British Antarctic Survey counts 63% fewer chinstrap nests on Zavodovski Island than in 2011

The 63% counts occupied nests ^[1]. The 2011 baseline that made Zavodovski famous was given in birds: about 600,000 chinstraps ^[2]. If birds fell in step with nests, roughly 222,000 chinstraps would be left ^[17]. That figure holds only if the two measures moved together.

Spread over 14 years, a 63% loss works out to about 7% a year, compounded ^[16]. The team describes the decline as steady and ongoing since 2011 ^[6]. Steady losses clear the obvious suspect. Mount Curry, the active volcano the birds nest beneath, last erupted in a major way in 2016 ^[7][6]. By then the decline had been running for five years.

Humpback whales were the second suspect. They eat large quantities of krill, but their dung fertilises the marine plants krill feed on, so they add to the chinstraps' food supply as well as taking from it ^[8].

The best evidence comes from the neighbours. Macaroni penguins nest on the same island, and their numbers rose sharply while the chinstraps fell ^[9]. The two species share the volcano and the whales but eat different things. That makes the comparison a rough natural control for anything happening locally. Tracked macaronis foraged in warmer water to the northeast, where the scientists think they were likely adding fish and other crustaceans to their diet ^[10]. Chinstraps eat a specialist diet and prefer cooler water, according to Norman Ratcliffe, who led the research ^[11].

Ratcliffe said that with the waters warming, the krill the chinstraps eat have moved southwards ^[12]. "Sea-ice cover to the south and a lack of suitable islands for nesting prevents chinstraps from following the food," he said ^[13].

The thing this doesn't tell you is how far the krill have moved. The BBC account of the study does not describe direct krill measurements, and the tracking sample was 20 birds ^[5]. The warming explanation fits the pattern, and the case against both local alternatives is strong. Still, the step from warm water to empty nests is the team's interpretation of that pattern.

The team says chinstraps may now be officially endangered ^[3]. One colony is a narrow base for a species-wide status, and trends at other colonies would have to point the same way. Ratcliffe called the Zavodovski shift the "biggest change in community composition ever observed at a single location in Antarctica" ^[14]. The study is published in Current Biology ^[15].