Aims: The main aim was to study seasonal variation in the mass and dimensions of nests of Tawny Pipits Anthus campestris.
Location: The study was conducted in Layna (Soria).
Methods: Seasonal variation in the mass and dimensions of 88 nests of Tawny Pipits was studied over five years. The data were evaluated against five hypotheses (involving predation, clutch size, incubatory efficiency, thermoregulation and sexual selection).
Results: The probability of predation was unrelated to outer nest diameter nor was there any relationship between internal diameter and the presence of unhatched eggs. Wall thickness and nest mass declined as the season progressed but there was no relationship between these variables and the mean environmental temperature during either seven or four days preceding the start of nest building. Clutch size did not correlate with outer nest diameter nor with nest mass but was positively related to nest volume.
Conclusions: It is concluded that the clutch size hypothesis, in which females are capable of adjusting nest size to clutch size, best explains the observed variation. This is the first demonstration of the applicability of this hypothesis to a ground-nesting, open-cup nest building species.
