Climate Warming is Killing Nestling Birds
Tree Swallows ©Autumn Turner
Earlier this year National Audubon along with a coalition of conservation organizations released a study (the 2025 U.S. State of the Birds report) that found recent large declines in bird numbers across the country and across many species. The continued conversion of habitat, both to urbanization and to farming, are the largest likely causes. A recent study out of UC Davis found that, especially in farmland, the nesting success of many common and widespread species is diminished by the more extreme heat waves associated with our warming climate. And as the climate warms further, the effect will only grow.
The UC Davis authors used a Cornell University database – Project NestWatch – of more than 150,000 nesting attempts to explore how extreme heat waves affect fledging success in various habitats in this century, compared to the late twentieth century. Species that build cup nests were more vulnerable than those using cavity nests. The main cause of increased failure rates seems to be exposure to sun and heat by nestlings, who have no way to cool themselves at their early developmental stage. Accordingly, nests in forests, insulated and shaded by tree canopies and other vegetation, showed no harm from extreme heat, and even a small increase in fledging success.
The study focused on common species that are habitat generalists, occurring across two or more land-cover types, including forest, grassland, farmland and urban settings. Virtually all of the cavity nests were in nest boxes. In our area, they were mainly tree swallows and western bluebirds, with small numbers of house wrens and house sparrows, and a few ash-throated flycatchers.
The authors suggested that the species in the study could be better protected from climate warming by the planting of scattered trees in agricultural and developed areas, to provide shade for nests. Also, shielding and insulating nest boxes can help birds cope with heat waves.
The article is “Agriculture and hot temperatures interactively erode the nest success of habitat generalist birds across the United States,” found here in the journal “Science.” The National Audubon report is at www.stateofthebirds.org. The citizen-science monitoring effort of Project NestWatch is at nestwatch.org. Have a look at them.
–Michael Perrone, Conservation Chair