Follow the boundary between the Caribbean Sea and Atlantic Ocean 40 kilometres north from St. John’s, Antigua — 1½ hours on the Excellence, Tropical Adventures Antigua’s bouncing, sea-spraying, rum-punch-stocked catamaran, which is how I travelled — and you reach Antigua’s sister island of Barbuda, a flat, green jewel with a population of just 1,800 and no more than a handful of tourists at any given time.

On this October afternoon, that solitary scattering of visitors is us. After disembarking and enjoying a swim on famed 17-Mile Beach, we’re invited to turn inland across the sand strip, narrow enough to throw a conch shell over, to the edge of massive Codrington Lagoon. There, as part of the Tropical Adventures Antigua “Barbuda by Sea” excursion, local guides with small speed boats and big smiles await. We strap PFDs around our torsos and motor off to find out what a “frigatebird” is.

Into the bird sanctuary

As small, shrubby islands begin to appear out of the brackish water, the lagoon takes on a dank, vaguely methane smell. In the denser mangroves where our guides tie up to buoys, the odour is also fishy — the result of the dietary preference of the giant black seabirds that throng the lagoon’s northwest corner, where they come to mate and nest every year between September and April. When the guides silence their motors, we’re surrounded by the birds’ amorous “drumming” and clicking sounds, their wheezing, grating signals.

Lead guide Tetworth Richardson stands in his boat to tell us about this place, a Wetland of National Importance and a national park since the early 1980s. The lagoon is home also to brown boobies, laughing gulls, the threatened Barbuda warbler, critically endangered leatherback and hawksbill turtles and many other species, but the most charismatic of them all are the magnificent frigatebirds, national bird of Antigua and Barbuda.

More than 5,000 nest here, says Richardson. “And it’s early in the season,” he adds, “so you can see the black-headed females hoverin’ above, checkin’ out the groups of males, who are inflatin’ their red throats, wavin’ their necks and clickin’ their beaks to say ‘C’mon down girls!’ ”

When a female chooses her mate, the pair settle onto a bough to meet and neck for a while; then, the male deflates himself and flies away to collect sticks and other materials for their nest, which he gives to the female to use in construction.