Brown pelican siblicide and the prey-size hypothesis

Abstract

We asked whether the brown pelican (Pelecanus occidentalis) practices facultative brood reduction and tested two predictions of Mock's (1985) prey-size hypothesis: (1) if chicks take food directly from the parental mouth, nestmates should compete aggressively;(2) aggression between nestmates should increase during the developmental transition from indirect feeding (parents deposit food on the substrate) to direct feeding (parents pass food from mouth to mouth). Eggs in two-egg and three-egg clutches were laid and hatched 2 days apart. Junior (second-and third-hatched) chicks, fed less than their nestmates, grew more slowly and died more frequently. Of non-hatchling victims 74% were underweight, 55% bore peck wounds and 43% were found outside the nest, implying starvation, aggression and expulsion from the nest. Aggression occurred in all observed sibships, and junior chicks were subordinate in eight of ten cases. A progressive transition from indirect to direct feeding occurred during the 30 days after hatching, but there was no evidence of an increase in pecking during the transition. In conclusion, the brown pelican practices facultative brood reduction, and chick loss is mediated by sibling aggression as predicted by the preysize hypothesis. Pecking during indirect feeds seems to contradict the prey-size hypothesis, but may serve partly to establish a relationship of dominance-subordinance.

Growth of Brown Pelican Nestlings Exposed to Sublethal Levels of Soft Tick Infestation

“…The growth rates we measured for both body mass and culmen length are similar to other available data on Brown Pelican growth (Schreiber 1976, Pinson andDrummond 1993). Significant relationships between hatching order and growth rates are common among asynchronously hatching species (Schew and Ricklefs 1998), have been observed previously in Brown Pelicans (Schreiber 1976, Pinson and Drummond 1993, Shields 1998, and were observed in each colony-year in this study.…”

“…It is typical for a breeding pair to produce a single three-egg clutch per season. Nestlings hatch asynchronously, with the first-hatched nestling usually benefiting from a size advantage and dominant position over its siblings (Pinson andDrummond 1993, Shields 2002). Nestlings are altricial and spend at least the first three weeks of development confined to the nest.…”

A kaleidoscope of mammal, bird and fish: habitat use patterns of top predators and their prey in Florida Bay

“…This mechanism of prey division may provide a less competitive foraging niche for cormorants in the mud and mudbank habitats of Florida Bay and help to explain their continued use of these habitats. However, the prey items of osprey (250 to 350 mm) and pelicans (64 to 257 mm) are within the same size range as dolphins, which increases the potential for competition between these predators for mullet and/or catfish (Poole 1989, Pinson & Drummond 1993. The contrasting habitat space of foraging and nonforaging dolphins indicates that the behavior states of all non-foraging dolphins (travel, socialize, rest or unknown) in Florida Bay do not require the same habitat characteristics as foraging habitat.…”

Parent–offspring conflict in avian families

“…In contrast, Brown Booby parents are apparently rarely or never able to feed both of their chicks to fledging, and the subordinate role implies a 90% drop in probability of fledging (De la Fuente, unpublished data), so junior chicks never accept subordination and, given the opportunity, fight all-out to kill their elder sibling. In between these two booby extremes are the Great Egret, the Brown Pelican (Pelecanus occidentalis) and the Cattle Egret, in which subordination involves cuts in fledging probability of 34, 62 and 76%, respectively, (Mock and Parker 1986;Pinson and Drummond 1993;Siegfried 1972), and whose subordinates resist domination by frequent attacking and fighting back but not by all-out aggression. The agonistic strategy of the dominant chick may be an evolutionary response to the strategy of the subordinate; a dominant Blue-footed Booby seems to husband its own inclusive fitness by attacking and threatening just enough to maintain its compliant broodmate's submissiveness, but a Brown Booby must kill its broodmate before it is big enough to mount a (predictable) lethal assault (Drummond 2006).…”