The size of adornments of male scarlet-tufted malachite sunbirds varies with environmental conditions, as predicted by handicap theories

Abstract

Scarlet-tufted malachite sunbirds, Nectarinia johnstoni, are endemic to the alpine zones of East African mountains. On Mount Kenya they feed almost exclusively on the nectar of Lobelia telekii flowers. Males are bright iridescent green, with red pectoral tufts and elongated central tail feathers. The pectoral tufts are used to mediate male-male competition for territories and the long tail may be important in mate choice. The pectoral tufts and the tails of males measured in both 1989 and 1990 increased between years. This increase could be due to age or to differences in condition of the males in the two years. There was an increase in the population tail length and pectoral tuft size of the same magnitude as the change seen in known males which suggests that an age effect is unlikely. The density of L. telekii inflorescences increased between the two years, so more food was available in 1990 than in 1989. Weather records and data from breeding success also suggest that conditions in 1989 were harsher than in 1990. The available information suggests that the size of pectoral tufts and tail length are influenced by environmental conditions rather than male age. © 1991 The Association for the Study of Animal Behaviour.