Peter Shaughnessy (South Australian Museum, North Terrace, Adelaide, Australia) has published in the open-access journal Marine Ornithology on evidence suggesting Southern Giant Petrels Macronectes giganteus are able to lay a two-egg clutch.
The paper's abstract follows:
“Females of the order Procellariiformes most often produce single-egg clutches. At Macquarie Island (54°S, 159°E) in 1959 during a field study of Southern Giant Petrels Macronectes giganteus, four or five nests (0.14–0.18% of all nests) contained two eggs or two chicks (Warham 1962). This species occurs in two plumage forms, a dark phase and a white phase. Inheritance of these forms is controlled by a single autosomal gene with two alleles, with white phase dominant to dark phase. At Macquarie Island in 1959, one nest contained two white phase chicks brooded by a white-phase adult, which Warham (1962) believed resulted from a two-egg clutch rather than from polygyny. Analyses using probabilities based on the inheritance pattern of plumage phases in Southern Giant Petrels and the frequency of white-phase birds at Macquarie Island in 1959 indicate that it was almost seven times more likely that the two white-phase chicks in the nest brooded by a white-phase adult resulted from a clutch of two eggs rather than from polygyny.”
White-phase Southern Giant Petrel, photograph by Markus Ritz
Shaughnessy, P.D. 2017. A two-egg clutch or polygyny? Two white-phase chicks in the nest of a Southern Giant Petrel Macronectes giganteus at Macquarie Island. Marine Ornithology 45: 43-46.
John Cooper, ACAP Information Officer, 13 February 2017