Jack-of-all-trades or master of one? Variation in foraging specialisation across years in Darwin’s Tree Finches (Camarhynchus spp.)

Abstract

Theory predicts that species should have wider foraging niches during conditions of resource scarcity. However, empirical evidence to date has shown mixed patterns, including studies of Darwin’s finches that have found narrower foraging niches during conditions of resource scarcity. Here, we compare foraging behaviour in three species of Darwin’s Tree Finches (Camarhynchus spp.) in a dry versus wet year on Floreana Island to examine the change in foraging breadth under conditions of resource scarcity. We provide descriptive data on diet, foraging substrate, technique, height, and foraging time across the Small Tree Finch (C. parvulus), Medium Tree Finch (C. pauper), and Large Tree Finch (C. psittacula). During dry versus wet years, we made the following predictions: (1) lower intraspecific niche breadth (that is, a higher level of specialisation), (2) lower interspecific overlap in all foraging parameters, and (3) longer foraging times due to relative resource scarcity. Our findings showed that the Small and Medium Tree Finches were generalist foragers, while the Large Tree Finch was a specialist. Resource specialisation varied across years of differing rainfall: both generalist species were less specialised (higher Shannon diversity index) during the dry year, while the specialist species was more specialised (lower Shannon diversity index).