Updating MacArthur's (1958) classic study: Morphological differences show past evolutionary pressure for efficient foraging in New World warblers (Parulidae)

Kristen M. Rosamond, Cody M. Kent and Thomas W. Sherry, Ecology and Evolutionary Biology, Tulane University, New Orleans, LA, Sarah Chieko Hunter, Biology, Delgado Community College, New Orleans, LA

Background/Question/Methods
New World wood warblers (Parulidae) represent one of the most dramatic adaptive
radiations in North America. However, the ecological bases for the morphological
differences among these species remain poorly understood, especially considering how
many foraging and habitat studies the family has inspired. MacArthur’s (1958) classic
ecological study demonstrated that foraging differences allow for coexistence of
breeding spruce-woods warblers, but he downplayed the importance of morphology
in shaping these relationships. We hypothesized the existence of relationships between
morphology and diet in a suite of parulid warblers. We combined a principal component
analysis (PCA) of 18 external morphological traits of 11 species of warblers with stomach-
content data from coexisting species in one breeding community in Louisiana and three
wintering communities in Jamaica.
Results/Conclusions
The primary morphological differences, corresponding with the first three PCA axes, were
body size, morphological adaptations for aerial foraging versus gleaning, and arboreal
versus ground-foraging adaptations. Our analysis revealed little morphological overlap
among the 11 species. Differences in diet among the warblers showed a significant
relationship to the first two PCA axes of morphological traits. For five coexisting, foliage-
gleaning species wintering in Jamaican wet limestone forest, larger warblers ate larger
beetles and Orthopterans but not larger ants. In analyses including all four communities,
species of warblers with aerial foraging morphologies consumed a greater proportion of
winged insects than other warbler species. These findings document prey selection
relevant to multiple subtle morphological differences among coexisting species. Overall,
our results indicate that food and foraging have likely played an integral role in the
morphological diversification and coexistence of species in the family Parulidae.