Demography and Breeding Ecology of the Critically Endangered Montserrat Oriole

Abstract. The Montserrat Oriole (Icterus oberi) is a critically endangered species, confined to a small range in the hill forests of the volcanic island of Montserrat in the eastern Caribbean. From 1998 to 2005 we studied its breeding biology and survival of adults, finding that the Montserrat Oriole has a smaller clutch, more extended parental care, and higher adult survival than do orioles nesting in the North Temperate Zone. Adults' probabilities of survival varied by year from 0.60 to 0.76 but were similar for both sexes. Average clutch size was 2.6 eggs (± 0.04 SE), and post-fledging parental care was 40 ± 5 days. We found nest success of 29% (n = 275 nests), and 87% of nest failures were due to predation by either introduced rats (Rattus sp.) or the native Pearly-eyed Thrasher (Margarops fuscatus). Most pairs initiated several nesting attempts after both failed and successful first broods, leading to an overall annual productivity of 1.2 fledged chicks per pair. Despite being able to raise up to thr...