From Millions, Ascension Island Sooty Tern Population Crashes 80% b/c Warming, Industrial Fishing

A population crash in a massive seabird colony on Ascension Island has been attributed to a poor diet after their normal food sources dried up. Numbers of sooty terns living in the UK overseas territory have plummeted by over 80 per cent in recent decades, from several million to just a few hundred thousand.

New research suggests the bird’s downfall is the result of poor quality food, after industrial fishing and climate change combined to rob them of their usual diet. Instead they have had to turn to relatively low-nutrient food such as squid, snails and locusts.

Scientists arrived at this conclusion after efforts by the RSPB to eradicate feral cats from the island failed to bolster tern populations. While other seabirds thrived once the feline threat was removed from their nesting grounds, tern populations continued to stagnate.

A clue came when the scientists noticed the contents of the bird’s regurgitated food did not consist of as much fish as they would have expected. To solve the mystery, the scientists obtained old museum specimens of sooty terns dating back to 1890, and analysed the chemical composition of their feathers to gain an insight into their diets.
They were able to chart a shift from eating mainly fish to lower-quality food, which coincided with their population decline.

EDIT

https://www.independent.co.uk/environment/seabirds-colony-fishing-climate-change-ascension-island-atlantic-ocean-a8758941.html