With its trademark flippers and comical mating dance, the blue-footed booby is one of the most commonly sighted birds on the Galapagos Islands.

But concerns that numbers of the seabird have declined by more than one-third on the islands in the last 40 years have prompted a British conservation charity to launch an unprecedented study of the booby to determine how threatened it really is.

"There is anecdotal and observational evidence that the numbers of boobies are in decline, but no one has detailed the population status since the 1970s when there were 30,000 breeding pairs on the islands," said Toni Darton, chief executive of the Galapagos Conservation Trust, which is funding the two year-review alongside groups in the United States and Switzerland.

"We now think there could be less than 20,000 pairs on the islands, but we need to try to gather the baseline data on these birds so we can start to understand why this decline is happening and what we can do to manage it."

First analysed by Charles Darwin on his visit to the archipelago in 1835, the boobies' ungainliness on land but precision in the sea possibly helped to influence the biologist's theory of evolution. With more than half of the species' population found on the first World Heritage Site, conservationists believe the bird's nesting and feeding areas have been destroyed through over-fishing, the introduction of alien species and the impact of the growing population and increased numbers of visitors to the islands each year.

The human population of the Galapagos is now seven times larger than it was in the 1970s, with almost 28,000 permanent residents thanks to a surge in immigration from mainland Ecuador. There were 165,000 visitors to the islands last year.

Hendrik Hoeck, 67, who used to work in the Galapagos as director of the Charles Darwin Research Station, blamed the shifting temperatures of water currents across the Pacific, known as the "El Niño" effect, for the reduced numbers of the marine-reliant species.

"I used to take tourists around the largest island, and we would see hundreds of pairs of boobies – now you don't often see more than 30. I would say population numbers in certain places are down by almost 70 per cent," he said.