Both Nothern and the similar Southern Giant-Petrel (Macronectes giganteus) are fairly common in South Georgia with Northern Giant-Petrel somewhat more common. Northern breed earlier than Southern and hybridization is limited (Brown et al. 2015). This situation differs from Macquarie Island where both species nest with different breeding schedules and no hybridization. Formerly the two species were lumped. The split into two species was proposed by Bourne & Warham in 1966 based on their observations at Macquarie.
Both species are about the same size and from a distance they can be mistaken for albatrosses. Southern Giant-Petrel is distinguished by its greenish bill tip vs. a reddish tip in M. halli. However this distinction can be difficult to see in the field depending on lighting and distance. Another difference is M. giganteus is polymorphic occurring in both dark and white forms, while M. halli occurs only in a dark morph. White morph M. giganteus are said to comprise less than 10% of the population. Canon PowerShot SX50 HS.
References:
Brown RM, Techow NMSM, Wood AG, Phillips RA (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(3): e0121688. https://doi.org/10.1371/journal.pone.0121688
Marchant, S. & Higgins, P.J. (coordinating editors) 1990. Handbook of Australian, New Zealand & Antarctic Birds. Volume 1, Ratites to ducks; Part A, Ratites to petrels. Melbourne, Oxford University Press .
Szabo, M.J. 2013. Northern giant petrel. In Miskelly, C.M. (ed.) New Zealand Birds Online. www.nzbirdsonline.org.nz