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resume/Holyoak1977a

Author: Holyoak, David T.; Thibault, Jean-Claude
Year: 1977
Title: Habitats, morphologie et interactions écologiques des oiseaux insectivores de Polynésie orientale
Journal: L’oiseau et la Revue française d’ornithologie
Volume: 47
Pages: 115-147

Abstract: The islands of eastern Polynesia have three resident genera of insectivorous woodland birds (Halcyon, Acrocephalus, Pomarea) and are visited by a migrant cuckoo (Urodynamis). The distribution , habitat preferences and feedind ecology of these genera are described and compared. The size, proportions and coloration of different populations of each of these genera are described and the following environmental correlations are noted: the measurements of Acrocephalus and Pomarea are directly correlated with the average size of the leaves of the native vegetation on different islands, but no such correlation is present with Halcyon, which uses different feedings techniques; Acrocephalus populations that are sympatric with Pomarea invariably have yelow underparts, whereas most other populations do not: the coloration of Pomarea is darker on islands with dense wet forest than on those with dry open woodland (fig.3); Acrocephalus and immature Pomarea are lighter coloured than adult Pomarea and they tend to inhabit more open and drier forest than adult  Pomarea. Adaptive explanations are offerd for these and other correlations and it is argued that the principal differences between populations on different island have evolved as adaptations to the local conditions. Ecological differences between the four genera are regarded as ecological isolating mechanism which are maintained by competition . The equilibrium theory of insular biogeography is criticized as the immigration and extinction rates for the birds and islands discussed appear only to represent boundary conditions, within which other factors determine species numbers. Inter-specific competition affected by adaptations to local conditions is thought to be of paramount importance in determining species numbers.