Evolution of winter molting strategies in European and North American migratory passerines

Abstract

Molt is critical for birds as it replaces damaged feathers and worn plumage, enhancing flight performance, thermoregulation, and communication. In passerines, molt generally occurs on the breeding grounds during the postbreeding period once a year. However, some species of migrant passerines that breed in the Nearctic and Western Palearctic regions have evolved different molting strategies that involve molting on the overwintering grounds. Some species forego molt on the breeding grounds and instead complete their prebasic molt on the overwintering grounds. Other species molt some or all feathers a second time (prealternate molt) during the overwintering period. Using phylogenetic analyses, we explored the potential drivers of the evolution of winter molts in Nearctic and Western Palearctic breeding passerines. Our results indicate an association between longer photoperiods and the presence of prebasic and prealternate molts on the overwintering grounds for both Nearctic and Western Palearctic species. We also found a relationship between prealternate molt and generalist and water habitats for Western Palearctic species. Finally, the complete prealternate molt in Western Palearctic passerines was linked to longer days on the overwintering grounds and longer migration distance. Longer days may favor the evolution of winter prebasic molt by increasing the time window when birds can absorb essential nutrients for molt. Alternatively, for birds undertaking a prealternate molt at the end of the overwintering period, longer days may increase exposure to feather-degrading ultra-violet radiation, necessitating the replacement of feathers. Our study underlines the importance of the overwintering grounds in the critical process of molt for many passerines that breed in the Nearctic and Western Palearctic regions.

Keywords: Nearctic; Passeriformes; Western Palearctic; molt; phylogenetic analysis.

FIGURE 1

Phylogeny of the 116 species and subspecies of Western Palearctic passerines. The color of the branches represents the average photoperiod where red is longer photoperiod. The red dots indicate the species doing a prealternate molt while the black dots indicate a prebasic molt on the overwintering grounds. We labeled the passerine families with more than 5 members. See supporting information for phylogeny with tips labeled with species name

FIGURE 2

Phylogeny of the 183 species and subspecies of Nearctic passerines. The color of the branches represents the average photoperiod where red is longer photoperiod. The red dots indicate the species doing a prealternate molt while the black dots indicate a prebasic molt on the overwintering grounds. We labeled the passerine families with more than 5 members. See supporting information for phylogeny with tips labeled with species name

FIGURE 3

Boxplots and bar graph of the significant predictors (parameter estimates did not overlap 0) associated winter prebasic (a, b), prealternate (c, d, e), and complete prealternate molts (f, g). Plots to the left (a, c) correspond to the results for the Nearctic passerines, and plots to the right (b, d, e, g, f) are the results for the Western Palearctic species