A Trade-Off between Reproduction and Feather Growth in the Barn Swallow (<i>Hirundo rustica</i>)

<div><p>Physiological trade-offs mediated by limiting energy, resources or time constrain the simultaneous expression of major functions and can lead to the evolution of temporal separation between demanding activities. In birds, plumage renewal is a demanding activity, which accomplishes fundamental functions, such as allowing thermal insulation, aerodynamics and socio-sexual signaling. Feather renewal is a very expensive and disabling process, and molt is often partitioned from breeding and migration. However, trade-offs between feather renewal and breeding have been only sparsely studied. In barn swallows (<i>Hirundo rustica</i>) breeding in Italy and undergoing molt during wintering in sub-Saharan Africa, we studied this trade-off by removing a tail feather from a large sample of individuals and analyzing growth bar width, reflecting feather growth rate, and length of the growing replacement feather in relation to the stage in the breeding cycle at removal and clutch size. Growth bar width of females and length of the growing replacement feather of both sexes were smaller when the original feather had been removed after clutch initiation. Importantly, in females both growth bar width and replacement feather length were negatively predicted by clutch size, and more strongly so for large clutches and when feather removal occurred immediately after clutch completion. Hence, we found strong, coherent evidence for a trade-off between reproduction, and laying effort in particular, and the ability to generate new feathers. These results support the hypothesis that the derived condition of molting during wintering in long-distance migrants is maintained by the costs of overlapping breeding and molt.</p></div

Length of the growing R₄ in relation to time since feather removal.

<p>Length of the growing R₄ relative to the length of the original R₄ in relation to time since removal of the original R₄. The continuous lines represent the Gompertz functions fitted to the data.</p

Mean (±SE) residual GBW of females in relation to clutch size.

<p>Mean (±SE) residuals of GBW of females from a regression on date and second-order polynomial terms of breeding stage in relation to clutch size. Numbers above bars indicate sample size.</p

Strength of the association between growing R₄ length and clutch size during the breeding cycle.

<p>Contouring of <i>t</i>-values associated to the effect of clutch size on LengthRe (see Methods) of females obtained in multiple regression models also including breeding stage and date at original feather removal (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0096428#pone-0096428-t005" target="_blank">Table 5</a>). <i>t</i>-values were obtained from models including ranges of breeding stages centered at different median values and differing in width (see SI.5). Thus, for example, the yellow dot indicates a model where breeding stages spanning from breeding stage 10 and 21, and centered on breeding stage 15.5 were considered. Red isopletes indicate breeding stage ranges where unsigned <i>t</i>-values were significant. The arrow indicates the range where the unsigned <i>t</i>-value was largest. The apex of the triangle denotes the t-value of the models including the entire range of breeding stages (i.e. all data points) (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0096428#pone-0096428-t005" target="_blank">Table 5</a>).</p

Length of the growing R₄ in relation to time breeding stage at feather removal.

<p>Variation of LengthRe (see Methods) in relation to breeding stage for the replacement feathers measured less than 40 days after removal of the original feather. The linear function fitted to females (F), the second-order polynomial function fitted to males (M), and the second-order polynomial function fitted to the two sexes pooled (F+M) are shown.</p

Length of the growing R₄ in relation to time since feather removal.

<p>Length of the growing R₄ relative to the length of the original R₄ in relation to time since removal of the original R₄. The continuous lines represent the Gompertz functions fitted to the data.</p

Length of the growing replacement feather in relation to clutch size.

<p>Linear model of the residuals of replacement R₄ length relative to original R₄ length on time since original feather removal (LengthRe in the text), on clutch size, breeding stage and date at original feather removal. For males, the second-order polynomial term of breeding stage is also included (see Results).</p

GBW of the replacement feather in relation to clutch size.

<p>Linear mixed model of GBW of replacement R₄ on clutch size, and breeding stage and date at original feather removal. For females, the second-order polynomial term of breeding stage is also included (see Results).</p

Length of the growing replacement feather in relation to time since removal.

<p>Linear models of the residuals of replacement R₄ length relative to original R₄ length on time since original feather removal (LengthRe in the text), on date and breeding stage for either sex separately or for the two sexes pooled. The statistics for the intercept and the first-order term for females are obtained from a model excluding the non-significant second-order polynomial term (in parentheses). Only re-growing feathers measured less than 40 days after plucking are considered (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0096428#pone-0096428-g002" target="_blank">Fig. 2</a>).</p

GBW of the replacement feather in relation to stage in the breeding cycle.

<p>Linear mixed models of GBW of the replacement R₄ on breeding stage in the pre-laying (breeding stage ≤ 0) or the post-clutch initiation period (breeding stage>0), and linear mixed model with second order polynomial terms of breeding stage testing for a difference in the relationship between GBW and breeding stage in either sex.</p