The extent to which interspecific competition structures species interactions and coexistence within communities, and the relevant mechanisms, are still debated. We focus on New World wood warblers (Parulidae), beginning with Robert MacArthur's iconic 1958 paper in which he shows how subtle foraging behaviors, purportedly linked to dietary differences, within spruce trees contribute to the coexistence of 5 spruce-woods warbler species. MacArthur coined the phrase “resource partitioning”, and profoundly impacted the field of Ecology for subsequent decades in diverse ways. To understand what MacArthur got right and what he missed, we reviewed both ecological and evolutionary approaches to questions of the origin and coexistence of competing species in the context of diet. We argue that an important, underappreciated, mechanism of competition among coexisting migratory warbler species, particularly in winter, is diffuse exploitation competition, based in part on our own studies of warbler diets in relation to foraging behavior, substrate use, bird morphology, and other traits. Our review and synthesis of interspecific competition and coexistence in warblers have important consequences, including our questioning of the importance and effectiveness of resource partitioning in birds. We also suggest a novel hypothesis for the success of warblers today in the Caribbean and other habitats, beginning with their relatively recent adaptive radiation and the ecological opportunity on Caribbean islands.
LAY SUMMARY
Robert MacArthur's pioneering study of New World wood warblers triggered immense interest in how competitor species partition resources to coexist.
By reviewing resource competition and use among coexisting warbler species we address MacArthur's impressive legacy and consider what he missed.
We integrate ecological and evolutionary information to argue that warblers coexist primarily by specializing on different foraging substrates (i.e., locations where food is sought) rather than partitioning resources themselves, which are diverse and substantially shared by demonstrably competing species.
By comparing warbler diets, foraging behavior, and morphology we argue that diffuse exploitative interspecific competition favors substrate specialization; and we suggest a novel evolutionary hypothesis to explain the adaptive radiation of these warblers, particularly in the Greater Antilles islands.
Diverse new data—especially diets—and approaches to warbler biology compel innovative perspectives on MacArthur's fundamental contributions to coexistence in competing species.
