Context-dependent effects of nestling growth trajectories on recruitment probability in the collared flycatcher

Nestling growth is known as an important determinant of fitness in altricial birds, but its evolutionary potential has been debated, and little is known about detailed patterns of current selection on growth. Relationships are often reported between nestling growth and attributes of nestlings and parents, but the interpretation of these depends on the advantages a given growth difference confers to the chicks. Increased growth may have positive, negative or context-dependent effects on offspring fitness, but these effects are largely unknown in natural populations. We measured growth trajectories of body mass in fostered broods of collared flycatchers (Ficedula albicollis) in 3 years of contrasting food conditions. We examined the growth of young and their recruitment probability to the breeding population in relation to year quality, hatching rank, sex, paternal age and paternal attractiveness. We also looked at the interactive effects of growth and intrinsic offspring attributes on recruitment probability. The predictors of nestling growth and those of recruitment did not agree. Moreover, the recruitment consequences of a given nestling growth rate were significantly influenced by nestling rank and paternal ornamentation. Differential recruitment effects of nestling growth in relation to parental traits and nestling attributes suggest that using growth as a generally applicable measure of nestling quality may not be justified. These findings also have implications for morphological evolution and the indicator value of sexual signals.