CULTURAL-EVOLUTION OF MALE SONG TYPES IN DARWIN MEDIUM GROUND FINCHES, GEOSPIZA-FORTIS

To examine the patterns and causes of cultural change in a bird species, the changes in frequency, between-generation transmission, and demographic success of different male song types were studied in a population of Darwin's medium ground finches, Geospiza fortis, on Isla Daphne Major, Galápagos over a 6-year period (1979-1984). Males in this population sang one of four simple discrete song types throughout their lifetimes and males with different song types differed in body size. Sons generally sang the same song type as that of their fathers, although the percentage of sons who learned non-paternal song types (25% of all father-son pairs) was higher than previously reported for this population. There were frequency-dependent changes in the relative numbers of different song types over time: the type that was most common at the start of the study declined significantly in frequency over the 6-year period while the three less common types showed equivalent increases in relative abundance. This non-random, pattern of cultural change was at least partially caused by both direct and indirect selection on male song type. Direct selection occurred because males with the less common songs survived longer; this relationship was independent od possible effects of body size on survival. Males with the less common songs also produced more yearling offspring. However, this difference is best explained as an indirect, correlated effect of a positive relationship between male body size and fecundity. Finally, males with less common songs also produced more sons who recruited to the breeding population, which is indirect evidence that the song type of a young male may influence his chances of becoming a breeder. This demonstration of fitness differences between males with different, song types argues that biological and cultural evolution are more closely coupled processes in these birds than in previously studied species. © 1990.