Fig. 1: Overview of the willow warbler system.

a Geographic structure of mean δ¹⁵N stable isotopes from winter-grown feathers (n = 64 sites). Color gradient shows extrapolation of δ¹⁵N values from low (orange) to intermediate (green), to high (blue). Low values associate with wintering in W-Africa, high with E-Africa. b Geographic structure for InvP-Ch1. c Geographic structure for InvP-Ch5. d Geographic structure for MARB-a based on a previous and less precise genotyping assay, AFLP-WW2. The data for b and c are from Lundberg et al.¹³ and for a and d from Bensch et al.¹². In b–d, progressing gradient orange-green-blue illustrates higher probability of birds carrying alleles fixed in acredula. e Tracks of allopatric acredula (n = 5) in blue and trochilus (n = 16) in orange. f Tracks of genetic acredula in blue (birds that have the MARB-a and are homozygotes for acredula alleles on invP-Ch1 and invP-Ch5, n = 5) and genetic trochilus in orange (birds that do not have the MARB-a and are homozygotes on invP-Ch1 and invP-Ch5 for trochilus alleles, n = 5). g Tracks of birds from the migratory divide with hybrid genotypes (n = 41). Hollow circles in e–g show estimated longitudes where birds crossed latitude 35°N (Mediterranean Sea). The lines connect individuals to their respective breeding site, Mediterranean crossing site and main winter site. Whiskers around locations in Africa in e–g show standard deviations in longitude and latitude from the main winter site. Crosses in a–d mark sampling sites. Source data are provided as a Source Data file.