Nest Construction Costs Affect Nestling Growth: A Field Experiment in a Cavity-Nesting Passerine

Nest construction effort is seldom taken into account in avian life history studies, although activity duration and the amount of nest material collected may affect the condition of parent birds and their subsequent capacity to care for dependent offspring. In the present work, we reduced the effort needed to construct the nest in a cavity nester, the Pied Flycatcher Ficedula hypoleuca, where females are the main builders. To this end, we placed stored complete nests in certain nest-boxes as soon as nest building was initiated, whereas stored complete nests were placed in the control nests-boxes as soon as nest building had ceased. Thus, no nest-box contained the original nest. Pairs collected more nest material and for longer periods in the control nests, although the experimental pairs still added some nest material after the manipulation. Nest-boxes were filmed for 1 to 3 h during incubation and on days 4 and 11 of the nestlings' life. Female attendance during incubation was negatively affected by nest mass, this parameter explaining almost half the variation in attendance, which suggests an effect of nest mass on incubation efficiency. The female provisioning effort on day 4 was significantly higher in the experimental nests, whereas other parental care variables were affected by date, weather conditions and brood size or brood mass. Mean tarsus length at fledging was significantly longer in the experimental nests. The savings by females in nest construction effort were expressed in higher provisioning at early nestling stages and in improved nestling growth.