Egg-laying and photorefractoriness in the European Kestrel Falco tinnunculus

Breeding and moult cycles were investigated in pairs of European Kestrels Falco tinnunculus exposed to four different photoperiods (LD regimes of 17.5:6.5, 15:9, 13:11, and 8:16). Laying, incubation and feeding of the young occurred in all four photoperiods, with all birds moulting after the breeding season. The time until the first egg, the duration of the period during which first clutches were laid, and the time until birds began to moult, were all negatively correlated with daylength.

Two groups of birds (LD 13:11 and 8:16) were held under constant conditions for 4.5 and 2 yr, respectively. After the first breeding/moult cycle, the LD 13:11 group did not begin a second cycle for the next 19 months, and only started laying again, and subsequently moulting, after exposure to an 8 h day for 6 weeks. The LD 8:16 group went through two breeding/moult cycles during the two years.

It is concluded that the development and expression of photorefractoriness is related to daylength, and that photorefractory Kestrels need short days to become photosensitive again. The possibility that the development of photorefractoriness is involved in the seasonal decrease in clutch size, found in both free-living and captive breeding Kestrels, is discussed.