Structure, regulation, and evolution of molt in passerines

Resumen

Molt is the process of plumage renewal by which birds maintain and adjust its functionality throughout their lifecycle. Multiple elements have been tackled in bird molt research (timing, duration, sequence, intensity, extent, feather growth rate, and plumage quality), but major gaps still exist on molt regulation, and especially on molt evolution. This thesis focuses on one molt element extensively recorded since mid-20th century but seldom studied as an individual trait: the set of feathers replaced after a given molt episode by one individual (here referred to as final molt phenotype). This is surprising because feathers differ in their function (e.g. signaling, thermoregulation, contribution to different flight functions, durability), costs of production, and morphology (e.g. exposure, mass, shape), all of which can be targeted by natural selection. Therefore, the final molt phenotype should be under strong selective pressures, suggesting that its regulation has been shaped during evolution to optimize plumage performance throughout the bird’s lifecycle. This thesis explores the potential of analyzing final molt phenotypes as is (instead of being analyzed partially or indirectly) to uncover underlying mechanisms of molt regulation and to provide insights on the evolution of molt in passerine birds. Following are the main findings presented in this thesis. Final molt phenotypes differed between the post-juvenile and the pre-breeding molts along the passerine phylogeny. A nested organization of final molt phenotypes suggested a rank of feather molt importance as underlying rule of molt. However,deviations from perfect nestedness were largely associated with the pre-breeding molt. Shared ancestry explained a large portion of final molt phenotype variation, likely due to constraints associated to plumage morphology, which is highly conserved in passerines. Phylogenetic analyses confirmed the phylogenetic independence of the pre-breeding molt and the strong phylogenetic signal of the post-juvenile molt. Further, they showed the overlooked relevance of environmental factors on the evolution of passerine molt, although their effect varied among taxonomic groups and molt episodes, thus highlighting the flexibility and adaptiveness of molt. Findings exposed in this thesis confirm the relevance of the final molt phenotype as a promising element to advance in our understanding of bird molt.