We compared support for 3 hypotheses that might explain observed morphological variation among islands of 4 species of Caribbean land birds: ecological release from competition and predation pressure, predation pressure from 1 novel predator species (small Indian mongoose, Herpestes auropunctatus), and climate. We measured wing chord, tarsus length, bill length, and mass of Bananaquits (Coereba flaveola), Black-faced Grassquits (Tiaris bicolor), Lesser Antillean Bullfinches (Loxigilla noctis), and Common Ground Doves (Columbina passerina) in Grenada, 2015–2017, and combined these measures with data from 23 other Caribbean islands collated from academic papers and researchers, for a total sample size of 6,518 individuals. We found the strongest support for the ecological release hypothesis, but each of our hypotheses received some support, suggesting that ecological release from competition, predation pressure from mongoose, and climate may all interact to influence morphological adaptations of birds to local conditions in the Caribbean.
LAY SUMMARY
The same species of bird can look different on different islands; for example, their bills, wings, and legs might be longer or shorter in different populations.
We studied four species of birds in the Caribbean (Bananaquit, Black-faced Grassquit, Lesser Antillean Bullfinch, and Common Ground Dove) to try to understand why they might vary among islands.
We evaluated whether reduced competition, risk of predation by the introduced small Indian mongoose, or temperature and precipitation might affect the morphology of birds on different islands.
Our results suggest that reduced competition and threats from native predators explained the most variation in morphology among islands, but there was some evidence that climate and predation from mongoose might also affect birds.
The local adaptations that different species have to their island homes highlight the importance of conserving species across all their diverse populations.
