The Significance of Song Variation in a Population of Darwin's Finches

Abstract

AbstractMales of Geospiza conirostris, the large cactus finch, on Isla Genovesa, Galápagos, sing a single, precisely copied song. There are two song types in the population, and these are sub-divided into 3 and 5 discrete song sub-types respectively. Adult males sing only one song sub-type throughout life. Sons sing the same song sub-type as their fathers' and do not copy the songs of either natal or breeding territory neighbours. Significantly more of the males holding territories with neighbours of unlike (heterotypic) song type obtain a female than males in territories with no heterotypic neighbour. Pairs in territories with a heterotypic neighbour fledge significantly more young than do pairs in territories with no heterotypic neighbour. Significantly more of those young born on territories with a heterotypic neighbour, that survive to become adults, obtain a mate, than do those surviving young born on territories with no heterotypic neighbour. The data are consistent with the hypothesis that the relative greater importance of song for species recognition in this population has made it advantageous for males to have a single, short, distinct, precisely copied and stable song. The necessity for recognizing both song types as conspecific has produced a unique mating pattern which favours pairs in territories with a heterotypic song neighbour. This allows young to associate visual and auditory cues of father and neighbouring male during the short period of imprinting. A small effective population size, combined with the observation that no female has been known to mate with a male of the same song sub-type as her father's, suggests a possible kin recognition system to avoid inbreeding.