites to breed earlier than if they fed themselves exclusively, and producing early broods appears to be a better fitness strategy than having larger clutches or additional broods. Juveniles from early broods survive better and attain higher social ranks in flock hierarchies, and dominant pairs have the highest overwinter survival rates as well as a better chance than subordinate pairs of obtaining a territory in the spring.