A seasonally breeding tropical bird lacks absolute photorefractoriness in the wild, despite high photoperiodic sensitivity

1. Photoperiod is an important seasonal cue for temperate seasonal breeders. In birds, increasing photoperiod typically stimulates reproductive maturation, while also triggering gonad regression through photorefractoriness at a later date. Curiously, many tropical birds show photoperiodic reproductive responses, even though day-length changes in their natural habitat are minimal. 2. Spotted Antbirds (Hylophylax n. naevioides) from a near-equatorial rainforest in Panama are highly sensitive to small photoperiodic changes at the onset of reproduction. We therefore tested whether these tropical birds use photoperiod as the primary signal for seasonal processes, as do most temperate birds, and terminate reproduction through photorefractoriness. 3. Male and female Spotted Antbirds captured during postbreeding moult significantly increased gonad sizes when exposed to a photoperiod of 22 h. Furthermore, males exposed to a naturalistic 1-h increase in photoperiod (to 13 h) also grew their gonads, suggesting that these birds retain photosensitivity at a time of year when most temperate birds are unable to grow their gonads. Hatch-year birds in all groups showed reproductive development indicating their capability to breed. Unlike in many temperate-zone birds, the moult of adult birds was not inhibited by gonad development. 4. The lack of photorefractoriness allows Spotted Antbirds to flexibly adjust the end of breeding to environmental conditions. Our findings support the view that photoperiodic mechanisms are evolutionary plastic and match the ecology of a particular species.