Role of Sibling Aggression in Food Distribution to Nestling Cattle Egrets (Bubulcus Ibis)

--Sibling a gression occurs in a wide variety of asynchronously hatching bird species. In some, •ights among siblings lead inevitably to death, in which case the benefits of winning are •lear. In species where sibling aggression s common but usually not fatal, the benefits gained by winning and the methods used to achieve them are less obvious. In a Texas colony of Cattle Egrets (Bubulcus ibis), sibling aggression was frequent but siblicide rare. Parents rarely interfer•ed with fights. Last-hatched chicks lost more fights and received less food than their elder siblings. Fighting limited the losers' immediate access to food and contributed to the senior sib's ability to monopolize boluses. These results are consistent with the hypothesis that monopolizable food can act as both a proximate and ultimate cause of sibling aggression. The main effect of sibling aggression lay in depressing food supplies to last-hatched chicks. Firstand second-hatched sibs accrued roughly equal feeding advantages. Received 19 August 1985, accepted 11 April 1986. MANY raptors, cranes, herons, and various other birds lay their eggs at intervals of a day or more and commence incubation before laying is complete. This pattern produces an asynchronous hatch (Gibb 1950, Inoue 1985), wherein the youngest typically grow more slowly than their senior nest mates and consequently are more likely to die as nestlings (see reviews in Lack 1954, 1968; Howe 1978; O'Connor 1978; Hahn 1981; Mock 1984a, 1985). These deaths may be due to starvation or to physical abuse inflicted by siblings (reviews in O'Connor 1978, Stinson 1979, Mock 1984a). As in other asynchronously hatching species, the youngest members of a Cattle Egret (Bubulcus ibis) brood typically grow more slowly than their nest mates and are more likely to die as nestlings (Blaker 1969, Siegfried 1972, Fujioka 1984, D. F. Werschkul unpubl. data). Although starvation appears to be the primary cause of death (Siegfried 1972), fights among siblings can result in mortality (Skead 1966; Blaker 1969; Siegfried 1972; Fujioka 1985a, b; Werschkul unpubl. data). The consequences of sibling fights may be direct (the victim dies from injuries) or indirect (the victim becomes too intimidated to feed). The indirect effects of sibling aggression on Cattle Egret nestling growth and mortality • Present address: Department of Zoology, University of Florida, Gainesville, Florida 32611 USA. patterns remain unclear. We attempted to estimate the advantages and disadvantages of aggression among Cattle Egret siblings. By creating asynchronous hatching, parents may facilitate adaptive brood reduction and thereby maximize their own reproductive output in the face of unpredictable food supplies (Lack 1947, 1954). If death of the youngest nestling enhances the survival chances of remaining brood members during food shortages, then the creation of competitive asymmetries among siblings can help maximize parental reproductive success. When food is abundant, all chicks are likely to survive, but if food is limited, the last-hatched siblings can be dispatched efficiently by nest mates. Parents thus may benefit because their surviving offspring gain food that might have gone to the doomed sibs. In many avian species, sibling aggression is not required for brood reduction, size and age differences being sufficient to enforce the starvation of the smallest sibs (Lack 1954, 1968; Howe 1978; Werschkul 1979; Ryden and Bengsston 1980; Hahn 1981; Mock 1984a; Inoue 1985). Theoretically, chicks that rely on nonaggressive brood reduction could avoid the costs of time and energy associated with fighting (Hamilton 1964, Hahn 1981). Therefore, for fighting to be advantageous, victors must accrue compensatory benefits, such as enhanced access to limited food. Mock (1984b, 1985) proposed that when food is delivered in monop768 The Auk 103: 768-776. October 1986 October 1986] Sibling Aggression in Cattle Egrets 769 olizably small units, selection can favor assault on junior sibs by senior nest mates. Sibling aggression appears to be widespread among ardeids (see examples in Blaker 1969, Milstein et al. 1970, Werschkul 1979, Fujioka 1985b, Mock 1985), but little is known about its effects on the distribution of limited food. Detailed demonstrations of the advantages of nestling aggression have been presented only for Great Egrets (Casmerodius albus; Mock 1985) and for a Japanese population of Cattle Egrets (Fujioka 1984, 1985b). Great Egret senior nest mates obtain significantly larger shares of food than younger siblings by (1) gaining more frequent access to the food source (parent's bill), (2) obtaining larger bolus portions, and (3) monopolizing more boluses. Elder Great Egret chicks also intimidate junior sibs and cause them to miss many food offerings. We studied intrabrood competition in Cattle Egrets. Our work parallels Fujioka's (1985b) descriptive study, which was complicated by variable brood sizes and small sample sizes. We used a single brood size, which allowed more robust statistical treatments. We provide new information on how aggression affects the relative feeding success among sibs by investigating the effects of aggression on the frequency of failed feeding attempts and the estimated amounts of food consumed by each chick. We propose the hypothesis that fighting is most intense between the two youngest sibs. This idea is based on the two fundamental attributes of brood reduction in asynchronously hatching birds. First, mortality falls disproportionately on the last-hatched individual, and second, following initial brood reduction, mortality risks drop for the remaining individuals (both points well documented for Cattle Egrets by Blaker 1969, Siegfried 1972, Fujioka 1984, Werschkul unpubl. data; also shown in related species by Werschkul 1979, Mock and Parker 1986). Furthermore, occasional "dominance reversals," in which one of the senior sibs comes to occupy the lowest social position in the linear intrabrood hierarchy, have been observed in other ardeids (Mock unpubl. data). Because the penultimate chick may be most vulnerable to such potentially fatal reversals, it might be expected to invest extra effort in reinforcing its supremacy. Therefore, we predicted that a disproportionate amount of fighting would involve the two youngest sibs. We also predicted that if survival depends on avoiding the most subordinate position, then fights between the penultimate and last-hatched chicks should be more intense (longer, involving more blows) than those of the other two dyads.