Harris, M.P. 1970. Abnormal migration and hybridization of Larus argentatus and L. fuscus after interspecies fostering experiments. Ibis 112: 488-498.
Between 1962 and 1966 eggs of Larus argentatus and L. fuscus were interchanged andalmost 900 young were reared by the wrong species. Many of these cross-fostered young were later recovered or retrapped on and away from the colonies.
The British population of argentatus is sedentary and ringed birds are not recovered outside Britain, whereas fuscus normally migrates. Many cross-fostered argentatus migrated to France, Spain and Portugal, areas where fuscus is common, but they did not migrate as far as the control fuscus. It is possible that these cross-fostered argentatus had followed their foster parents when these migrated, but this is unlikely as the cross-fostered fuscus also migrated although their foster parents would have remained in Britain.
Despite wide ecological and behavioural overlaps, interbreeding between L. argentatus and L.fuscus is exceedingly rare. However, as a result of cross-fostering experiments, 31 and 40 mixed pairs were found on Skokholm in 1968 and 1969 respectively. Although some of the birds involved were unringed it is probable that all the adults in mixed pairs had been cross-fostered. Other cross-fostered birds were found mated with their own species and it appears that the sex of the imprinted birds was important. Female gulls will usually only mate with males of their own species, or in the case of the cross-fostered birds, with males of their foster species. Males will mate with either species.
Evidence is given that suggests that the colour of the mantle and wings is important in species recognition at long range, and the colour of eye-ring and join of the mandibles for recognition at short range. The role of voice is uncertain but general behaviour is probably unimportant.
The original paper can be purchased here
A later paper is downloadable here Dr M. P. Harris, C. Morley & G. H. Green (1978): Hybridization of Herring and Lesser Black-backed Gulls in Britain, Bird Study, 25:3, 161-16