Northern and Southern Giant Petrels (Macronectes halli and M. giganteus) breed sympatrically on five island groups: South Georgia, the Prince Edward Islands, Iles Crozet, Iles Kerguelen and Macquarie Island. Hybridization between these species has been reported on some of these islands. In a recent paper, Brown et al. (2015) provide the first evidence that the hybrid offspring of these species are fertile and that back-crossing occurs. A known hybrid male, that was ringed as a chick, was obserbed breeding and successfully raised five chicks with a female Northern Giant Petrel.
More information on hybridization in the Procellariiformes can be found here.
Brown, R. M., Techow, N. M. S. M., Wood, A. G. & Phillips, R. A. (2015). Hybridization and Back-Crossing in Giant Petrels (Macronectes giganteus and M. halli) at Bird Island, South Georgia, and a Summary of Hybridization in Seabirds. Plos One 10.