The timing of reproduction is a fundamental aspect of life history, yet the breeding seasons of most birds of the world, i.e., those in the tropics, remain poorly understood. Here we use more than 3,000 mist-netting records and 300,000 citizen scientist observations collected over six years to characterize the nesting, incubation, fledging, and juvenile stages of the breeding season on Singapore Island in Southeast Asia’s equatorial rainforest zone. The breeding season was compared with climate variables and food availability to identify possible proximate and ultimate causes. Breeding was seasonal and began just after the rainiest months of the year, when insect abundance was highest and when masting events were most likely to occur. While true photoperiod varied little throughout the year, overcast weather in November–December and sunnier weather in February–March caused average daily sunshine to increase by several hours at the onset of the breeding season in all six years. Our data suggest that subjective daily sunshine hours, which correlate with photoperiod at higher latitudes but not in the tropics, may be the actual proximate trigger of breeding activity in most of the world’s birds.