The role of development, parental behavior, and nestmate competition in fledging of nestling Tree Swallows

Fledging (i.e. leaving the nest) in altricial birds is a major step toward independence. The timing of this important event may be influenced by nestl ing development, parental behavior, and sibling interactions. In this study we examine the effect of these factors on fledging in Tree Swallows (Tachy cineta bicolor). Wing length explained a significant proportion of the vari ance in fledging age across broods. Similarly, within broods, longer-winged nestlings fledged before shorter-winged nestlings. Feeding rate per nestli ng did not decrease in the period leading up to fledging, as might be expec ted if parents stimulated fledging by decreasing their feeding rate. Parent al activity near the nest varied in that the frequency of parental passes a nd hovers within 1 m of the nest opening increased in the period before fle dging. Brood size also explained a significant proportion of the variance i n fledging age independent of wing length, although the direction of the re lationship was not consistent across years. Finally, longer-winged nestling s spent more time in the nest opening and initiated fledging of the brood m ore often than did shorter-winged nestlings. Overall, our study suggests th at nestling development is the most important determinant of fledging age i n this species. Once a critical wing length has been reached, sibling inter actions, and possibly parental behavior, may influence the timing of fledgi ng.