Conservation of Large, Nomadic Populations of White Ibises (Eudocimus albus) in the United States


We compiled published and unpublished records of large nestings of White Ibises (Eudocimus albus) in the United States between 1930 and 1993. The resulting database provides evidence of at least four major geographic shifts of the breeding population of White Ibises during the period. Banding returns and colony histories demonstrate that the movements were not migratory. The extremely fast growth of most White Ibis breeding colonies indicates that local recruitment could not have fueled the large increases in breeding numbers in colonized areas. Ibises seem to colonize wetland areas that have good feeding resources and to abandon areas that lose the ability to supply this resource. Mechanisms of attraction to new sites and repulsion from degraded ones may work alone or in tandem. Degradation of breeding sites frequently occurs through natural processes, such as hurricanes, stochastic weather patterns, and wetland conditions, and the nomadic behavior of White Ibises appears to be an obligate life-history feature. Although surveys throughout the range of the U.S. population of White Ibises have never been comprehensive, available records indicate minimum breeding populations of 125,000 pairs in 1933, 170,000 pairs in 1976, and 51,000 pairs in 1991. The U.S. population as a whole appears to be decreasing. White Ibises share a suite of population, social, and movement characteristics with a number of nomadic species. Conservation strategies for these species may differ fundamentally from those targeting more sedentary species. Nomadic species may typically depend on large populations to find food and to stimulate breeding and are therefore likely to decline abruptly and unpredictably as habitat is lost or degraded. Rapid population declines may occur at population levels well above those predicted by genetic and minimum viability models. Because of its nomadic habit, the survival of this species may depend on a regional planning approach. To conserve this species we recommend a continuing commitment to the monitoring, assessment, and preservation of a mosaic of fresh and estuarine wetlands in the southeastern U.S.; the maintenance of natural disturbances needed to produce pulses of food in these wetlands, and the coordination of management efforts throughout the US., as well as between the U.S. and Cuba.

Conservation Biology