Population studies assume that individuals belonging to a study population are homogeneous for natal and breeding origin, although this assumption is rarely tested. We tested for heterogeneity in stable–isotope profiles (δ15N, δ13C, δD) of feathers grown in the African winter quarters from a Danish breeding population of adult barn swallows, Hirundo rustica. Deuterium isotope values did not provide useful information on population segregation of wintering swallows in Africa. However, both δ15N and δ13C values showed a clearly bimodal distribution with 6% belonging to one category and the remaining birds belonging to another category, resulting in this population comprising three categories of birds. Adults belonging to the two categories of δ13C isotope profiles differed weakly in morphology for several different characters. The frequency and the size of second broods differed between categories of δ13C isotope profiles. Phenotypes of nestlings from the first brood in terms of tarsus length, body mass and T–cell response differed significantly between the two δ15N isotope categories, suggesting that conditions during winter carried over to the breeding season at least as late as the first brood. Polymorphism can be maintained only if fitness is similar for birds from categories of isotope profiles. We suggest that fluctuating selection or migration–selection balance may maintain the observed polymorphism.