Sexual selection, feather wear, and time constraints on the pre-basic molt explain the acquisition of the pre-alternate molt in European passerines

Abstract

Avian feathers need to be replaced periodically to fulfill their functions, with natural, social, and sexual selection presumably driving the evolution of molting strategies. In temperate birds, a common pattern is to molt feathers immediately after the breeding season, the pre-basic molt. However, some species undergo another molt in winter-spring, the pre-alternate molt. Using a sample of 188 European passerine species, Bayesian phylogenetic mixed models, and correlated evolution analyses, we tested whether the occurrence of the pre-alternate molt was positively associated with proxies for sexual selection (sexual selection hypothesis) and nonsexual social selection (social selection hypothesis), and with factors related to feather wear (feather wear hypothesis) and time constraints on the pre-basic molt (time constraints hypothesis). We found that the pre-alternate molt was more frequent in migratory and less gregarious species inhabiting open/xeric habitats and feeding on the wing, and marginally more frequent in species with strong sexual selection and those showing a winter territorial behavior. Moreover, an increase in migratory behavior and sexual selection intensity preceded the acquisition of the pre-alternate molt. These results provide support for the feather wear hypothesis, partial support for the sexual selection and time constraints hypotheses, and no support for the social selection hypothesis.

Keywords: birds; feather wear; pre‐alternate molt; sexual selection; social selection; time constraints.

FIGURE 1

Sexual selection scores for 188 European passerine species with and without pre‐alternate molt. Every dot represents a species. Mean ± 95% CI values are denoted with vertical lines. Feather molt in November or December was considered pre‐alternate.

FIGURE 2

Percentage of European passerine species with pre‐alternate molt in relation to (a) migratory behavior (1 = resident; 2 = resident to short‐distance, resident to partially migratory, or resident to eruptive; 3 = altitudinal, short‐distance, partial migrant, or resident to migratory; 4 = migratory to short‐distance, or migratory to resident; 5 = migratory), (b) type of habitat (1 = desert; 2 = savannah, steppe, cliffs, or high mountain; 3 = scrub, tundra, or grassland; 4 = riparian area, groves, or wetland; 5 = open woodland; 6 = forest), (c) aerial foraging (0 = unimportant; 1 = important; 2 = the most important foraging technique), and (d) winter gregariousness (1 = territorial; 2 = nongregarious; 3 = moderately gregarious; 4 = gregarious). Feather molt in November or December was considered pre‐alternate.

FIGURE 3

Flow diagrams showing correlated evolution between the occurrence of the pre‐alternate molt and (a) the strength of sexual selection, (b) migratory behavior, (c) aerial foraging, and (d) winter territoriality in European passerine species. Feather molt in November or December was considered pre‐alternate. For information on dichotomization of variables, see main text. Bayes factors (BF (95% CI)) indicate the probability of correlated evolution between the two traits. Transitions of one dichotomous character holding the state of the other constant are represented by arrows and named as q(ij). The associated mean rate parameter for each transition is shown. Arrow width is proportional to the magnitude of the mean rate parameter within each diagram. For two dichotomous characters, there are four possibilities of co‐occurrence: 00 (1), 01 (2), 10 (3), and 11 (4). The number in parentheses is the identity of each combination of characters (1, 2, 3, or 4), the first number within each pair (0 or 1) represents the state of one character, and the second number the state of the other character. The dependent model considers eight transition rate parameters that describe the probability of all possible single changes among the four states: q12, q21, q13, q31, q24, q42, q34, and q43. For example, q12 refers to the transition from 00 to 01 and q43 to the transition from 11 to 10. Posterior distribution and percentage of visits assigned as zero (Z‐value) for every rate parameter are shown in Figures S1–S4.

FIGURE 4

(a) Sexual dichromatism scores for 83 European passerine species with partial or complete pre‐alternate molt. Every dot represents a species. Mean ± 95% CI values are denoted with vertical lines. (b) Percentage of European passerine species with complete (as opposed to partial) pre‐alternate molt in relation to migratory behavior (2 = resident to short‐distance, resident to partially migratory, or resident to eruptive; 3 = altitudinal, short‐distance, partial migrant, or resident to migratory; 4 = migratory to short‐distance, or migratory to resident; 5 = migratory). In both figures, feather molt in November or December was considered pre‐alternate.

FIGURE 5

Flow diagrams showing correlated evolution between the extent of the pre‐alternate molt (partial/complete) and (a) sexual plumage dichromatism or (b) migratory behavior in European passerine species. Feather molt in November or December was considered pre‐alternate. For information on dichotomization of variables, see main text. Posterior distribution and percentage of visits assigned as zero (Z‐value) for every rate parameter are shown in Figures S5 and S6 and remaining information as in Figure 3.