Nest mass in forest tits (Paridae) increases with elevation and decreasing body mass, promoting reproductive success

Based on the concept of the extended phenotype, bird nest characteristics can serve as indicators for adaptations to changing environmental conditions. We examined how the nest mass of three cavity-nesting tit species (Paridae) varied across 22 mixed forests in Germany in response to elevation, canopy openness, and species body mass. We predicted that nest mass should increase with elevation and canopy openness, due to thermoregulation being more demanding in colder or warmer climatic conditions, and decrease with body mass, as larger species have greater thermoregulatory capabilities. To test these predictions and to assess the consequences of nest mass variation for reproductive success, we recorded nest mass, clutch size, and pre-fledging brood size in 576 standardized nest boxes. Nest boxes were installed along an elevational gradient of approximately 1000 m a.s.l., either in forest gaps with fluctuating microclimatic conditions or in closed forests with buffered microclimates. We found that nest mass increased by ~ 60% along the elevational gradient, but the effect of canopy openness on nest mass was not significant, while nest mass decreased along the ranked species from the smallest Periparus ater to the medium-sized Cyanistes caeruleus and the largest Parus major. Structural equation modeling revealed that heavier nests were associated with larger clutch sizes, which in turn resulted in larger pre-fledging brood sizes. Altogether, our results suggest that forest tits adjust nest construction in response to macroclimatic conditions, thereby compensating for the thermoregulatory challenges posed at higher elevations and their small body size. This strategy may be critical for maintaining reproductive success in changing environments.