||The White-breasted Thrasher (Endangered), a facultative cooperative breeder endemic to Martinique and St. Lucia, was rare before resort development on St. Lucia destroyed habitat containing 34% of the global population. I studied thrashers within undamaged forest and habitat fragments on the development site in 2006-2008 in order to document immediate effects on the population's viability and to understand the relationship between habitat loss and cooperative breeding. Thrashers continued to occupy the landscape in high densities, nesting in fragments as small as 0.23 ha, though data suggests increased post-disturbance emigration from the development site. Short-term reproductive success did not change, but due to a significant increase in cooperative breeding after habitat loss, individual success is decreasing. This research further demonstrates the plasticity of cooperative breeding systems as well as provides a broad dataset for guiding thrasher management and future decisions about further development and habitat protection on St. Lucia.