Behavioral ecology of Darwin's finches: Song and mating patterns in a bimodal population of the medium ground finch (Geospiza fortis)
Darwin’s finches of the Galápagos Islands are a model system for the study of evolution by natural selection and ecological speciation. Previous studies suggest that selection on bill morphology due to feeding may have secondary consequence on song, an important cue in species recognition. In Chapter 1, I document differences in songs between nine species of Darwin’s finches on Santa Cruz Island. Songs of individuals species are distinct, with sister species having the greatest number of song differences in comparison to species that are further removed phylogenetically. Additionally, differences in song features between species correlate with beak and body size. These results led me to examine song differences within a highly variable population of the medium ground finch, Geospiza fortis, on Santa Cruz Island.
The medium ground finch at El Garrapatero, Santa Cruz Island, features large and small beak morphs with relatively few intermediates. As in other Darwin’s finches of the Galápagos Islands, these morphs presumably diverged in response to variation in local food availability and inter- or intra-specific competition. In Chapters 2--4, I address whether the two beak morphs produce distinct mating signals (songs), mate assortatively, and have higher fitness than intermediate sized birds (e.g., produce more offspring). I find that birds with longer, deeper, and wider beaks (large morphs) produce songs with significantly lower minimum frequencies, maximum frequencies, frequency bandwidths, and vocal performance. The two morphs show strong positive assortative pairing, a pattern that holds over three breeding seasons and during both dry and wet conditions. In addition, there is reduced gene flow between the morphs, as revealed by genetic variation at ten microsatellite loci. Documenting offspring success was complicated by the introduction of a parasitic fly, Philornis downsi. Nestlings in nests with P. downsi had extremely high mortality. Parasitism rates were equal across all beak morphs, however the combination of mortality due to both parasitism and predation resulted in fewer small morph fledglings and more large morph fledglings than expected in 2005. In 2006 only two birds fledged, and they were the offspring of a single pair with intermediate sized beaks; the fitness of small and large morphs was zero. Nestling size and growth rates did not differ between parasitized and unparasitized nests. These findings together have interesting implications for speciation and population divergence in this group of birds.