The phenology of molting, breeding and their overlap in central Amazonian birds

Th e energetically challenging periods of molting and breeding are usually temporally separated in temperate birds, but can occur simultaneously in tropical birds, a condition known as molt – breeding overlap. Here, we document great variation in the timing and duration of molting and breeding, and in the extent of molt – breeding overlap, among 87 species of understory passerines in central Amazonia. We analyzed molt and breeding from 26 871 birds captured over a 30-yr period near Manaus, Brazil. Although most species typically bred during the late dry season (about October through January), many thamnophilids apparently bred year-round, whereas a few other species from a variety of families bred mainly during the wet season (about January through May). Of all breeding birds with an active brood patch, 12.7% were simultaneously molting. Molt – breeding overlap was more frequently observed among suboscines (13.3%), especially thamnophilids (23.0%), than oscines (6.4%). Some families had  5% molt – breeding overlap frequency, including Tyrannidae (4.4%), Tityridae (0.0%), Pipridae (1.5%), Turdidae (0.0%), and Th raupidae (0.0%), indicating that not all tropical species exhibit molt – breeding overlap. Among 31 well-sampled species (n  15 brood patches), variation in molt – breeding overlap frequency was positively correlated with each species ’ average duration of fl ight feather replacement (range 98 – 301 d). We also measured feather growth rates of individual birds in nine species; in fi ve of these, slower-growing feathers increased with an individual ’ s probability of having molt – breeding overlap. Among furnariids, molt – breeding overlap occurred either at the beginning or end of the molt cycle, suggesting that physiological mechanisms typically separate molting from breeding. Th amnophilids showed a much diff erent pattern; molt – breeding overlap occurred at any stage of feather replacement, apparently not regulated to be independent of breeding. Th ese results reveal substantial life-history variation among Amazonian birds. Future work to resolve the physiological regulation of molting and breeding in tropical birds will greatly contribute to understanding these patterns and their relevance to avian diversity.