Availability of food resources to chicks or to parents poses a potential ecological limit to the growth rate in species that are either self-feeding or provide for their young. Foraging time is strictly limited by the number of hours (generally daylight) during which individuals can be active. Thus, for example, tropical species have fewer hours to forage during the breeding season than do species at higher latitudes (Lack 1954). Of course, food abundance also influences the rate of food acquisition. Time devoted to one activity cannot be given to another, unless two activities can be performed simultaneously (Dunham et al. 1989; Ricklefs 1991). Because foraging and brooding occur in different places or are otherwise incompatible, time must be allocated between them. Visser (1991) showed that the degree of homeothermy and weather conditions together determine the time allocated for feeding and brooding in shorebird chicks. Warm weather allows for more foraging time and thus higher growth rates in chicks (Beintema and Visser 1989a, 1989b; see chapter 5). This limit is partially under parental control through variation in foraging effort. Having limited access to resources, parents must decide how much food to gather and what proportion of it to deliver to their offspring or to self-maintenance. We assume that parents will limit a loss of personal condition for the sake of their offspring, particularly if they have a reasonable expectation of living to breed again (Williams 1966; Roff 1992; Stearns 1992). At this level of constraint, allocation may be determined primarily by the requirements of the parent, the remainder being delivered to the brood. Parents can feed a small number of chicks at a high rate per chick or a larger number of chicks at a lower rate. To the extent that the rate of growth influences resource requirements of the chick, this creates a potential trade-off between growth rate of the chick and fecundity of the parent (Ricklefs 1984; see chapter 15). Additional factors that influence optimization of this trade-off are the probability of survival of the chicks and the number of broods that a parent can rear each season, both of which should be inversely related to the length of the development period and hence directly related to rate of growth. Finally, sibling competition may affect the distribution of food among chicks and may favor rapid development, resulting in high food requirements per chick and fewer offspring per brood, in direct conflict with the optimum strategy of the parent (Trivers 1974; Werschkul and Jackson 1979; Ricklefs 1982; see chapter 15).

Level 2 The second level of limitation is the individual chick's capacity to utilize available resources. For example, level-2 constraints may reflect the capacity of the chick's digestive tract to assimilate food. In this situation, the design of the chick must resolve two kinds of alternatives. One concerns the allocation of body tissue to digestion and assimilation, which would control the rate of resource acquisition by the individual but also constrain other functions (Konarzewski et al. 1989, 1990). The second concerns the allocation of resources between growth and other functions, such as self-maintenance, temperature regulation, activity, and energy storage (Dunham et al. 1989; Ricklefs 1991). As in the 266