Life History and Environmental correlates of molts and color
change
Phenotypic evolution is the result of repeated interactions between
selective pressures and pre-existing structures available for selection
to act upon, in addition to neutral drift. While Gould and Lewontin
(1979) has been widely criticized for oversimplifying adaptation
hypotheses (Pigliucci and Kaplan 2000), a general point that it explains
well is that selection can only work upon biological features that
exist, and that contemporary uses for a biological structure may not
fully explain why that structure originally evolved. While it may make
intuitive sense that prealternate molt is “for” a breeding plumage,
and indeed some naming conventions (e.g. prenuptial molt,
prebreeding molt) imply this causative relationship; it is important to
disentangle direct and indirect causation when attempting to understand
how selection interacts with phenotypic evolution over time (Hardenberg
and Gonzalez-Voyer 2013). Phylogenetic path analysis produced two top
models, both of which found that the extent of prealternate molt was
driven directly by migratory distance and cumulative annual day length..
This suggests that seasonal dichromatism is connected indirectly to
migratory distance through prealternate molt (Fig 3b). The models
suggested that seasonal dichromatism was determined by the presence of
the prealternate molt and foraging stratum, with birds foraging in more
open strata experiencing more extensive prealternate molts and seasonal
dichromatism. This generally agrees with previous findings that sexual
selection operates more strongly in canopy birds, which tend to be more
visually oriented (Gomez and Théry 2004, Shutler and Weatherhead 1990),
resulting in brighter plumages (Shultz and Burns 2013). From a
structural standpoint, canopy birds may also experience greater solar
exposure. Indeed, one of the few tropical groups of birds with a known
prealternate molt are the becards (Pachyramphus ; Johnson and
Wolfe 2018) which show identical alternate and basic plumages, and
inhabit canopy and forest edge habitats. Importantly, breeding season
foraging stratum, when combined with extent of prealternate molt,
strongly predicted extent of seasonal dichromatism, but did not by
itself predict extent of prealternate molt (table 1). This suggests that
selective pressure on plumage color acts on seasonal dichromatism only
after prealternate molt has evolved for other reasons and then provides
a structural canvass for sexual selection to paint upon.
Past studies have found that sexual dichromatism can evolve through the
loss of a gaudy plumage among female migratory birds (Simpson et al
2015). Similarly, we find that at least in some cases, year-round
monochromatism evolved through loss of the prealternate molt. It is
important to consider phylogenetic context in the evolution of different
types of dimorphism because trait gains and losses may mean different
things over evolutionary time. For example, Simpson (2015) found sexual
dichromatism in warblers stems from loss of bright coloration in
females, and Hoffmann et al (2009) found a similar pattern in oriole
plumage. From the perpsective of migratory distance, Winger and Lovette
(2011) showed that resident warblers were more likely to be examples of
lineages that had lost long-distance migration. We found gains and
losses of both seasonal dichromatism and prealternate molt, and,
importantly, we found that losses of long-distance migration were
associated with loss of prealternate molt. Froelich et al. (2005) and
Tökölyi et al. (2008) proposed that the relationship between migratory
distance and seasonal dichromatism is caused by earlier breeding in
resident species which limited their ability to molt; however, resident
species do not appear to be limited in their molts when compared to
migrant birds, as they show increased molt-breeding overlap (Johnson et
al. 2012) and protracted molts (Kiat et al. 2019, Terrill 2018).
Furthermore, it is likely that migrant birds are limited in their molt
timing, as they generally complete prealternate molt before beginning
spring migration (Pyle 1997b). Without a prealternate molt, nonmigratory
warblers are often the same color throughout the year, and resident
warblers fall into two categories, those that are either gaudy all year,
or cryptic all year. These findings suggest that variable needs for
feather color alone are not strong enough to maintain a biannual molt in
these birds, without an external force acting on the structural
integrity of their feathers, and long-distance migration directly
impacts structural integrity. Furthermore, pressures that affect the
latitudinal gradients in sexual dichromatism and seasonal dichromatism
likely differ because each is derived from a different mechanism. While
sexual dichromatism can be associated with the prebasic molt and
result in a year-long plumage aspect, seasonal dichromatism results in
discrepancies between the prebasic and prealternate molt,
and results in seasonally variable plumage aspects.