US Bird Nest Success Threatened by Heat, Agriculture

Maximum temperature extremes reduce the nesting success of birds across the United States by nearly 50% in agricultural landscapes but not forests, according to a new study based on more than 20 years of citizen-science nest-monitoring data. The findings show that future warming may exacerbate the negative effects of habitat conversion on bird fitness, especially among species of conservation concern in human-dominated landscapes. Habitat conversion and climate change are fundamental drivers of biodiversity loss worldwide. However, the effects of each are often analyzed in isolation, despite the understanding that the fate of many species will ultimately depend on how these dual stresses interact. By removing insulating tree canopies or other complex microhabitats, many forms of habitat conversion (e.g., agricultural expansion) can expose species to more pronounced climate extremes. For example, agricultural landscapes regularly reach temperatures more than 10 degrees Celsius higher than in nearby natural habitats. As a result, agricultural and other human-dominated landscapes may become less hospitable for species, including habitat generalist birds, who are particularly sensitive to temperature extremes. Despite this, the interactive effects of climate and land-use change aren't fully understood. Using data from Project NestWatch – a citizen-science nest-monitoring program – Katherine Lauck and colleagues examined how extreme heat influences birds' fledgling success in different land-use types within the continental U.S. Lauck et al. analyzed 152,863 nesting attempts by 58 bird species across 23 years at sites in forests, grasslands, agricultural settings, and developed areas. They compared nest success to temperature extremes recorded for each region, during each attempt. They report that the probability of a nest successfully fledging at least one young bird declined by 46% during extreme warm temperature anomalies in agricultural settings. In contrast, extreme heat increased the reproductive success of birds in forested areas by 14% during extreme warm temperatures, perhaps due to cooling from the forest canopy or a reduced need for thermoregulatory care. Importantly, Lauck et al. note, however, that this positive relationship does not suggest that climate change is an overall benefit to forest birds, overall. The findings also show that birds that build exposed cup nests and species of higher conservation concern, like the oak titmouse or chestnut-backed chickadee, for example, were particularly vulnerable to heat extremes in agricultural landscapes.