The Consequences of Brood Size for Breeding Blue Tits II. Nestling Weight, Offspring Survival and Optimal Brood Size

Abstract

(1) Two hypotheses first proposed by Lack (1947, 1948, 1954) to account for the evolution of brood size were examined: (i) the average brood size of a population maximizes the number of surviving offspring, and (ii) in nidiculous birds, the ability of parents to feed the young limits brood size. A revised form of Lack's first hypothesis was also investigated, one which would incorporate the effect current reproduction has on subsequent parental survival. To examine these hypotheses, in 1978, 1979 and 1980 breeding pairs of blue tits (Parus caeruleus) were randomly assigned one of five brood sizes: 3, 6, 9, 12 or 15, and the fate of offspring and parents was determined. (2) Nestling weight significantly and consistently decreased with increasing brood size. Nestlings from the largest broods were 8-12% lighter at day 10 of the nestling period than those from the smallest broods; between days 10 and 13 the former did not catch up in weight with the latter nor did they fall further behind. Nestling weight at day 10 or 13 of the nestling period was correlated with subsequent survival in 1978 but not in 1979. The survival of individual offspring was independent of brood size except that in 1978 (but not in 1979) nestlings from broods of fifteen survived less well than young from smaller broods. The number of surviving offspring per brood was maximized for broods of twelve in 1978 and for broods of fifteen in 1979. These results do not support either of Lack's original hypotheses, though they are consistent with the view that investment in offspring reflects the balance of costs and benefits to the breeding individual. (3) A revision of Lack's first hypothesis was tested: the optimal brood size maximizes the population growth rate, AL. In 1978 and 1979, the optimal brood sizes were twelve and fifteen (or possibly larger), respectively. Since the average brood sizes were c. nine and ten, respectively, the revised form of Lack's first hypothesis is not confirmed (4) These results underline the importance of two factors, either of which may account for the apparent discrepancy between observed and calculated optimal brood size. (i) Time and energy constraints may act to limit brood size during the laying period, not during the feeding period. (ii) If survival probabilities (especially of offspring) are unpredictable from year to year, restrained reproductive effort may be favoured. During this study, temporal variability in the reproductive ecology of this population was apparent, but a much longer study is required to assess adequately its significance for the reproductive strategy of the blue tit.