Functional traits and foraging behaviour: avian vampire fly larvae change the beak and fitness of their Darwin's finch hosts

1. The avian beak is a key morphological trait used for foraging. If parasites alter beak shape, we may expect changes in host foraging behaviour. Larvae of the avian vampire fly Philornis downsi cause naris enlargement in Darwin's finch nestlings when first and second instar larvae consume keratin, blood and tissue from inside the beak of the developing host. This naris malformation persists into adulthood, where nares that are >15% of total beak length are considered enlarged. 

2. We measured effects of parasite-induced naris enlargement on foraging behaviour, foraging niche overlap and body condition in Darwin's finches on Floreana Island. Foraging behaviour was ranked by the stress per foraging technique exerted on the beak and ranged from least stress for ‘gleaning’ to most stress for ‘chip off bark’. 

3. Naris enlargement occurred in 34% of adult birds. The most common foraging technique differed among species: medium tree finches (Camarhynchus pauper) often chipped off bark to extract subsurface prey, small tree finches (C. parvulus) often gleaned surface prey from foliage, hybrids gleaned prey from bark and foliage, and small ground finches (Geospiza fuliginosa) mostly foraged on the ground. In C. pauper, birds with naris enlargement did more gleaning and less subsurface prey excavation. Foraging niche across species was most similar in birds with naris enlargement. Finally, body condition was lower in insectivorous tree finches with malformed beaks. 

4. A novel aspect of this study is the idea that parasite-induced alterations to phenotype affect ecological processes and interspecific interactions at large temporal and spatial scales. The parasitism occurs early in life but the ecological effects of this parasitism, if causative, are happening later. Read the free Plain Language Summary for this article on the Journal blog.