The feather wear vs. variables needs hypotheses
The variable needs hypothesis , which proposes that prealternate
molt evolves in response to birds’ needs to change colors, predicts
coevolution of prealternate molt and seasonal dichromatism. We found
discrepancies in the best fit models of evolution, timing, pattern and
external correlates of evolution of between the prealternate molt and
seasonal dichromatism. The character that we studied with the strongest
evidence for selection, as interpreted by the ratio of likelihood for an
Ornstein-Uhlenbeck model to a Brownian Motion model, was presence of
prealternate molt. This may imply that prealternate molt itself is under
stronger selection (Butler and King 2004) than the color change it
produces, which fit slightly better to a model of Brownian motion. We
interpret this as support for the feather wear hypothesis ,
because this hypothesis predicts stronger selection on molt patterns
than on seasonal dichromatism. The life history characteristics that
best predicted prealternate molt were migration distance, day length,
and solar radiation experienced on the breeding grounds (Fig 1, table
1). Top models for seasonal dichromatism all included prealternate molt
and foraging stratum on the breeding and nonbreeding ranges. Combined
with the results of the path analysis, we interpret these results as
evidence for prealternate molt and seasonal dichromatism evolving along
separate pathways.
The feather wear hypothesis invokes preadaptation in the
relationship between prealternate molt and seasonal dichromatism. Thevariable needs hypothesis may predict synchronous evolution of
prealternate molt and seasonal dichromatism, whereas the feather
wear hypothesis predicts that prealternate molt should precede seasonal
dichromatism and correlate with separate external parameters When we
investigated the evolutionary timing of these characters, prealternate
molt appeared to arise before seasonal dichromatism, and in more species
and feather regions (Fig. 4). The idea that a character can evolve in
response to selection for one function, and then be co-opted to serve
another, has been well-explored in evolutionary biology (Bock 1959).
Preadaptation has been implicated in the evolution of a wide array of
evolutionary novelties (Cheney and Seyfarth 2005, Ketola et al. 2013,
Schiestl and Cozzolino 2008, Quiñones and Pen 2017), and is an important
phenomenon to understand when investigating how traits evolve. Our
ancestral state reconstruction suggests that prealternate molt is a
preadaptation, rather than an adaptation for seasonal dichromatism. We
do not present these results as a rebuttal to variable needs for feather
color. Clearly, needs for feather colors vary with life-history and
latitudinal gradients in social behavior (Friedman et al. 2009), needs
for crypsis on migration (Simpson et al. 2015) and nest stratum (Martin
and Badyaev 1996), though we found little support for a relationship
between nest stratum or nest type and prealternate molt or seasonal
dichromatism. Our findings suggest that latitudinal gradients likely do
play a role in the evolution of color change in feathers once
prealternate molt is present. From these results, we propose a two-step
pathway for the evolution of disparate breeding and nonbreeding plumages
in warblers: a biannual molt evolves in response to structural needs for
feathers, and then serves as a preadapted mechanism for seasonal
dichromatism.
Immediacy of needs helps explain why structure may influence molt more
than needs for color change. Structural functions provided by feathers
are more immediately necessary for survival of birds than colors that
function for social signaling. Without feathers, chicks are
poikilothermic (Whittow and Tazawa 1991) and reliant on their parents
for warmth. In adult birds, worn feathers directly influence survival
through decrease of important functions such as flight (Swaddle et al.
1996). Timing of molt appears to be so important that experimentally
malnourished birds will undergo a molt in spite of losing up to 40%
body mass in the process, instead of delaying molt (Murphy et al. 1988).
Because of more immediate implications on survival, it may make sense
that selection on feather structure is stronger than on color change,
and that selection on feather structure may be more likely to influence
the evolution of molt strategies.