Apparent annual survival of staging ruffs during a period of population decline: Insights from sex and site-use related differences

The ruff Philomachus pugnax, a lekking shorebird wintering in Africa and breeding across northern Eurasia, declined severely in its western range. Based on a capture-mark-resighting programme (2004–2011) in the westernmost staging area in Friesland (the Netherlands), we investigated changes in apparent annual survival in relation to age and sex to explore potential causes of decline. We also related temporal variation in apparent survival to environmental factors. We used the Capture- Mark-Recapture multievent statistical framework to overcome biases in survival estimates after testing for hidden heterogeneity of detection. This enabled the estimation of the probability to belong to high or low detectability classes. Apparent survival varied between years but was not related to weather patterns along the flyway, or to flood levels in the Sahel. Over time, a decline in apparent survival is suggested. Due to a short data series and flag loss in the last period this cannot be verified. Nevertheless, the patterns in sex-specific detectability and survival lead to new biological insights. Among highly detectable birds, supposedly most reliant on Friesland, males survived better than females (Φ_HDmales = 0.74, range 0.51–0.93; Φ_HDfemales = 0.51, range 0.24–0.81). Among low detectable birds, the pattern is reversed (Φ_LDmales = 0.64, range 0.37–0.89; Φ_LDfemales = 0.73, range 0.48–0.93). Probably the staging population contains a mixture of sexspecific migration strategies. A loss of staging females could greatly affect the dynamics of the western ruff population. Further unravelling of these population processes requires geographically extended demographic monitoring and the use of tracking devices.