Feather wear and structural needs
Photodegradation is a primary source of feather structure atrophy in
feathers (Ito et al. 2017, Pearlstein et al. 2014). The main variables
that predict extent of prealternate molt are migration distance, day
length, and foraging stratum. Migration distance likely affects feather
degradation through increased overall day length (Fig. 1a).
Long-distance migrants experience longer days overall because they
experience long summer days in the temperate zone, but escape short
winter days. For example, the longest-distance migrant in our dataset,
and one of the most seasonally dichromatic species with one of the most
extensive prealternate molts, the Blackpoll Warbler (Setophaga
striata) experiences an average of 1.7 more hours of daylight each day,
or 621 more hours of ultraviolet exposure each year, when compared to
the species exposed to the least amount of ultraviolet radiation, the
Masked Yellowthroat (Geothlypis aequinoctialis ), which also shows
no seasonal dichromatism and no prealternate molt. Additionally, many
warbler species exhibit prealternate molts that do not result in
seasonal dichromatism, this phenomenon may seem paradoxical from the
standpoint of hypotheses focused on coloration as the evolutionary
catalyst for the prealternate molt, but makes sense within the context
of the feather wear hypothesis.
Evidence from other taxa outside the New World Warblers provides
additional context for the relationship between prealternate molt and
seasonal dichromatism. The most extensive prealternate molts in birds
occur in three species of long-distance migrants that breed, winter, and
migrate in open, solar-exposed environments: Franklin’s Gull
(Leucophaeus pipixcan ; Howell 2010) , Bobolink
(Dolichonyx oryzivorus ; Renfew et al. 2011) and Willow
Warbler (Phylloscopus trochilus; Underhill et al. 1992) .
Bobolink shows seasonal change in feather color, but Willow Warbler does
not, and Franklin’s Gull only shows a partial plumage color change. In
these species, anecdotally, migration distance and habitat better
predict prealternate molt than color change. The Willow Warbler is an
extreme example: this species completely replaces all feathers twice a
year, but the basic and alternate plumages are indistinguishable.
Further research into this phenomenon should expand beyond the new world
warblers to other groups of birds, as well as attempt to measure
relative feather degradation rates in association with life history,
habitat, and environment in birds, and study groups with more variable
social systems. Other resident species of birds with strong variable
selection on feather color, such as Ptarmigans (Beltran et al. 2018) may
indeed have molts that evolve solely for variables needs of feather
color. Overall, our results demonstrate the importance of molt
strategies in the functional diversification of feathers and illuminates
the value of considering interactions between different functional
requirements for birds in the evolution of feather function.