Effects of temperature and time constraints on the seasonal variation in nest morphology of the Thorn-tailed Rayadito (Aphrastura spinicauda)

Environmental adjustment is the most invoked explanation for intra-seasonal variation in bird nest morphology. However, time constraints may also be important, coming as a trade-off between the costs of nest building and the requirement to coincide a breeding attempt with maximal food supply. We describe the seasonal variation in nest morphology of the Thorn-tailed Rayadito (Aphrastura spinicauda) in a sub-Antarctic population in southern Chile, and investigate its relation with the seasonal fluctuation of ambient temperature (an ‘environmental adjustment’ hypothesis) and time constraints (an ‘optimal time frame’ hypothesis). As the breeding season progressed, rayaditos spent fewer days building their nests, built smaller nests, and used less animal-derived insulating material. After statistically removing the effects of daily temperatures on nest building periods and nest morphological measurements, we observed no seasonal trend in the amount of insulating material used, supporting an ‘environmental adjustment’ explanation. However, the nest building periods, nest depth, and nest dry weight still showed a seasonal trend, favouring an ‘optimal time frame’ hypothesis. Our study shows that both temperature fluctuations and time constraints can affect different components of the nesting ecology of birds, and that nest morphology is the consequence of distinct non-mutually exclusive forces.