Andrew Radford

Foraging and vocal communication in the sexually dimorphic red-billed woodhoopoe

Institution: University of Cambridge, U.K.
Supervisors: NB Davies
Details: Ph. D. 2003 (Completed)

Current Address: School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG (Jun 2007)
Email: Andy.Radford @ bristol.ac.uk (remove spaces)

Subject Keywords: Foraging competition, vocal communication, conflict resolution, group living, niche partitioning

Species Keywords: Green (red-billed) woodhoopoe Phoeniculus purpureus

Abstract:

The benefits to group living are well known, as cooperation may lead to higher success than can be gained by an individual acting alone. However, group living also brings associated costs, such as greater competition over resources and mating opportunities. I investigated how conflict within and between groups influenced the foraging behaviour and vocal communication of the cooperatively breeding red-billed woodhoopoe, Phoeniculus purpureus, in the riverine forests surrounding Morgan’s Bay, Eastern Cape Province, South Africa. Thirty colour-ringed groups, ranging in size from two to eight adults and up to four dependent young, were studied from November 1999 to March 2002. Groups tended to feed as a close-knit unit. Foraging competition was reduced through both intrasexual and intersexual niche partitioning. First, breeding individuals were dominant over helpers of the same sex and prevented them from foraging in prime areas. Second, adult males had longer bills than females and used different foraging techniques as a consequence. Fledglings of both sexes initially foraged in a fashion similar to adult females, but young males began to forage using adult male strategies when their bill attained a certain length. I found no evidence that the sexual dimorphism in bill length arose from sexual selection. Foraging competition was further mediated through sexually dimorphic vocalisations, which regulated spacing between feeding individuals. Young males and females produced calls of a similar frequency on fledging, but the male voice ‘broke’ after a period of 4-5 months, coinciding with growth of a longer bill and a switch to adult male foraging strategies. Conflict also arose when groups moved to a new feeding area. Dominant individuals tended to initiate movement with a mobilisation vocalisation, which generally elicited immediate following by others. Subordinates occasionally moved away silently by themselves, which resulted in increased foraging success at new feeding sites before the arrival of dominants.
Group members combined in the year-round defence of well-defined, all-purpose territories, which remained relatively stable in size between seasons despite large fluctuations in group size. Although all group members participated in the raucous vocal rallying displays given during territorial encounters, individuals contributed differing amounts depending on the composition of the intruding group. Playback experiments demonstrated that groups gave longer rallies in response to larger groups, and that individuals contributed more in response to same-sex intruders. Rallying therefore seemed to serve two main functions. First, it determined the likelihood of a successful temporary intrusion: owners tended to win short contests, while larger groups dominated extended ones. Second, as a result of the sexually dimorphic vocalisations, rallies indicated group composition and the possibility of breeding vacancies. Since subordinates tended to participate more than dominants, they may have more to lose from intrusions or the addition of new group members.

Published Papers: