Biannual cycles of reproduction and molt are rare in birds, with most species exhibiting a single annual cycle. We studied the breeding and molting phenology of the Cinnamon-bellied Flowerpiercer (Diglossa baritula), a nectar feeding passerine, in western Mexico. Monthly captures from December 1994 to May 1998 revealed clear biannual breeding and molting cycles. The two breeding seasons took place from January to April (dry season) and from July to October (rainy season). Their July to October breeding season coincided with the single breeding season exhibited by resident hummingbirds at our study site. Even though flowerpiercers and hummingbirds use the same food resources, hummingbirds only molt and breed once a year. Male flowerpiercers breed and molt flight feathers twice a year, whereas females appear to breed only once. A higher proportion of females bred in the rainy season (90%) than in the dry season (30%), and they underwent a complete molt only during the rainy season. Proportions of males in breeding condition during the dry and the rainy season were similar (86 and 92%). Some males (40%) showed an incomplete molt in the dry season, while most males (80%) underwent a complete molt in the rainy season. Differences in proportions of breeding and molting individuals between the dry and the rainy season suggest that the dry season cycle is limited by food availability. We hypothesize that D. baritula individuals use soft fruits in addition to flowers to fuel their breeding during the dry season.
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Vol. 35 • No. 2