Life-History Consequences of Avian Brood Reduction

Abstract

-Studies of avian brood reduction characteristically focus on the short-term consequences of hatching asynchrony for offspring (e.g. number and sizes of fledglings), but a truly comprehensive brood-reduction theory needs to incorporate long-term fitness effects for parents if trimming family size leads to lessened parental effort thereafter. A simple model shows that a brood-reduction strategy is more likely to be favored by natural selection when early losses of one or more brood members in poor years (expedited by parental manipulation of hatching asynchrony) lead to significantly diminished parental work levels. Field workers should design experiments to assess the effects of brood reduction on parental work levels, parental survivorship, and/or future fecundity; they could do so simply by borrowing the experimental field techniques already employed in studies of avian reproductive costs. Received 9 February 1993, accepted 10 May 1993. DAVID LACK (1947, 1954) suggested that parent birds may create more offspring than they can normally rear as a hedge against uncertain food. By starting incubation before laying has been completed, parents handicap last-hatched offspring, facilitating their selective elimination if food proves short. Conversely, when food is bountiful, the full brood may be reared. Lack's hypothesis has attracted considerable attention in recent years. Field tests are easily performed, usually involving experimental manipulation of the normal pattern of hatching intervals followed by measurement of the consequences for number and quality of surviving offspring. The results of such work appear uneven and have spawned considerable controversy (Clark and Wilson 1981, Amundsen and Stokland 1988, Magrath 1990, Amundsen and Slagsvold 1991, Pijanowski 1992, Konarzewski 1993, Forbes and Mock 1994). Experiments on the value of hatching asynchrony have focused mostly on how differing reproductive strategies affect recruitment from the current brood. In this regard they parallel the literature on another classic Lack hypothesis, his model of optimal clutch size. There he suggested that parent birds should not maximize the number of nestlings in each breeding attempt, but rather should maximize the number of offspring surviving to recruit into the breeding population (Lack 1954, 1966, 1968). Fundamental to this argument is a trade-off between the number and quality of offspring in a given brood. Although few would argue that Lack's clutch-size model did not represent an important conceptual development, it was quickly recognized as being incomplete. In part, its failure to account for all observed variation inspired refinements of the basic model, most notably recognition that parental reproductive costs may favor smaller clutch sizes (Williams 1966a, b, Charnov and Krebs 1974). Although Lack's hatching-asynchrony hypothesis (usually called the brood-reduction hypothesis, following Ricklefs 1965) predates his clutch-size model, it has not undergone similar scrutiny and amendment. Indeed, the brood-reduction hypothesis per se received little formal theoretical attention until relatively recently (Temme and Charnov 1987, Forbes 1991a, b, Forbes and Ydenberg 1992, Pijanowski 1992, Konarzewski 1993). These contributions have added much-needed mathematical rigor to the original argument, while exploring the roles of environmental variability, sibling rivalry, predation, and egg failure, as well as the cost of tracking variable resources on the evolution of brood-reduction strategies. Not surprisingly, field studies of hatching asynchrony and brood reduction have maintained the same short-term focus. Lack's broodreduction hypothesis is characteristically tested by perturbing the degree of hatching asynchrony (the presumed parental manipulation), then assessing how broods with trimmed (and sometimes exaggerated) hatching spreads compare with sham-manipulated control broods in terms of fledgling production. The rationale for this protocol is simple: the Lack argument predicts that synchronized broods should exhibit