Not one but two! Two-egg Southern Giant Petrel clutches spotted on Nelson Island.

A Southern Giant Petrel sits alongside its two chicks in the nest; photograph by Júlia Finger

Chilean seabird ecologist Júlia Finger has provided ACAP Latest News with an account of a rare sighting of two two-egg Southern Giant Petrel clutches spotted at Harmony Point, Nelson Island in maritime Antarctica. The clutches were observed whilst Julia and fellow team members were conducting fieldwork for a Chilean Antarctic Institute (INACH) project in the austral summer of 2022.

When asked about her reaction to the discovery Júlia stated, "I've been working with giant petrels for the last 10 years, so it was a huge surprise to see two-egg clutches for the first time after all these years. But if there's one thing that giant petrels can do well is to surprise us with novel behaviours (which later I found iweren't that novel)! The team was excited with the unusual find and also happy to see that the two chicks looked healthy and well-nourished. We could only hope for a two-chick fledge!"

Her official record states in translation: 

On 23 January 2022 we first observed two Southern Giant Petrel Macronectes giganteus nests with two-egg clutches at Harmony Point, Nelson Island (maritime Antarctica). One nest had two chicks about two weeks old. One was younger than the other and was being guarded underneath the body of the male. The other was lying beside the adult. Chicks had similar body size to other chicks at the area. The other nest had two eggs, but we believe at least one was spoiled because it had a crack. Also, hatching was two weeks overdue. As we were about to leave the area a few days later, we did not check if the pair were able to successfully raise two chicks until fledging.

Two-egg clutches are rare for the species. In 1962, John Warham described the frequency of occurrence to be 0.14 to 0.18% (4 - 5) of all nests at Macquarie Island.  Harmony Point holds a population of ca 480 nests, and after checking all nests from a distance, we assume the frequency of two-egg clutches in this population to be at least 0.41%. Warham also states that the birds were capable of covering both eggs during incubation.  However, he wasn't able to confirm if the brood patch covered both eggs. Nevertheless, three of the nests Warham found did not hatch and in the other two nests that did, neither of the pairs succeeded in rearing both chicks. Other sites with records of two-egg clutches of Southern Giant Petrels are Îles des Pétrels in Terre-Adélie, Antarctica  (Prévost 1953), where one nest among 120 was found to have two eggs; and Anvers Island, Western Antarctic Peninsula, where five nests were recorded (Shaughnessy 2017).

The second Southern Giant Petrel clutch containing two eggs; photograph by Júlia Finger

Two-egg clutches are also known to occur among albatrosses, with ACAP covering an account from Chris Jones and Michelle Risi of an Atlantic Yellow-nosed Albatross pair with a two-egg clutch on Gough Island in 2015. 

With thanks to Júlia Finger for translating her account.

REFERENCES:

Ryan, P.G., Cuthbert, R. & Cooper, J. 2007.  Two-egg clutches among albatrosses. Emu 107: 210-213.

Warham, J. 1962. The biology of the Giant Petrel Macronectes giganteus.  Auk 79: 139-160.

Prévost J. 1953. Note sur l'écologie des pétrels de Terre Adélie. Alauda 21: 205-222.

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. [click here for ACAP review]

2 November 2022