Abstract
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 (Hirundo rustica) 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.
Highlights
The simultaneous expression of major organismal functions is often constrained by limiting energy, material resources or time [1] [2]
Because of its energetic costs, feather biosynthesis may to have to be traded against allocation to reproduction, which is highly demanding [14], [15], [16], [17], [18], [19]
The width of the growth bars on the replacement feather decreased the later removal of the original feather was relative to the day of first clutch initiation, whereas this was not the case for males
Summary
The simultaneous expression of major organismal functions is often constrained by limiting energy, material resources or time [1] [2]. The diverse molt strategies that birds have evolved [12], [20], [21] may reflect selection for optimization of annual routines by reducing the impact of any trade-offs arising from overlap of molt with reproduction and migration [5], [22], [23], [24], [25]. In species breeding in temperate boreal latitudes, summer molt appears to be the ancestral state, while winter molt is a derived state that has evolved in species that winter south of the Sahara [29], [30] Winter molt in these species possibly results from tightness of annual routines, whereby early onset of autumn migration prevents post-breeding molt, and/or from conditions in the winter quarters favoring this alternative molt strategy In addition to periodic partial or complete molt, birds can replace feathers that may be accidentally lost [32], [33], suggesting strong selection for maintenance of aerodynamic and insulatory plumage integrity
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