Knowing how long animals live is really central to understanding and determining the status of a population. But it is not easy gathering this information, especially for animals that live a long time and need to be studied for many decades. An exciting observation at Gardner Island is shedding some light on longevity of south polar skuas.
On 23 November 2015 a trio of Davis expeditioners (Vicki Heinrich, Louise McMahon and Clive McMahon) sighted a banded south polar skua (Stercorarius maccormicki) on a trip to Gardner Island. Gardner Island is a small island with a large population of breeding Adélie penguins just off Davis station in Eastern Antarctica (Prydz Bay region, 68°35’ S, 77°52’ E). From a set of zoomed-in photographs and some post processing magic we were able to gather some vital information from the band including the number and information on who to contact. It was immediately obvious that the skua was not a local given that information on the band asked observers to contact the US Wildlife Service. This bird was sighted again on 13 December 2015, guarding its nesting partner. Details about the band and photos were sent to an American colleague who replied that the skua was banded as a chick at Cape Crozier on Ross Island in the Ross Sea (77°27’ S, 169°14’ E) around 1968–1970, and was re-banded at Cape Crozier in the early 1980s. This bird is at least 45 years old, has relocated approximately 5500 kilometres away from its natal colony and is still breeding! This sighting on Gardner Island and a sighting of a skua at Davis station back in 2007, also a bird banded at Cape Crozier, is especially interesting as it adds to the growing information that indicates that philopatry (the tendency of an organism to stay in, or return to, its home area) among seabirds is a myth. With respect to skuas, a detailed study of skua demography at Cape Crozier indicated that that colony was maintained by immigration, birds raised elsewhere being attracted by the bounty in food at Crozier. A few Crozier-raised skuas, however, have been sighted elsewhere, including Pointe Geologie in Adélie Land (Dumont d’ Urville).
Some interesting skua biology
Skuas tend to be monogamous and both parents share incubation, chick rearing and nest and chick defence. The nests are bare scrapes where most often two greenish eggs are laid some time between November and December. The eggs hatch after 26–34 days, and the chicks fledge after a further 45–50 days. Young birds return to their natal breeding colonies after about four years but don’t breed until aged between seven and nine years.
While it is difficult to sex skuas observations from the long-term study at Cape Crozier indicate that females are larger and lighter in colour than the males.
Most of what we know about skua demographics comes from a long-term capture-mark-recapture study in the Ross Sea at Cape Crozier*. From this pioneering study we know that juvenile (to age four when the birds return to the breeding colonies) and adult survival is high, over 9 percent. Such high survival rates and the fact the birds probably live a long time combine to ensure that most populations are fairly stable. From our observations at Gardner Island we know that that south polar skuas can live to at least 45 years, approximately the age of banded skuas remaining at Cape Crozier. As we note above, we also know that skuas are philopatric (return to previous nest) once they begin to breed but that young, pre-recruits can be attracted to move from their natal colony to breed elsewhere.
*Ainley, D.G., Ribic, C.A. & Wood, R.C. (1990) A Demographic Study of the South Polar Skua Catharacta maccormicki at Cape Crozier. Journal of Animal Ecology, 59, 1–20.