Theo Meijer

Brood size manipulations in the kestrel (Falco Tinnunculus) : Effects on offspring and parent survival

(1) Brood reductions and enlargements were carried out in kestrel nests to evaluate the consequences of raising different numbers of nestlings for both the offspring and the parents. (2) Brood enlargements caused increased daily hunting activity of the parents, reduced growth rate of the nestlings, increased nestling mortality and enhanced weight loss in the female parent. Brood reductions caused an increased food intake by the nestlings, in spite of (non-significantly) reduced parental hunting activity. Local survival of the parents was negatively correlated with the experimental change in brood size. (3) A review of the literature on brood enlargements is presented, showing that parents were able to raise more young till fledging than their natural broods in twenty-nine out of forty altricial bird species investigated. Negative effects of brood enlargements on parental survival or future reproduction were established in eight out of twelve species investigated. (4) The results are consistent with the theory that parental work for the offspring entails an inherent reduction in future reproductive output and that natural broods, by being smaller than the maximum number of nestlings that can be raised, maximize the total reproductive output.

Seasonality of clutch size determination in the Kestrel Falco tinnunculus: An experimental approach

Egg numbers in Kestrel clutches decline from 6.5 to 3.2 with progressive laying date. Laying Kestrels did not respond to eggs added to the nest after the second egg. Egg removals starting on day 3, 5 or 7 of laying demonstrated that responsiveness (production of extra eggs) ceased ca 4 d before the normally final egg. Thus clutch fixation occurred sooner after the first egg in late than in early nests. Continuous records of nest temperature were used to establish incubation frequency of the female throughout laying in experimental and control nests. Incubation rose from 0 to 98% of time in the course of the laying phase. This rise started sooner and was more rapid in late nests and in nests with added eggs, and delayed in response to egg removal. The higher level of incubation tendency in late breeders at the onset of laying may be instrumental in more rapid follicle resorption and hence in clutch size reduction.

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.