Asynchronous Broods Research Articles

Clutch-Size Dependent Asynchronous Hatching and Brood Reduction in Junco hyemalis

Clutch-Size Dependent Asynchronous Hatching and Brood Reduction in Junco hyemalis

Hatching Asynchrony in American Goldfinches: An Experimental Study.

I examined Lack's (1954, 1968) hypothesis that asynchronous hatching is an adaptive response to food shortage during the breeding season by comparing growth and survival of nestlings in asynchronous and artificially created synchronous broods of American Goldfinches (Carduelis tristis). I also examined the effects of seasonal and environmental factors on nestling growth and survival and on hatching asynchrony. Nestlings in asynchronous broods were more likely to diverge in mass and did so at a faster rate than those in synchronous broods. The lightest nestlings of asynchronous broods grew more slowly than their heavier nestmates and than all nestlings in synchronous broods. Brood reduction occurred more frequently in asynchronous broods. Survival and growth rates decreased throughout the breeding season and during inclement weather. Hatching intervals increased during the season. When brood reduction or differential growth among nestmates occurred in asynchronous broods, suggesting that the energy available for growth was limiting, heavy nestlings in asynchronous broods grew nonsignificantly faster than heavy nestlings in concurrent synchronous broods. This trend implies that when insufficient food is delivered to nestlings, asynchronous hatching may provide a slight advantage for older nestlings. Growth rates of all nestlings, however, were greatest in highly synchronous broods. Explanations other than growth of young must be invoked to explain why extreme synchrony is not more common in goldfinches. I discuss constraints on the normal pattern of hatching asynchrony characteristic of this species. The proximate mechanism for differential feeding and brood reduction is discussed.

Brood reduction in the Blue‐eyed Shag Phalacrocorax atriceps

Brood reduction is common in a population of Blue‐eyed Shags on Signy Island, South Orkney Islands. This paper describes possible adaptations which may reduce the brood. In clutches of three, the last egg was smaller, and hatched 2.4 days later than its siblings. Whilst 78–84% of first and second (‘A’ & ‘B’) chicks fledged, only 11 % of ‘C’ chicks did. In a sample of artificially synchronized broods chick survival was as high as in normal asynchronously hatching broods, but there were more cases of total brood loss. The age at which the C chick died was related inversely to the length of the A‐C hatching interval. Relative differences in sibling weights were highest during the first 12 days, when most of the C chick deaths occurred. At this age the daily food requirements of each brood of three was one‐tenth that of each brood of two just prior to fledging. It is suggested that C chicks were unable to compete effectively for a food supply which was limited by the parents, rather than by the environment. The asymptotic weight attained by A chicks was inversely related to brood size, and was greater than that of B or C chicks. Normal asynchronous broods produced at least one heavy (A) chick and one medium weight (B) chick, whilst in synchronized broods the asymptotic weight attained was similar to that of B chicks in normal broods.

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

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.