Dataset: Sanderlings (Calidris alba) of two different age classes at the moment of individual colour-marking at four non-breeding sites that were or were not observed during migration following the capture

The data file contains data of Sanderlings (Calidris alba) of two different age classes at the moment of capture and individual colour-marking at one of four wintering areas that were or were not observed during migration following the capture. Each individual is indicated with a unique number in column “BirdID”. The column “country” indicates which of the four wintering areas (as depicted in Fig. 1 in the manuscript), with GB indicating England, PT indicating Portugal, MR indicating Mauritania and GH indicating Ghana. The “age” of each bird was either juvenile (<1 year old) or adult (>1 year old). Whether an individual was observed during migration, i.e. at least 2 latitudinal degrees north of its average winter location between 15 March – 15 October, in the migration period following capture is indicated with a 0 (not observed) or 1 (observed) in the column “observed”. Further details can be found in the methods section in the manuscript.