Visual and nutritional food cues fine-tune timing of reproduction in a neotropical rainforest bird.

Food may act as a proximate factor in the regulation of avian seasonal breeding. Food cues could provide particularly important seasonal information to birds living in variable tropical environments, but this has not yet been tested. Spotted antbirds (Hylophylax n. naevioides) inhabiting a humid forest in central Panama (9 degrees N) likely use changes in the tropical photoperiod to time reproduction on a long-term, seasonal basis. We predicted that these insectivorous birds also adjust reproduction to short-term cues such as food availability because the onset of the rainy season and the resulting increase in insect abundance varies considerably between years. To test this prediction, prior to their breeding season (when they had half-maximal gonads), we either exposed captive male spotted antbirds to an ad libitum standard diet only or added live crickets to this diet. Males that received live crickets significantly increased gonad sizes within 3 weeks over controls on the standard diet. Moreover, in six additional experiments cricket availability always increased song rate, usually within a few days. The stimulatory effect of live crickets on song activity may function independent of nutritional aspects: Freshly killed crickets, providing similar nutritional content as live crickets, did not stimulate the birds' song activity. However, song activity increased to intermediate levels when live crickets were shown under a clear plastic wrap, i.e., when birds could see but not eat crickets. We hypothesize that the opportunity to see and handle live insects stimulates song and reproductive activity in these birds. Our data indicate for the first time that a tropical rainforest bird can use food cues to evaluate the suitability of local environmental conditions for breeding. J. Exp. Zool. 286:494-504, 2000.