Diving duck census efficiency based on monitoring of individually marked females: the influence of breeding stage and timing of census.

Capsule: Observations of individually marked females of three diving duck species showed that the stage of breeding season and census month most affected their resighting probability in the breeding season. Aims: To evaluate the efficiency of a diving duck census in the breeding season based on an analysis of the resighting probability of individually marked female ducks. Methods: We used monitoring data from 2004 to 2015 of individually marked females of Common Pochard Aythya ferina, Tufted Duck Aythya fuligula and Red-crested Pochard Netta rufina in South Bohemia, Czech Republic. Using a model selection approach, the resighting probabilities of females were modelled incorporating covariate parameters, such as stage of breeding, census month, year, numbers of counted individuals, individual species, water surface area and proportion of emergent vegetation. Results: We demonstrated the significant effect of the stage of breeding, year and census month on the resighting probability of females, while the number of individuals, water surface area and proportion of emergent vegetation did not contribute to the best models. Moreover, the three investigated species with different local population sizes did not differ in resighting probabilities. The incubation period of the females (i.e. June) corresponded to the lowest detection probability in the study area. Conclusions: Our approach of using individually marked females contributed to modelling to show census accuracy for diving ducks was substantially enhanced by avoiding counting females during the incubation period. Ultimately, these findings are crucial to increase the accuracy of their breeding population size estimation. [ABSTRACT FROM AUTHOR]