Active Warming of Open Cup Nests Reveals Contrasting Effects of Temperature on Development and Size in Wild Songbirds

Abstract

Temperatures during development can affect bird physiology and morphology. As the climate changes, understanding these impacts across diverse sets of species in natural systems is increasingly important. Previous studies of the relationship between temperature and bird development have largely relied on field experiments using nest boxes, or have warmed eggs ex situ in an incubator. Fewer studies have tried to manipulate the temperature of open cup nests, largely limiting experiments to cavity-nesting species that are willing to nest in nest boxes. We developed an active warming device that consistently and accurately warms nests to a specified level above the ambient temperature and maintains this elevated temperature throughout the incubation period. We then used this method to warm the nests of three North American passerines—gray catbirds (Dumetella carolinensis), American robins (Turdus migratorius), and northern cardinals (Cardinalis cardinalis)—2C above ambient temperature throughout the incubation and nestling period, and to quantify the impacts of warming on developmental rate and size. We find contrasting effects of the experimental warming across species, with warming resulting in faster development and smaller size in catbirds and slower development and larger size in robins. These contrasting responses may be explained by differences in the relationships between the ambient temperature at the study site and the optimum developmental temperature for each species. Our results show that large scale comparative analyses of warming-mediated developmental plasticity are needed in order to better-understand observed impacts of global warming on birds.