Response of Hawaiian Moorhens to broadcast of conspecific calls and a comparison with other survey methods

Accurate population surveys are critical for effective management of species of conservation concern. Traditional visual and aural surveys are ineffective for behaviorally cryptic species such as rails, but incorporating call-broadcast into surveys can increase their detection rate. From 2004 to 2006, we surveyed wetlands (N = 67) on Kauai and Oahu for endangered Hawaiian Moorhens (Gallinula chloropus sandvicensis) with the goal of comparing the effectiveness of visual and aural observations to that of call-broadcast surveys. We evaluated six different Hawaiian Moorhen calls, including the squeal call of young moorhens. We also compared the results of surveys conducted using the broadcast of Hawaiian Moorhen calls to those of extended time (75 min) surveys and surveys where the calls of the North American subspecies of moorhen (G. c. cachinnans) were broadcast. We found that broadcast of Hawaiian Moorhen calls increased detection rates by 56% on Kauai and 30% on Oahu. Territorial and chick-distress calls elicited the greatest response. We also found a nonlinear positive relationship between the estimated population of Hawaiian Moorhens at a wetland and the improvement in detection due to call-broadcast, suggesting social facilitation of responses. Survey periods of 60 min produced results similar to those obtained using call-broadcast. However, long survey periods require more time than call-broadcast surveys and increase the likelihood of double counting individuals. Broadcast of the calls of the North American subspecies of moorhen failed to increase detection rates above those obtained using visual and aural surveys. Our results suggest that the population of Hawaiian Moorhens is larger than previously estimated, but is likely well below the 2000 individuals recommended for removal from the Endangered Species list. We recommend the use of call-broadcast during surveys of Hawaiian Moorhens to improve estimates of population sizes and trends.