Reduced avian body condition due to global warming has little reproductive or population consequences

Climate change has strong effects on traits such as phenology and
physiology. Studies typically assume that climate-induced trait changes
will have consequences for population dynamics, but explicit tests are
rare. Body condition reflects energy storage and may directly affect how
much can be invested in reproduction and survival. However, the causal
pathway by which decreased body condition impacts population dynamics has
never been quantified across multiple populations and species. Therefore,
we lack a general understanding of the consequences of changes in
condition for variables more relevant for conservation, such as population
size. Using structural equation modeling, we investigate how
temperature-induced changes in body condition affect reproduction, and the
subsequent impact on population growth rates of 19 bird species across 80
Dutch sites over a 21-year period. Warmer temperatures were associated
with decreased body condition, which led to both decreased and increased
reproduction at different sites, cancelling out any overall effect. The
indirect effect of temperature on population growth (via body condition
and reproduction) only explained within-species variation in the total
effects of temperature on population growth. Instead, the direct effect of
temperature on population growth (unrelated to condition and reproduction)
was the most important pathway underlying the total effects of temperature
on population growth, suggesting that unknown variables are mediating this
effect. About half of the species are expected to increase under global
warming, but this variation was not associated with any species
characteristic. Overall, body condition responses to global warming are
common, but their consequences on reproduction and subsequently population
growth contribute relatively little to the total temperature impacts on
population dynamics. Given that warming temperatures have strong effects
on population dynamics, understanding the pathways via which temperature
impacts population dynamics will be crucial for our ability to predict
climate change effects in the future and improve conservation efforts.