Patchiness in prey levels increases vulnerability of Critically Endangered Northern Bald IbisesGeronticus eremitaon their Syrian breeding grounds

Abstract

SummaryA small colony of Northern Bald IbisGeronticus eremitabreeding in the Syrian desert is the only wild remnant of the eastern population of this ‘Critically Endangered’ species. Since its discovery in 2002, the colony has not increased in size despite being protected on the breeding grounds and exhibiting good productivity until recently. Although it appears that the population is being limited during migration and/or wintering, maintaining maximum breeding productivity is a priority for saving this colony to offset poor survival away from Syria. The ibises have a large home range but forage at a relatively small number of sites within it, despite having access to larger areas of apparently suitable habitat. We sampled potential prey using transects, pitfall traps and searches under stones at sites used by the birds to compare with unused sites. Analysis showed that used sites were twice as rich in vertebrate and invertebrate prey than the surrounding areas. Prey levels halved over the course of the breeding season, but we found that they remained higher in the preferred locations compared with the unused areas. Sites closer to the breeding cliff tended to have lower levels of prey available, which perhaps explained the long commutes that the adults undertook when foraging.This work highlights the vulnerability of the birds at this site. Degradation of the patches they use could reduce food supply below critical levels and alternative sites may be hard for the birds to identify, being relatively scarce. The birds are also vulnerable to shortening of the season in which adequate food is available. There is already little time post-fledging to prepare for migration. Low food availability later in the season makes the birds dependent on key resources found around local reservoirs and these have failed recently. Breeding failures in 2008 and 2009 may have been mitigated by improved feeding conditions on the breeding area. The successful management of the site for ibises should include measures to improve their food security through range management, and possibly reservoir rehabilitation.