Hatching Asynchrony and Brood Reduction
photograph of a Great Horned Owl by Rohan Kamath	Whether eggs in a single clutch will hatch simultaneously or sequentially over an extended period of time is determined by the onset of incubation. In many birds, including most precocial species, incubation does not begin until the last egg has been laid, resulting in all of the eggs hatching within a few hours of each other. In contrast, many other birds begin incubation prior to laying the last egg of the clutch. This results in asynchronous hatchings separated by anywhere from a few hours to several days, depending on how soon incubation commences following the start of egg laying. In altricial and semialtricial species, asynchronous hatching gives the first chicks to
leave the egg a head start at vigorously begging for food and successfully attracting parental attention. The influential British ornithologist David Lack viewed the evolution of asynchronous hatching as a parental strategy for raising the largest number of offspring that food resources will allow when the abundance of food for the chicks cannot be predicted at the time that eggs are laid. The matching of offspring number with food availability is thus achieved by means of brood reduction: with asynchronous hatching, the smallest chick or chicks do not garner "their fair share" and will only survive in years of abundant food. Elimination of the smallest chicks usually occurs by starvation as result of competition with their larger siblings for parental feeding, from overt parental neglect, or psychological and physical intimidation by their larger siblings. Brood reduction by means of starvation routinely occurs in almost all of our gulls and raptors, and is commonly seen in birds as diverse as cormorants, herons, egrets, terns, Red-cockaded Woodpeckers, Curve-billed Thrashers, some corvids, and Common Grackles. Size disparity among chicks is exacerbated in gulls by the laying of different-sized eggs in which egg weight typically decreases with laying order. Eagles and boobies exhibit "obligate siblicide," in which the larger chick invariably kills its smaller sibling. For example, of more than 200 records of two-egg clutches followed in the Black Eagle of southern Africa, only one record exists of both chicks surviving to fledging. Obligate siblicide also occurs among pelicans, owls, and cranes. In obligate siblicide, which occurs even when food supplies are abundant, the second egg serves as insurance against loss of the first egg from infertility, predation, or damage, rather than as a means of rearing two chicks. In the 25-40 years since Lack's studies, information about many additional species has accumulated, and hatching asynchrony is now thought to be more common than synchronous hatching among altricial birds. Ornithologists now think that asynchronous hatching is not a strategy for achieving brood reduction in many species. They have advanced other hypotheses to explain its evolution. Sociobiologists Anne Clark and David Wilson argue that high nest predation rates can encourage the evolution of asynchronous hatching as a means of minimizing the total amount of time that eggs and nestlings spend in the nest. Asynchronous hatching also can be interpreted for some insectivorous species as an adaptation to "speed up" hatching so that at least some of the nestlings can capitalize on rapidly (but unpredictably) peaking food resources (such as outbreaks of forest caterpillars). This "new view" of the adaptive significance of hatching asynchrony helps to explain the increasing egg weight often seen from first to last egg laid among many asynchronously hatching passerines. These weight differences represent differing proportions of nutrients and energy invested in different eggs within clutches, with the greatest investment going into the last egg of the clutch. This pattern is clearly inconsistent with the notion of facilitating brood reduction, and appears to be a means of compensating for the delay in hatching and development by providing the last chick with more resources initially.
SEE: Incubation Time; Precocial and Altricial Young; Eggs and Their Evolution; Variation in Clutch Sizes; Average Clutch Size. Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.