Effect of photoperiod on incubation period in a wild passerine, Sylvia atricapilla

Time required for avian embryos to develop is influenced by incubation temperature and the amount of time adults incubate eggs. Experiments on poultry indicate that photoacceleration, the light-induced stimulation of embryonic development, decreases the length of the incubation period as embryos receive more light. We hypothesized that eggs of wild birds exposed to longer periods of light should also have shorter incubation periods. We tested whether photoacceleration would occur in a species of open-cup nesting passerine, the blackcap Sylvia atricapilla. We artificially incubated blackcap eggs under four different photoperiods, four hours of light (4L) and 20 h of dark (20D), 12L:12D, 20L:4D, and a skeleton photoperiod (1 h light, 2 times per day) that framed a 20 h day. While incubation periods were accelerated with increasing photoperiod length, the differences among photoperiods of 4, 12 and 20L were weak. Embryos exposed to skeleton photoperiods developed as fast as those exposed to 20L and significantly faster than those exposed to 4L and 12L treatments. Skeleton photoperiods may most closely approximate natural patterns of light exposure that embryos experience during dawn and dusk incubation recesses typically associated with adult foraging. If our results from this species also occur in other wild birds, exposure to different day lengths may help explain some of the variation in the observed seasonal and latitudinal trends in avian incubation period.