Abiotic conditions shape spatial and temporal morphological variation in North American birds

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.