Whooping crane demographic responses to winter drought focus conservation strategies

Focusing conservation strategies requires identifying the demographic parameters and environmental conditions affecting the growth of animal populations most. Therefore, we examined relationships between population demographics and winter drought (1950-2011) for endangered whooping cranes (Grus americana) wintering in Texas, USA. We modeled winter loss and its contribution to annual mortality as functions of winter drought, determined recruitment needed to maintain population growth after drought, and identified which demographic parameters underpin this population's growth. Previous research assumed winter loss (i.e., birds missed in subsequent surveys) represented mortality. We show that loss includes temporary emigration to upland habitats, early migration, and incomplete detection. Despite this, we maintained this assumption to evaluate the relevance of winter mortality to population growth. We found that winter loss (β^=-0.308, SE = 0.042) and its contribution to annual mortality (β^=-0.318, SE = 0.047) increased with drought severity (Palmer hydrological drought index; PHDI). Given average recruitment (0.145, SD = 0.090), this population increases 1.2% (95% CI = −2.9% to 4.2%) after extreme drought (PHDI = −4). No recruitment must occur for 3 years with moderate to severe drought (PHDI < −2.5) to delay species' recovery ≈7 years. This scenario has not occurred since population monitoring began in 1938. Of the demographic parameters we examined, winter loss explained population growth least (14.4%; 95% CI = 3.6-35.8%), and it was partially compensatory. Breeding-migratory mortality explained 42.2% (95% CI = 19.1-61.5%) of population growth and recruitment 49.9% (95% CI = 20.6-75.2%). Our results focus conservation on breeding and migratory periods, and deemphasize winter mortality and drought. On the wintering grounds, conservation of whooping cranes should emphasize maintaining coastal, upland, and interior habitats for this population.