Since hybridization in birds is so rare in nature, it is unusual to have the opportunity to study what happens when two species interbreed. Blue-winged Warblers (Vermivora pinus) and Golden-winged Warblers (V. chrysoptera) produce hybrids wherever they occur together. Furthermore, such hybrids are fertile and there is pairing between these hybrids and both parental species. When we learned that both species breed in the northern Kettle Moraine State Forest, we decided to see what sorts of interactions between the two species were occurring there. Our objective initially was to determine if pairing was random between the species, e.g. were Blue-wings pairing with Golden-wings as frequently as they were with members of their own species.
Ficken, M.S. 1968. Bird hybrids in the Kettle Moraine. Field Station Bulletin 1(2): 7-12.