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.