Nest Survival in a Large Ibis-Heron Colony during a Three-Year Decline to Extinction

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Abstract
I studied nest survival of seven species of Ciconiiformes on a dredge-spoil island in South Carolina, USA. In 1984, about 24,450 pairs nested in the colony. By 1986, only 3,682 pairs remained, and the site was abandoned in 1987. White Ibis (Eudocimus albus) numbers declined rapidly, while heron populations were stable, or decreased slightly. In 1985 the total nest success (percentage of nests producing climb-out young) of herons (47%) did not differ from that reported from other mixed-species colonies in the Southeast. In 1986, the nest success of most species was lower than that reported for any other viable colony. Nest survival of White Ibises was significantly lower than that of most of the heron species throughout the three years, and in 1986, ibises produced no fledglings, while total nest success for all heron species was 18%. Most nest mortality was caused by Fish Crows (Corvus ossifragus) and large mammals. Interspecific differences in nest survival were correlated with differences in nest heights and nest stability, characteristics that probably reflect ease of access to predators. In most White Ibis-heron colonies, ibises have lower success than herons. Other than the higher mortality of White Ibis of White Ibis nests from drought and flooding, their low survival, compared to heron nests in the same colony, may be explained in part by different susceptibility to predation. Although most species were subjected to intense predation for two years, they did not abandon the colony immediately, and the lack of response of these populations appears to be related to (1) site tenacity of more successful, nuclear species and (2) the limited opportunities for information exchange in a large colony dispersed through a dense canopy.

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