Quantifying environment-morphology relationships is important not only for understanding the fundamental processes driving phenotypic diversity within and among species, but also for predicting how species will respond to ongoing global change. Despite a clear set of expectations motivated by ecological theory, broad evidence in support of generalizable effects of abiotic conditions on spatial and temporal intraspecific morphological variation has been limited. Using standardized data from over 250,000 captures of 105 landbird species, we assessed intraspecific shifts in the morphology of adult male birds since 1989 while simultaneously measuring spatial morphological gradients across the North American continent. We found strong spatial and temporal trends in average body size, with warmer temperatures associated with smaller body sizes both at more equatorial latitudes and in more recent years. The magnitude of these thermal effects varied both across and within species, with results suggesting it is the warmest, rather than the coldest, temperatures driving both spatial and temporal trends. Stronger responses to spatial – rather than temporal – variation in temperature suggests morphological change may not be keeping up with the pace of climate change. Additionally, as elevation increases, we found that body size declines as relative wing length increases, likely due to the benefits that longer wings confer for flight in thin air environments. Our results provide support for both existing and new large-scale ecomorphological ‘rules’ and highlight how the response of functional tradeoffs to abiotic variation drives morphological change.

Bird morphology data were collected as part of the Monitoring Avian Productivity and Survivorship (MAPS) program, a collaborative long-term bird-banding project operating across North America. Data were obtained from 1124 banding stations (Fig. 1), each consisting of 6–20 mist nets, over the period 1989–2018 (though most stations operated during only a subset of this period). Banding stations were operated 6–12 times per year, from May 1 to August 28, encompassing the breeding season for most birds in North America. Records consist of only male birds classified as After Hatch Year (captured at least one breeding season after the hatch year of the bird). All records with body mass or wing length measurements that were more than five median absolute deviations [MAD] away from the median were excluded, as these likely represented measurement or data entry errors. If an individual was captured more than once in a season, only measurements taken during the initial capture were included. Only species for which data were available for at least 375 captures (post data filtering) were included.

Data from the Monitoring Avian Productivity and Survivorship (MAPS) program are curated and managed by The Institute for Bird Populations and were queried from the MAPS database on 2019-10-16.