The structure and function of nests of Long-Tailed Tits Aegithalos caudatus

1. The aim of this study was to investigate the structure and thermoregulatory function of nests of the Long-Tailed Tit, Aegithalos caudatus. 2. The feather lining of Long-Tailed Tit nests represents a major portion (41%) of the total nest mass. 3. The mass of feathers varied among nests and declined through the breeding season, but there was no seasonal loss of nest insulation quality because of increasing ambient temperatures. 4. In an experiment to investigate the seasonal decline in the feather mass of nests, feathers were added to nests at an early stage of the lining phase of nest construction. Nest structure and insulating properties were then examined following nest completion. 5. The total mass of feathers in treatment and control nests did not differ significantly and there was no significant difference in their nest insulation quality. 6. Our results demonstrate that Long-Tailed Tits adjust their nest-building behaviour according to the nest's thermal environment. Moreover, nest structure appears to be adjusted to prevailing environmental conditions rather than being a function of feather availability or time constraints.