Clutch Size, Nest Size, and Hatching Asynchrony in Birds: Experiments with the Fieldfare (Turdus Pilaris)

Abstract

Breeding strategy of an open—nesting passerine bird, the Fieldfare (Turdus pilaris) was studied in a woodland in Norway during four successive breeding seasons. Most birds laid clutches of five or six eggs; only 1% laid more than six. For 100 nests, eggs and/or chicks were added to raise the final number to seven hatchlings. Parents that laid a clutch of five eggs showed a equal skill in raising broods of seven, as did those who laid six. In enlarged broods, when only small age differences existed among the hatchlings, their survival rate was higher than that of enlarged asynchronous broods. In the enlarged synchronous broods the nestlings the subsequently died did so at a greater age than did those in the asynchronous broods. Brood size frequency for synchronous broods, recorded 8 d after brood enlargement, deviated from that expected under a random mortality pattern, i.e., mortality more resembled in "all—or—one" type. Hatching success was not lower for larger clutches (provided that the eggs hatched synchronously); the parents showed great skill in raising enlarged broods, and these nests were not subjected to a much greater degree of predation than were nests that contained small clutches and broods. The Fieldfare therefore normally rears a smaller—sized brood than could be nourished. More pairs that were provided with an artificial, large—sized nest managed to raise a brood of seven than did pairs that attempted to rear seven in their own original nest. Brood reduction in birds may not be adaptive, as argued by Lack and others, but may result from overcrowding. The reason why wider nest cups are not built in nature may be related to problems involved in hatching the eggs successfully; hatching losses were greater in enlarged nests than in natural nests.