The avian beak is a key morphological trait used for foraging. If parasites alter beak shape, we expect changes in host foraging behaviour. Nestling Darwin’s finches on the Galápagos Islands experience naris size enlargement caused by Philornis downsi larvae. We predict (1) altered foraging behaviour in birds with malformed beaks from P. downsi. To test the evolutionary implications of naris malformation, we predict (2) changes in foraging niche overlap between sympatric species, and (3) worse body condition in birds with malformed beaks. Our focal species are small tree finch (Camarhynchus parvulus), medium tree finch (C. pauper), hybrid tree finch (Camarhynchus spp.), and small ground finch (Geospiza fuliginosa) on Floreana Island. With the exception of hybrid finches, Darwin’s finch species with malformed beaks exhibited increased surface prey removal and decreased hidden prey extraction during foraging. Naris size enlargement, and different beak size traits per species, was associated with worse body condition in Camarhynchus finches. Divergence can be rapidly undone when introduced parasites alter functional morphological traits that create ecological distinctiveness in species.

Darwin's finches were mist-netted, banded for later field identification, and measured. Foraging data were collected as first foraging observations of Darwin's finches during transect sampling on Floreana Island, Galapagos. For a sub-set of foraging observations of color-banded birds, we have information on body size and naris size in relation to foraging behavior. 

Each row represents a unique foraging observation per bird collected from ad hoc observations during transect sampling. Sheet two provides explanations for the column headings.