Work by international researchers has helped to shine a light on the secretive life of Bermuda’s national bird, the cahow.

Jeremy Madeiros, the principal scientist (terrestrial conservation) for the Department of Environment and Natural Resources, wrote in the latest edition of the newsletter Envirotalk that a variety of research projects into the endemic seabirds were under way to learn more about their time spent at sea.

The seabirds spend the bulk of their life at sea, but return to Bermuda in the autumn for their courtship period and to nest, with couples laying a single egg.

As part of one study, 43 adult breeding cahows were fitted with small GPS trackers to learn about their trips to find food for their chicks.

“Cahows have a ‘tag team’ approach during egg incubation; one adult stays in the underground nest burrow incubating the egg while the other goes to sea to feed, after which the foraging bird returns to take over incubation while the other goes to feed,” Mr Madeiros said.

“Later, during chick-rearing, both adults carry out lengthy foraging trips to gather food for their growing, ever-hungry chick.

“They only spend a few hours with the chick during the feeding visits and return immediately to the open ocean to continue foraging.”

He said the results showed the birds travelling “almost unbelievable distances” from the island as part of foraging trips, with one bird travelling 1,979 miles one way during a single foraging trip.

On average, the birds would fly the still-significant distance of 908 miles one way.

During the egg incubation period, the foraging trips would last an average of 11 days and six hours, while during the early chick-rearing period the trips would last an average of six days and eight hours.

“This is obviously due to the need to feed the chick regularly,” he said.

“Observations carried out over the past two decades show that cahow chicks are fed on average only two to three times a week, with both male and female adults sharing feeding duties and egg incubation equally.”

The studies have also shown that cahows regularly forage off the coast of Nova Scotia and Newfoundland, leading to a proposal to have cahows declared a Canadian endangered species to enhance protections.

“This is important for the continued recovery of the cahow because there are numerous areas along the Nova Scotia Shelf that have been identified for oil and gas exploration and the intense lights and gas flares associated with offshore oil platforms tend to attract seabirds, which leads to collision injuries,” Mr Madeiros said.

Researchers also studied what the birds were feeding on using genetic bar coding analysis, revealing that cahows target deep-water species.

“The taxonomic diversity was remarkable, and included 16 fish and six squid species,” he said.

“These prey are presumably taken at night during diurnal vertical migration when many species that normally live at depths of 1,000m or more migrate to the surface to feed on plankton before returning to the depths before sunrise.

“Overall, the results contribute critical new knowledge on the foraging behaviour plasticity of the cahow, which can help to predict how a small population of an endangered species responds to climate-related changes in wind patterns and oceanic processes in the North Atlantic Ocean.”

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Researchers also looked into if organic pesticides have had an impact on the species and found evidence of several such pesticides including the long-banned DDT in blood and feather samples.

“DDT was confirmed in less than 30 per cent of the samples but DDE, a common breakdown product from DDT, was confirmed in all of the samples,” Mr Madeiros said.

“The presence of DDT and DDE is a cause for concern because both affect calcium uptake in various bird species, which results in thinner eggshells that are more easily broken during incubation.”

The study also found hexachlorobenzine, another banned pesticide linked to cancer and birth defects, in half of tested cahows.

“These findings may partly explain why there is a relatively high level of egg failure in Bermuda’s breeding population of cahows (41 per cent to 52 per cent each year),” he said.

“DDT was banned in the 1970s and HCB was banned in 2001. However, these fat-soluble chemicals have the ability to persist within the environment and bioaccumulate within organisms.”

Mr Madeiros noted that further studies into the lives of cahows are under way, which will help to expand knowledge of the seabirds.

As part of one study, 37 fledgelings were fitted with geolocator tags in 2022, with the tags to be retrieved when they return to their nesting colonies after three to five years.

“We have been developing a fairly good understanding of the oceanic range and main feeding areas used by adult cahows, but we have no idea where newly fledged birds go,” Mr Madeiros said.

“Do they use the same areas as the adults? Do they explore the ocean basin more widely to build a map in their memories of productive oceanic areas at different times of the year?

“The first of these tagged birds should return over the next two to three years and hopefully begin providing answers to some of these questions.”

Cahows, also known as Bermuda petrel, were largely wiped out by introduced predators and hunting by early English colonists, and by the 1620s the species was believed to be extinct.

However, the species was rediscovered in 1951 with a handful of the birds found nesting on four rarely visited rocky islets, sparking efforts to rebuild the population.