South Polar Skua breeding populations in the Ross Sea assessed from demonstrated relationship with Adélie Penguin numbers

Deborah J. Wilson; Phil O’B. Lyver; Terry C. Greene; Amy L. Whitehead; Katie M. Dugger; Brian J. Karl; James R. F. Barringer; Roger McGarry; Annie M. Pollard; David G. Ainley

doi:10.1007/s00300-016-1980-4

USGS · 70192085

South Polar Skua breeding populations in the Ross Sea assessed from demonstrated relationship with Adélie Penguin numbers

Deborah J. Wilson·Phil O’B. Lyver·Terry C. Greene·Amy L. Whitehead·Katie M. Dugger·Brian J. Karl·James R. F. Barringer·Roger McGarry·Annie M. Pollard·David G. Ainley

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Abstract

In the Ross Sea region, most South Polar Skuas ( Stercorarius maccormicki ) nest near Adélie Penguin ( Pygoscelis adeliae ) colonies, preying and scavenging on fish, penguins, and other carrion. To derive a relationship to predict skua numbers from better-quantified penguin numbers, we used distance sampling to estimate breeding skua numbers within 1000 m of 5 penguin nesting locations (Cape Crozier, Cape Royds, and 3 Cape Bird locations) on Ross Island in 3 consecutive years. Estimated numbers of skua breeding pairs were highest at Cape Crozier (270,000 penguin pairs; 1099 and 1347 skua pairs in 2 respective years) and lowest at Cape Royds (3000 penguin pairs; 45 skua pairs). The log–log linear relationship ( R 2 = 0.98) between pairs of skuas and penguins was highly significant, and most historical estimates of skua and penguin numbers in the Ross Sea were within 95 % prediction intervals of the regression. Applying our regression model to current Adélie Penguin colony sizes at 23 western Ross Sea locations predicted that 4635 pairs of skuas now breed within 1000 m of penguin colonies in the Ross Island metapopulation (including Beaufort Island) and northern Victoria Land. We estimate, using published skua estimates for elsewhere in Antarctica, that the Ross Sea South Polar Skua population comprises ~50 % of the world total, although this may be an overestimate because of incomplete data elsewhere. To improve predictions and enable measurement of future skua population change, we recommend additional South Polar Skua surveys using consistent distance-sampling methods at penguin colonies of a range of sizes.

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Deborah J. Wilson, Phil O’B. Lyver, Terry C. Greene, Amy L. Whitehead, Katie M. Dugger, Brian J. Karl, James R. F. Barringer, Roger McGarry, Annie M. Pollard, David G. Ainley. 2016-06-08. South Polar Skua breeding populations in the Ross Sea assessed from demonstrated relationship with Adélie Penguin numbers. https://doi.org/10.1007/s00300-016-1980-4

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