Bill dimorphism and foraging niche partitioning in the green woodhoopoe

Abstract

1. Cooperatively breeding green woodhoopoes, Phoeniculus purpureus, forage mainly as close-knit groups, creating opportunities for intrasexual and intersexual foraging competition. 2. Adult males foraged on wider branches than adult females, probably as a consequence of their larger body size (5-8%). Moreover, adult males spent more time scaling bark and probing the ends of broken branches, while adult females preferred pecking. There was no intersexual difference in the use of hole probing or surface gleaning. 3. Intersexual differences in foraging technique probably resulted from morphological differences: adult male bills were 36% longer than those of females, with no overlap between the sexes. In support of the specialization hypothesis: (a) birds were more likely to forage in close proximity to a member of the opposite sex; (b) there was more intrasexual than intersexual aggression during foraging; (c) lone females did not change their foraging behaviour from when in close proximity to a male; (d) the niche breadth of both sexes was similar; and (e) juveniles foraged in ways predicted from their bill lengths. On fledging, bills of juvenile males and females were the same length as those of adult females, and all juveniles fed like adult females. After 4 months, the bills of juvenile males exceeded those of adult females, and they began to forage like adult males. 4. Adult males brought different invertebrate taxa and heavier prey to the nest than females. These differences were a consequence of the different foraging techniques used, because both sexes collected the same types of prey when using the same technique. 5. When all group members foraged together, dominant adults spent more time hole probing than subordinates of the same sex. This intrasexual difference probably resulted from interference competition, as dominants and subordinates did not differ in bill length, and dominants excluded subordinates from prime feeding areas. 6. To compensate, subordinate adults did not increase their foraging time or collect larger prey items than dominants, but more often left the group to forage alone. When alone, subordinates spent more time hole probing and experienced higher foraging success than when in a group.