Chapter 5 - Monitoring Recruitment and Abundance of the Aransas-Wood Buffalo Population of Whooping Cranes: 1950–2015

Abstract

The Aransas-Wood Buffalo Population of Whooping Cranes (Grus americana), has been enumerated using aerial surveys on its wintering grounds since winter 1950–51. When aerial surveys began, the flock consisted of fewer than 30 individuals that wintered on the Blackjack Peninsula and Matagorda Island in Aransas and Calhoun counties of Texas, USA. Since that time, the Aransas-Wood Buffalo flock has dramatically increased its population size and the area occupied during winter. Sixty-five years of aerial surveys have provided much information that has aided whooping crane conservation. Data collected during aerial surveys have been used to estimate population size and trends, estimate recruitment and survival rates, track the population’s range expansion, identify areas in need of conservation, and have been the primary means of monitoring the recovery of this endangered species. Despite the importance of these surveys and the widespread use of the data, a formal protocol, including objectives, methods, sampling frame, data analyses, and reporting procedures, was not completed until March 2014. Survey methods should be driven by objectives, so that the data will be collected in a manner designed to meet particular information needs, and ensure that conclusions drawn from them are valid and defensible. Here we attempt to provide a posthoc description of the aerial survey methods used at the Aransas NWR from winter 1950–51 through winter 2010–11, drawing heavily from Stehn and Taylor’s (Stehn, T.V., Taylor, T.E., 2008. Aerial census techniques for whooping cranes on the Texas coast. In: Proceedings of the North American Crane Workshop, vol. 10, pp. 146–151) account of the aerial survey methods employed from winter 1982–83 through winter 2010–11. We discuss how aerial survey data have been used to understand Whooping Crane demographics and population dynamics and to improve Whooping Crane conservation efforts. In addition, we provide a description of the improvements made to the survey beginning in winter 2011–12. Finally, we discuss how these recent improvements (e.g., hierarchical distance sampling) yield more rigorous estimates of abundance and improve planning and conservation efforts by associating local Whooping Crane abundance with environmental conditions.