Establishing the right period to estimate juvenile proportions of wintering Sanderlings via telescope scans in western Scotland

Telescope scanning of flocks of shorebirds to age individuals by plumage characteristics has increasingly been used to assess the proportion of first-year birds in local populations. To standardise measurements of the proportion of juveniles for a local wintering population it is necessary to know when this can be measured without possible biases due to migration or moult. We investigated how the proportion of juvenile Sanderlings Calidris alba changed between mid-Jul and early Nov 2009–2011 on the Island of Tiree, Scotland. We suggest measuring juvenile proportion of locally wintering Sanderlings in NW Europe between mid-September and the end of October, because migration has terminated by then and juveniles can still be aged using field characteristics. During this time of the year the proportion of juveniles at Tiree was similar between the three years (6–9%).