Bird breeding season linked to sunshine hours in a marginally seasonal equatorial climate

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.