Evolutionary exposition from plumage pattern in African Accipiter

Abstract

Louette, M. 2000. Evolutionary exposition from plumage pattern in African Accipiter. Ostrich 71 (1 & 2): 45–50.

Plumage characteristics were superficially taken into account by Wattel (1973) for his division of the genus Accipiter; Savalli (1995) formulated hypotheses to explain plumage colouration evolution in birds in general.

I examine here variation of plumage coloration and pattern in the 14 African species of Accipiter, in order to evaluate their phylogenetic and/or functional values.

I study isolated populations in this respect:

— A.francesae: the derived Comoro Islands taxa are remarkably different from the Madagascar stock in the disappearance of adult sexual dimorphism and by the ‘reversion’ to an ancestral juvenal pattern.

— A. tachiro: the derived Pemba Island taxon is more colourful in adult plumage than the Tanzanian savanna-dwelling stock. The Bioko Island taxon derived from Cameroonian forest stock and is only slightly different therefrom. Juvenal plumage is remarkably variable among central African forest races, supposedly because in sympatry with other Accipiter species, the juvenile needs to be recognised by conspecifics.

From a general examination of the African species it appears that:

	— Adults can be dimorphic (cryptic females) or monomorphic (both sexes colourful or both sexes cryptic). Bright adult plumage serves sexual attraction: in dense environment better visibility of bright colour is on balance advantageous over crypsis.
	— Juvenal plumages are mostly highly cryptic; ventrally, the basal colour occurs in two phases within species and the pattern is variable among species: it can be streaked, barred or spotted. In most African Accipiter certain bright colours present in the adult are announced in the juvenile plumage.

It is assumed that the cryptic juvenal plumage serves to minimise harassment by territorial adults, but in some species, the juvenile resembles the adult female, the reason for this mimicry is not yet established. Juvenal and adult plumages being subject to rapid evolutionary change, they seem to be of limited use in phylogeny.