Birders were treated to an avian drama as a cahow at the Nonsuch Island sanctuary was caught on camera in a subterranean showdown with another of the island’s iconic seabirds.

Nonsuch Expeditions, which oversees cahow conservation on the offshore nature reserve, uses the infrared CahowCam to monitor the birds in their underground nests — and, in the process, saved the day when a prospecting longtail tried to muscle in and steal the burrow at the expense of its chick.

While the white-tailed tropicbird succeeded in ousting the smaller cahow from the den, the occupant put up enough of a fight for a happy ending after conservationists rushed to intervene.

The saga, which illustrates the competitive side of birds under pressure to raise their young, was described online by J.P. Rouja of Nonsuch Expeditions.

Mr Rouja keeps watch during cahow breeding season with Jeremy Madeiros, the terrestrial conservation officer with the Department of Environment and Natural Resources.

The cahow who opted to fight the longtail, Zephyr, has been reported on before: its successful launch out to sea as a youngster in 2020 was watched online by viewers worldwide.

In the latest blog, Mr Rouja wrote: “The livestreams have allowed our regular observers to gain an understanding of what normal feeding visit behaviours should look like, and over the past few nights, what appeared at first to be the parents returning was, in at least one case, prospecting birds that spent a few hours somewhat aggressively attempting to brood and harass, but not feed, the chick.”

When one of the cahows could be seen still underground with the chick on Sunday morning instead of heading out to sea, they debated heading out to identify it.

The blog added: “As we were jointly watching online, shockingly, a tropicbird entered the nest, fought briefly with the adult, and started harassing the chick.”

Online footage showed the cahow taking on the longtail before getting driven back into the tunnel to the nest by the interloper.

Many chicks fall victim to longtails in the battle for nest spaces — but Mr Madeiros was able to dash to his boat in Flatts and get to the scene fast enough to dislodge the intruder longtail “just in time to save our chick”.

The adult female cahow that intruded on the nest was relocated to an empty burrow.

Checking the bird’s identification band, Mr Madeiros discovered it was none other than Zephyr returning to the same burrow where she had hatched.

Given the cahows’ habit of coming back to the same burrows to breed, the conservationists concluded that Zephyr had “returned just in time to save her sibling”.

To protect the burrow from future invasion attempts, Mr Madeiros camouflaged the exposed nest entrance with plants to hide it from longtails in the air since the birds, which are limited in their ability to walk once on the ground, spot targets from above and “try to crash-land into their nest openings”.