Lack (1954; 40-41) has pointed out that in species of birds which have asynchronous hatching, brood size may be adjusted to temporal fluctuations in food availability. Parents generally feed the most active of their brood, that is, the largest (oldest) and the strongest. When such nestlings are replete, they become inactive and food is then passed on to the smaller members of the brood. When food availability is high, the parents are capable of satisfying all of the brood; when low, only the largest or first hatched are satisfied and the others starve to death. Thus, because of size differences due to asynchronous hatching, the parents are able to distinguish their young and can distribute limited food resources to the best advantage. If differentiation were not possible, food would be distributed randomly with each nestling having equal probability of being fed. During a poor year, all of the young would be underfed, a condition presumably less advantageous than having fewer well-fed young. Brood reduction is generally thought of as being prevalent in the hawks and owls and in some sea birds. It is the purpose of this paper to demonstrate the phenomenon in a passerine, the Curve-billed Thrasher (Toxostoma curvirostre). Up to a point, the parent can increase its rate of food delivery to the young by increasing its own effort. When food availability is low, it presumably takes a greater effort to deliver food at a given rate than during a more favorable period. It is convenient to think of the rate as being constant for any given level of food availability. Although probably unrealistic, this assumption is not impractical if we consider that regardless of the food availability there is an optimum effort which the parent can devote to gathering food. For different resource levels, this optimum effort will yield different rates of food delivery to the young. The energy requirement for the growth of the nestling increases with age up to a certain point after which it decreases (fig. 1). Thus, the parents are increasingly taxed not only with the addition of more young to the brood, but also with time. In a good year, the parents are able to keep up with the increasing demands and can deliver food at the maximum rate required by a full brood of nestlings. In a poor year, the requirement of the entire brood may exceed the delivery capacity of the parents long before the peak requirement is reached. When this happens, the smallest receives no food and dies. If, several days later, the requirement of the reduced brood again exceeds the delivery capacity of the parents, the smallest of the remaining birds has already been starved to death. It is theoretically possible that not one nestling would end up well fed in extremely hard times, but the birds would not be likely to breed under such conditions.