The factors affecting the timing of nesting in tropical birds remain poorly understood. We investigated the phenology of White-throated Magpie-Jay (Calocitta formosa) nesting in the dry forest of northwest Costa Rica, a region characterized by a severe 5-month dry season followed by a very rainy wet season. We examined whether nesting was associated with climate, diet, risk of parasite infection to nestlings, risk of nest predation, and opportunity to re-nest. Groups of White-throated Magpie-Jays nested repeatedly, initiating nests over a 7-month period that spanned the transition from the dry to early wet season. The diet of adults and the composition of food fed to broods varied seasonally in parallel with changes in vegetation condition and climate associated with the transition from dry to wet season. Fledgling transition to nutritional independence occurred exclusively in the wet season when caterpillars and other arthropods were a large component of the diet. The timing of groups’ last nests was associated with an increase in nestling infections by Philornis botflies. We argue that progress at understanding tropical birds’ nesting seasons will be made by looking beyond diet and climate at the time of nesting, to additional factors such as the conditions during the post-fledging period of offspring development, temporal patterns of risk of parasite and pathogen infection to nestlings, and temporal patterns of nest or fledgling predation risk.
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