Components of Niche Width in a Territorial Bird Species: Habitat Utilization in Males and Females of the Chaffinch (Fringilla coelebs) on Islands and Mainland

Here we have discussed how interspecific competition will affect intrapopulation variation in traits (morphological and/or behavioral) involved in resource utilization. We concluded that decreased intensity of interspecific competition will lead to an increase in variation within a population. This increase in variation may be continuous or discontinuous (e.g., sexual dimorphism). For a territorial species within a given macrohabitat the increased variation is expected to take the form of sexual dimorphism. This prediction was tested by a comparison of the habitat utilization of the chaffinch, a territorial species, on small islands in a lake and the surrounding mainland. Both the number of potentially competing species and food (arthropod) abundance decreased from mainland to islands; there was a greater decrease in competitors than in food abundance. Components of niche width did show different patterns on the islands and the mainland: on the islands, but not on the mainland, there were significant differences between males and females in habitat utilization. Thus, on the islands, as compared to the mainland, the between-sex component of the population niche width was relatively large and the within-sex component relatively small. Sexual dimorphism entails an adaptive increase in variation within the population in that it reduces intrapair competition.