BRANTA — Andrew Radford
Foraging and vocal communication in the sexually dimorphic red-billed woodhoopoe
Institution: University of Cambridge, UK
Supervisors: NB Davies
Details: PhD 2003 (Completed)
Address: School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG (Jun 2007) Email
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
Radford, A.N. 2005. Group-specific vocal signatures and neighbour-stranger discrimination in the cooperatively breeding green woodhoopoe. Animal Behaviour 70: 1227-1234.
Radford, A.N. & du Plessis, M.A. 2004. Territorial vocal rallying in the green woodhoopoe: factors affecting contest length and outcome. Animal Behaviour 68: 803-810.
Radford, A.N. 2004. Vocal mediation of foraging competition in the cooperatively breeding green woodhoopoe,Phoeniculus purpureus. Behavioral Ecology and Sociobiology 56: 279-285.
Radford, A.N. 2004. Incubation feeding by helpers influences female nest attendance in the green woodhoopoe,Phoeniculus purpureus. Behavioral Ecology and Sociobiology 55: 583-588.
Radford, A.N. 2004. Voice breaking in males results in sexual dimorphism of green woodhoopoe calls. Behaviour141: 555-569.
Radford, A.N. 2004. Vocal co-ordination of group movement by green woodhoopoes (Phoeniculus purpureus).Ethology 110: 11-20.
Radford, A.N. & du Plessis, M.A. 2004. Extreme sexual dimorphism in green woodhoopoe bill length: a case of sexual selection? Auk 121: 178-183.
Radford, A.N. & du Plessis, M.A. 2004. Green woodhoopoe territories remain stable despite group-size fluctuations. Journal of Avian Biology 35: 262-268.
Radford, A.N. & du Plessis, M.A. 2003. Bill dimorphism and foraging niche partitioning in the green woodhoopoe.Journal of Animal Ecology 72: 258-269.
Radford, A.N. 2003. Territorial vocal rallying in the green woodhoopoe: influence of rival group size and composition. Animal Behaviour 66: 1035-1044.
Radford, A.N. & du Plessis, M.A. 2003. The importance of rainfall to a cavity nesting species. Ibis 145: 692-694.