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. We hypothesized the existence of relationships between parulid morphology and diet. 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. 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.

Among the most rapidly declining birds in continental North America, grassland birds evolved with American bison (Bison bison) until bison nearly became extinct due to overhunting. Bison populations have subsequently rebounded due to reintroductions on conservation lands, but the impacts of bison on grassland nesting birds remain largely unknown. We investigated how bison reintroduction, together with other land management and climate factors, affected breeding populations of a grassland bird species of conservation concern, the Bobolink (Dolichonyx oryzivorus). We quantified population changes in Bobolinks over an 18-year period in conservation grasslands where bison were reintroduced, compared with adjacent grasslands grazed by cattle and where hay was harvested after the bird breeding season. Four years after bison reintroduction, the bison population in the study area had doubled, while Bobolink abundance declined 62% and productivity declined 84%. Our findings suggest that bison reintroduction as a conservation strategy may be counterproductive in grassland fragments where overgrazing, trampling, and other negative impacts drive declines in grassland breeding birds. Where bird conservation is an objective, small grassland reserves may therefore be inappropriate sites for bison reintroduction. To maximize conservation benefits to birds, land managers should prioritize protecting grassland birds from disturbance during the bird breeding season.

Synopsis: Biodiversity–disease studies typically focus on how changes in community composition (e.g., species richness, abundance, and functional or phylogenetic metrics of biodiversity) affect disease risk. In doing so, these studies tend to overlook intraspecific variation in the organisms comprising the community. Yet, intraspecific variation, which occurs to varying degrees both within and between communities, could also modulate biodiversity–disease relationships. One important driver of intraspecific variation is the microbiome. By directly and indirectly influencing health and susceptibility to infection and disease, microbiomes are integral to organismal functioning. Thus, the microbiome plays a crucial role in host quality. We define host quality as an integration of host traits related to parasite exposure, establishment, growth, and infectivity, which ultimately shape parasite fitness. The microbiome can impact host quality via a variety of mechanisms including host size and developmental stage, immune function, reproduction, nutrient acquisition, and behavior. However, the potential for such microbiome-driven changes in host quality to trigger cascading effects on community-level processes, specifically by altering parasite transmission dynamics and community competence, has not been well explored. Here, we examine and illustrate a pathway by which the microbiome may influence variation in organismal biology (i.e., host quality) of hosts in communities. Furthermore, we consider how major anthropogenic drivers of microbiome shifts, such as climate change, pollution, land use change, and domestication, might influence this pathway and thereby alter outcomes. Future studies bridging microbiome and disease ecology research will provide opportunities to unify these concepts across scales and between the plant and animal domains. To date, most microbiome research has focused on humans, crops, and laboratory animals. However, to better understand the potential for knock-on ecological effects of microbiomes, more attention must be paid to the microbiomes of wild plants and animals. Ultimately, more experimental and theoretical data are needed to clarify how the microbiome impacts host quality and disease dynamics, as well as how anthropogenic factors continuously reshape these relationships.