Family Planning in the Great Tit (Parus Major): Optimal Clutch Size as Integration of Parent and Offspring Fitness

Abstract

AbstractFitness variations due to natural variation in the size of the first clutch and its laying date were estimated using Fisher's reproductive value for both the clutch (Vc) and the parent (Vp) in a population of great tits. In order to test the hypothesis that individual birds maximize their reproductive value by the choice of clutch size, artificial variation in brood size was introduced and the consequences in terms of reproductive value estimated. Maximal Vc, computed on the basis of natural variation in clutch size, occurred at a clutch size of 15.2, and increased slightly with laying date (Fig. 1A) Vp increased with natural variation in clutch size and decreased with date (Fig. 1B). The total reproductive value V (= Vc+Vp) was maximal at a clutch size of 15.4 (Fig. 1C), substantially higher than the population mean clutch size (9.2). The components of the reproductive value of the clutch (Vc) that were negatively affected by manipulation were the survival of the nestlings and the recruitment rate. The reproductive value of the parent (Vp) was negatively affected only through the probability of having a second clutch. Maximal Vc computed on basis of artificial variation in clutch size, occurred at a clutch size of 10.0, and also increased with date (Fig. 1D). Vp decreased with artificial variation in clutch size (Fig. 1E causing the clutch size maximising reproductive value V to shift to a value of 9.4 (Figs 1F, 3), very close to the population mean clutch size (9.2). It is concluded that the majority of great tits produces the number of eggs (9-10) that maximizes their individual fitness, even though those individual birds laying 15 eggs have the highest reproductive value in the population. The fact that birds laying very large clutches have the highest reproductive value points in the direction of a selection pressure towards larger clutches. Yet, over the last 30 years clutch sizes have not increased in the study population. This apparent contradiction is discussed. Either no genetic variation in clutch size is involved, or a complex polymorphism exists.