Conclusions:
Based on the observation that prealternate molt often involves feathers
that appear to be identical between basic and alternate plumage, Pyle
and Kayhart (2010) proposed that prealternate molt may evolve to replace
sun-exposed feathers, then later be co-opted for seasonal dichromatism.
We examined this hypothesis across warblers and found that it better
explains patterns of evolution of prealternate molt than color change
alone. Here, we present evidence that selection on coloration and
structure interact in complex ways to influence the evolution of molts
and plumages in warblers. Namely, we find that color change poorly
explains the evolution of the molts that produce these changes. This
suggests that biannual molt acts as a preadapted platform for color
change, instead of evolving in direct response to needs for color
change. These results provide a more nuanced understanding of plumage
evolution in birds by incorporating the mechanism for plumage
generation. Rohwer and Butcher (1988) made a novel contribution to our
understanding of delayed plumage maturation in birds by arguing that
molt must be understood first in order to understand plumage maturation
in birds. They found that the breeding-season driven hypotheses lose
support when molt is studied, and that the limitations of preexisting
molts explain delayed plumage maturation in birds better that social
selection on the breeding grounds. Our results largely agree with this
study, in that we find needs for plumage change do not appear to be able
to influence the evolution of molt strategies, instead they only
influence the phenotypes of feathers produced within molt strategies
that have evolved for other reasons. Similarly, needs for seasonal color
change may be present in species, but the translation of that need into
phenotype may be limited by the extent of prealternate molt. Following
our results and those of Rohwer and Butcher (1988) that hypotheses about
the role of social selection on feather color may look different when
viewed through the lens of molt, we encourage other authors studying the
evolution of plumage to consider molt strategies when attempting to
understand mechanisms of feather evolution.
Feather color has attracted much attention, especially into selective
processes that may have produced the diversity of coloration present in
birds’ feathers (Darwin 1871, Wallace 1891, Chaine and Lyon 2008, Li et
al. 2010, Payne 1984). Both natural and sexual selection play roles in
the colors of feathers as well as diversification of birds (Barraclough
et al. 1995, Møller and Cuervo 1998, Stoddard and Prum 2007, Marcondes
and Brumfield 2019). However, the molts that produce these feathers have
been largely ignored in these studies. Birds show a diverse array of
molt strategies (Stresemann and Stresemann 1966), but how and why
different species of birds have different strategies for the timing and
patterns with which they replace their feathers remains poorly known.
How selection interacts with molt, and not just feather phenotype, is an
essential question because molt is the underlying mechanism of feather
production. How molts provide limitations and opportunities for seasonal
change may be of widespread importance for understanding evolution of
avian color at a broader scale. For example, some juvenile birds are
brightly colored, and then lose this bright coloration in the highly
conserved preformative molt that occurs shortly after fledging (Pyle
2009). It may be that the preformative molt provides an opportunity for
these chicks to respond to selection from parental choice in the nest
(Lyon et al. 1994) without being “stuck” in a bright plumage for their
entire first year of life.
The needs for seasonal dichromatism likely vary by latitude and social
system in birds (Friedman et al. 2009, Simpson et al 2015), but these
results suggest that they may not be the primary factor influencing the
evolution of prealternate molt in the New World warblers. A major study
into global variation in seasonal plumage coloration in birds found that
seasonal color change is more uncommon than predicted by social systems
and predation risk (McQueen et al. 2019). We believe our study sheds
some light on this conundrum. From the viewpoint of the feather
wear hypothesis , the answer to this problem is that needs for seasonal
color change alone are not strong enough to influence molt patterns in
many species, and so seasonal color change can only evolve within the
context of pre-existing molts. This is similar to how pre-existing molts
limit the phenotypic realization of plumage maturation (Rohwer and
Butcher 1988). A two-step relationship between a birds’ needs for
feather color change and the response of phenotypic evolution to those
needs may not necessarily be unexpected. Selection for color and
structure on feathers likely interact in complex ways. For example,
sexual selection on feathers may act as a “bridge” between peaks on
natural selective landscapes for feather structure (Persons and Currie
2019). Our results provide evidence for similar “bridges” across
adaptive landscapes, where naturally selected molts may provide bridges
between spaces on a social selection landscape, in this case between
year-round monochromatism, and seasonal dichromatism. Further research
into the interplay between different types of selection on the evolution
of molts and plumages in birds could consider groups with disparate
social systems, as well as quantification of feather degradation. We
suggest that molt should be considered when attempting to understand the
evolution of plumages in birds.