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.