Today’s blog is by Alexis Osborne, Field Assistant on Gough Island

We have spent two years on Gough Island and have observed many unusual things, but Gough never ceases to amaze us. In December 2018, while monitoring the Critically Endangered Gough bunting Rowettia goughensis, I noticed a Brown skua Stercorarius antarcticus incubating an unusual egg at Gonydale, an inland region at c. 500 m elevation on Gough Island.

The egg was uniformly white (with no blotches) and was notably smaller than a typical skua egg. Skua eggs are usually pale brown with darker brown spots and blotches. Upon closer inspection the skua was intensely defending the nest. What a bizarre site to see.

Brown Skua calling loudly while defending an assumed MacGillivray’s Prion egg at Gonydale on Gough Island © Alexis Osborne

This behaviour reminded my other team mates, Chris and Michelle, of similar observations when they studied skuas at Marion Island in 2017, where they found instances of skuas incubating a Macaroni penguin Eudyptes chrysolophus egg and a Kelp Gull Larus dominicanus egg on another occasion. This behaviour had never been documented on Gough Island. On the next visit to the skua nest I measured the egg to compare it with egg dimensions of other species that breed on Gough. Based on the egg dimensions, time of year and location, we concluded that the foreign egg was likely from a MacGillivray’s Prion Pachyptila macgillivrayi (one of the prey species of Brown Skuas).

This situation may remind some readers of cuckoos who often lay their eggs in the nests of other birds, tricking these other birds to raise their chicks for them. However, it is highly unlikely that a MacGillivray’s Prion (or any other bird on Gough for that matter) is beguiling enough to trick a skua like that.

Brown Skua chick next to a typical skua egg in the pipping stage on Marion Island © Christopher Jones

There are three other suggestions that have been proposed to explain this phenomenon:

1) a food item carried to or regurgitated at the nest mistakenly joined the clutch, and are incubated as a would-be-egg;

2) egg-like items (pseudo-eggs) are rolled from the nest surrounds into the nest bowl to increase the clutch size. Such ‘pseudo-eggs’ are thought to be important incubation stimuli especially for birds with larger clutches, for example where gulls incubating three-egg clutches have been seen to resettle less frequently with longer incubation bouts;

3) pseudo or foreign eggs are mistaken for eggs that have rolled out of the nest bowl, and that the fitness cost of ignoring them is higher than the cost of incubating a potentially infertile egg.

This observation on Gough Island was coupled with Chris and Michelle’s prior observations on Marion Island and lead us to write a short scientific paper on the subject. All three instances were probably cases of mistaken food, since all foreign eggs were eggs of prey species, where the skua raided a nearby nest and managed to carry the egg unbroken to its own nest, whereupon the incubation instinct took over and the egg was incorporated into its clutch.

Two young Brown Skua chicks with a suspected Kelp Gull egg on Marion Island © Christopher Jones

This behaviour appears to be unproductive since none of the foreign eggs mention actually hatched any chicks and it may be a behaviour associated with inexperienced breeders. This unusual observation has shown us that there is still much more to discover on Gough Island.

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The Gough Island Restoration Programme is being carried out by the RSPB in partnership with Tristan da Cunha, BirdLife South Africa and the Department of Environmental Affairs in South Africa, and Island Conservation. The programme is part-funded by the RSPB, the UK Government, the National Fish and Wildlife Foundation and other generous individuals and organisations.

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