Body size decline with climate warming? Maybe not in temperate songbirds...

Submitted by editor on 23 April 2018.

By Nicolas Dubos

Recent observation of decreases in the body size of endothermic animals lead to the proposal that body size decline is the third universal response to climate warming (after distribution range and phenological shifts). However, the mechanisms underlying body size change through time are still poorly understood. Two hypotheses may explain how climate change can affect body size: (1) direct effects of temperature change on thermoregulation (with a selective advantage for the smallest in the warmest conditions) and (2) indirect effects of climatic conditions on food availability (with negative effects on body growth). Since then, an increasing number of studies showed that not all species exhibited body size declines, and some species could even become larger. In addition, the body size of a given species may vary differently between populations, which calls the need to assess body size variation on the basis of a large number of sites.

We analysed interannual variation in the body size of 41 songbird species across France (257 sites) between 2000 and 2014. We first assessed whether body size variation was synchronous between species and between sites. We then tested whether body size variation was better explained by spring temperature anomaly or by local variation in net primary production.

We found that body size variation was largely asynchronous between species, and between sites, suggesting that avian size was more dependent on local drivers than global ones.
Juvenile body size was best explained by net primary production, suggesting a higher importance of food availability than thermoregulatory mechanisms in this temperate system. Contrarily to expectations, juvenile birds were slightly larger in hot years. We did not detect any environmental effect on adults, which supports that the effects of net primary production and temperature affected body size through effects on body growth (and not size-dependent mortality). Overall, these results suggest that climate warming would affect positively the body size of birds in temperate systems.