The grass is always greener

Abstract

Millions of migratory and resident passerines molt in southwestern North America each summer, when ecological productivity spikes in response to the North American Monsoon. Although a monsoon-region molt is a critical event in the annual cycle of many species, the benefits of molting in the monsoon region are unclear. I used museum specimens to compare molt dynamics of the Vermilion Flycatcher (Pyrocephalus rubinus) in relation to the timing and location of the monsoon rains, and between northern and southern monsoon latitudes. In the north and the south, populations did not vacate arid non-monsoon areas to molt in adjacent monsoon areas, unlike several other lowland western passerines with more northern distributions. Onset, rate, and intensity of molt were similar between populations in and out of the monsoon region at the same latitude. By contrast, molt started significantly later and was significantly faster at northern latitudes than in the south, suggesting a strong, latitude-based temporal influence on molt dynamics, which was correlated with later nesting in the north. Thus, for this insectivorous passerine, the monsoon region may simply provide adequate habitat for molting at a low latitude, rather than abundant resources important for molt, as has often been proposed to explain the evolution of molt-migration to the monsoon region by western migrants. Similarities in foraging ecology of Vermilion Flycatchers and other western passerines may make these results applicable to other species, helping to clarify the role of the monsoon region in the evolution of molt schedules in North American passerines.