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Stopover of migrating birds: simultaneous analysis of different marking methods enhances the power of capture-recapture analyses

Salewski, Volker, Thoma, Marco, Schaub, Michael
Journal für Ornithologie 2007 v.148 no.1 pp. 29-37
Palaearctic region, Passeriformes, color, data collection, migratory birds, models, oases, probability, stopover sites, Mauritania
Analyses of stopover parameters of migrating birds with Cormack-Jolly-Seber (CJS) capture-recapture models often suffer from low precision due to sparse data sets. Low recapture rates result in low power to detect violations of the underlying assumptions and factors influencing stopover behaviour. We studied stopover behaviour of Palearctic migrant passerines in an oasis in Mauritania, West Africa. Using capture-recapture data and systematic observations of colour-ringed birds, we analysed the effect of increased sample size on probability of stay and recapture probability and the influence of a possible trap response on these parameters. We analysed capture-recapture data with the conventional CJS model and compared the results with those from a multistate model using in addition resighting data. The analyses including resighting data resulted in a higher precision of the estimates of the probabilities of stay compared to analyses using only capture-recapture data of the same individuals. Moreover, the power to detect transients was substantially enhanced. Capture had no effect on the estimates of probability to stay and recapture probability; birds did not leave the stopover site or avoid nets as a reaction to capture. The estimates of probability of stay were up to 15.7% higher when resighting data were included, probably due to the higher power to detect transients and the elimination of the bias induced by non-random temporary emigration when both data types are considered. As a consequence, stopover duration would have been underestimated when only the capture-recapture data were available. We conclude that additional resightings of colour ringed birds can produce more accurate results needed for enhancing our understanding of stopover ecology of migrants.