RESEARCH ARTICLE Hybridization and Back-Crossing in Giant Petrels (Macronectes giganteus andM. halli) at Bird Island, South Georgia, and a Summary of Hybridization in Seabirds

Hybridization in natural populations provides an opportunity to study the evolutionary pro-cesses that shape divergence and genetic isolation of species. The emergence of pre-mat-ing barriers is often the precursor to complete reproductive isolation. However, in recently diverged species, pre-mating barriers may be incomplete, leading to hybridization between seemingly distinct taxa. Here we report results of a long-term study at Bird Island, South Georgia, of the extent of hybridization, mate fidelity, timing of breeding and breeding suc-cess in mixed and conspecific pairs of the sibling species,Macronectes halli (northern giant petrel) andM. giganteus (southern giant petrel). The proportion of mixed-species pairs var-ied annually from 0.4–2.4 % (mean of 1.5%), and showed no linear trend with time. Mean laying date in mixed-species pairs tended to be later than in northern giant petrel, and al-ways earlier than in southern giant petrel pairs, and their breeding success (15.6%) was lower than that of conspecific pairs. By comparison, mixed-species pairs at both Marion and Macquarie islands always failed before hatching. Histories of birds in mixed-species pairs at Bird Island were variable; some bred previously or subsequently with a conspecific partner, others subsequently with a different allospecific partner, and some mixed-species pairs re-mained together for multiple seasons. We also report the first verified back-crossing of a hy-brid giant petrel with a female northern giant petrel. We discuss the potential causes and evolutionary consequences of hybridization and back-crossing in giant petrels and summa-rize the incidence of back-crossing in other seabird species