Combining multiple data sources to estimate autumn migration timing and hunting exposure of woodcocks in Central Europe

1. During migration, individuals of migratory species occurring in a specific area can include a mixture of local breeding birds and migratory conspecifics from distant populations. In birds that are legally hunted during migration, the unknown extent of mixing between populations can be problematic for management.
2. The Eurasian woodcock (Scolopax rusticola) is a forest-dwelling wader with a wide Palearctic distribution, where migratory individuals from north-eastern Europe mix with central European birds during autumn and winter. Because woodcocks are hunted in autumn and winter in Central Europe, poor knowledge of the timing of autumn migration complicates an assessment of to what extent hunting affects local breeding populations.
3. Here, we quantified the timing of autumn migration of woodcocks breeding in a fully migratory population in Switzerland using ringing and tracking data (n = 110 bird-years). Additionally, we estimated the proportion of non-local woodcocks shot in Switzerland using stable hydrogen isotopes (δ2H) in flight feathers of 842 individuals. We used a multistate mark-resight model to estimate the weekly probability of migration departure from late July until the end of December, and developed a hierarchical assignment model to estimate the probability of shot woodcocks to be of non-local origin, accounting for date and location of the harvest.
4. We found that 95% of the local Swiss population had departed by 7 December (95% credible interval 28 Nov–18 Dec), but that >50% of local woodcocks were exposed to hunting pressure during the legal hunting period in October and November. We estimated that 90.2% (48.8%–99.8%) of shot woodcocks were of non-local origin, but assigning individuals to an origin was highly uncertain.
5. Practical implication . The late autumn migration departure in Switzerland exposes many locally breeding woodcocks to hunting pressure, and the proportion of local birds being shot was mostly determined by the dilution of local birds by more numerous migrants. A delay of the hunting season after November may further reduce the probability of shooting local birds, but to ensure the viability of migratory populations the hunting effort should be coordinated along the entire flyway.