Clarification of molt strategies in three Empidonax flycatchers

We studied plumages and molt patterns of Yellow-bellied (Empidonax flaviventris), Least (E. minimus), and Alder (E. alnorum) flycatchers, primarily on the breeding grounds but with additional data from wintering grounds and a migration stopover site, in order to clarify our knowledge of molt strategies in these species. We determined that these 3 species have a similar overall molt strategy: (1) a partial-to-incomplete preformative molt begun on the breeding grounds, suspended over migration, and completed on the wintering grounds; (2) a limited-to-partial first prealternate molt on the wintering grounds that can overlap the final stages of the preformative molt; (3) a complete definitive prebasic molt begun on summer grounds and completed on wintering grounds; and (4) a limited-to-partial definitive prealternate molt that may overlap with the final stages of the prebasic molt. Our study clarifies previous terminologies and allows these molts to be interpreted in an evolutionary framework. We suggest that observed differences in molt extents among species may be linked to differences in wintering latitude and habitat. The broadleaf forest preferred by Yellow-bellied Flycatcher likely retains insect biomass later into the winter season than the scrub habitat preferred by Least Flycatcher, and this may promote more extensive preformative feather replacement in that species. The much more extensive preformative molt of Alder Flycatcher may be due to its wintering farther south, where there is greater annual solar exposure, perhaps resulting in higher rates of feather degradation. Our data also suggest that the extent of prealternate molt in these species may vary with proximate factors on the winter grounds affecting individuals, rather than body condition at the beginning of winter.