In many kinds of animals, parental care is essential for the survival of newborn progeny, yet it is sometimes terminated by the parent long before the offspring reach full independence. The ecological causes for such desertion are diverse, but center on the evolutionary principle of maximizing reproductive success. In other words, parents may be able to raise more young in the long run by truncating investment when the cause is already lost (for example, during a drought, if the food base deteriorates dramatically). Recent field studies of birds (egrets and herons) have identified another condition under which parents appear to abandon already-hatched young. When brood size drops from three or four chicks to just one surviving nestling, parents may forsake it and use the remainder of the summer breeding season to renest, perhaps thereby producing a larger brood. The signal for such a switch in parental behavior is presumably the number of chicks in the nest. This idea has been supported by observational data on two species, but has not been tested experimentally. The present study aims to fill that void with a set of complementary field experiments, using an extremely common species (the cattle egret). In the first experiment, brood size will be artificially reduced from three chicks to just one, by transferring the newly hatched nestmates to undersized broods nearby, where they are accepted readily. Controls will consist of broods whose young are handled the same amount, but then returned to their own nest. Dr. Mock predicts that parents will desert the singletons more frequently than the control broods. In order to explore whether this effect decreases as the remaining amount of breeding season wanes, the first experiment will be run three times throughout the nesting period, with the prediction that the later singletons are less likely to be abandoned than those occurring early in the season. If so, parents would have to take both number of chicks and time of season into account. The final experiment involves the behavior of the nestlings themselves. In this and related species, chicks hatch at different times, and the slightly older and larger nestlings persecute their younger siblings, even to the point of killing them. If becoming a singleton is a perilous condition (increasing the risk of parental desertion), then chicks may reduce their rate of fighting siblings when brood size drops from three to two. This will be tested by removing the youngest of three chicks from nests kept under continuous observation and then replacing it three days later. If aggression depends on the number of nestmates, its frequency should dip when only two chicks are present, but return to baseline levels when the full brood is restored.
