We also sampled fall migrants along the western and eastern flyways to collect tail feathers. In autumn, tail feathers either have grown in the nest for first-calendar-year birds or have been molted on the breeding grounds after reproduction and before fall migration for older birds. Hence, fall migrants have tail feathers grown on their breeding grounds, so we used the deuterium concentration in these summer-grown tail feathers to assign fall migrants to potential breeding grounds. We sampled migrants in France in August to September of 2012 to 2015 in two ways: first, wild migrants were captured, ringed, and released by ringers, and second, wild migrants were captured by hunters but seized by police, placed in a wildlife rescue center, and sampled at ringing before release into the wild. Seized birds comprised both wild birds recently captured and live dummies kept in captivity for 1 year or more. Seized birds were sampled (one tail feather; n = 274) when being ringed at the care center just before being released in the wild. We divided feather data from these seized birds in three groups: (i) first-calendar-year individuals (n = 34; aged using a combination of molt contrasts, body feather streaking, and characteristic pointed tail feathers; these birds hatched in the year, so in September, they have retained feathers grown in the wild in the summer at their hatching site), (ii) attested decoys [n = 21, feathers grown in captivity, including all individuals with coloration anomalies (one or more white secondary or primary, fuliginous body coverts; this sample also includes one male kept in a care center for more than 1 year)], (iii) and unknown status (n = 179; which should be a mix of nonobvious decoys and wild recently captured adults). Whatever the status of all individuals, they all have hatched in the wild (there is no captive breeding of the species), but their feathers have grown either in the wild on their hatching/breeding grounds (if recently caught) or in captivity (for nonobvious decoys).

Along the eastern flyway, we captured fall migrants in Kuwait in September 2014 and spring migrants in Kuwait and Israel in April 2015. We collected a tail feather from each bird, first to obtain DNA to assign migrants to a geographically defined genetic breeding cluster. We also used deuterium concentration in the summer-grown tail feathers to assign fall migrants to potential breeding grounds. Spring migrants have body coverts molted on the wintering grounds, so spring migrants captured in Israel and Kuwait also provided additional cover feathers for identifying potential wintering grounds of birds following the eastern flyway.

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