‘Pirate birds’ force other seabirds to regurgitate fishy food. Their flying habits could spread deadly bird flu

Finding food at sea is not easy. Seabirds often stay aloft, scanning the rough waters for elusive prey. Most seabirds catch fish, squid, or other prey in the first few meters of seawater. Scavengering is common.

But there are other tactics. Frigatebirds, skuas, and gulls rely on the success of other seabirds. These large, robust birds pursue, harass, and attack their targets until they regurgitate or drop their newly captured prey. They are the pirates of the seabird world, stealing hard-earned meals from other species. This behavior is known as kleptoparasitism, from the ancient Greek word kléptēs, meaning thief.

This strategy is brutal, effective and a key behavior for these important seabirds. But as our new study, published in Conservation letters Armed robberies pose major risks to thieves. The new strain of bird flu is killing millions of birds and we have discovered that kleptoparasitism can spread the virus very easily.

Food thieves at sea

It’s not that frigatebirds, skuas, and gulls can’t hunt. They can and do catch their own food. But hunting fish and squid is hard work. It’s much easier to use extortion tactics to get food from other seabirds.

These tactics have allowed these birds to successfully feed themselves. They hover around the breeding grounds of birds such as gannets and terns waiting for a tired parent to return from the sea with a harvest of food.

For the targeted seabirds, these kleptoparasitic birds are just another threat. The world’s 362 species of seabirds are found in every ocean and on many islands. At sea, they feed on fish and squid. When they nest or rest on islands, their nutrient-rich guano shapes soil and plant communities, defining entire ecosystems.

But the situation is dire. Nearly half of seabird species (155) are now classified as near threatened or critically endangered on the IUCN Red List. Of those for which trends are known, 56% have declining populations.

The threats they face are formidable. Invasive predators such as mice and rats devour eggs or chicks on breeding islands. Many are accidentally caught by fishing boats, while overfishing depletes their prey. Then there is climate change, habitat loss and many other threats, including disease.

Seabirds are generally long-lived. They often raise only one chick every one or two years. Many species breed in only a few locations. It takes many years for them to reach maturity. Together, these characteristics slow recovery from population declines.

Parasites and viruses

Three years ago, a more deadly strain of the bird flu virus emerged. This HPAI H5N1 2.3.4.4b strain has spread around the world, killing at least 280 million wild birds. This strain can also infect and kill marine mammals such as seals.

The acronym “HPAI” stands for “highly pathogenic avian influenza,” meaning the virus can easily cause severe and fatal disease. The strain has become an animal pandemic (formerly, a panzootic). It has reached Antarctica, but not yet Australia or the rest of Oceania.

We know that seabirds are particularly at risk. Our new research has shown that kleptoparasites are at even greater risk than other seabirds.

During the summer of 2022 in the Northern Hemisphere, the virus killed about half of the world’s great skuas (Stercorarius labbe).

Food theft can help spread the virus. When a great skua harasses a gannet and forces it to regurgitate food, the skua is given a meal of fish coated in saliva. If the gannet is contagious, its saliva will likely have a high viral load.

Once infected, these pirate birds can spread the disease more quickly. Skuas, frigatebirds and gulls can travel great distances in polar regions and the tropics. They can transmit the disease to their mates, chicks and other seabirds.

This means we could see outbreaks in new populations or new locations, hundreds or even thousands of kilometres away. We have already seen signs of this in skua populations in both the northern and southern hemispheres, with brown skuas (Stercorarius antarcticus) and great skuas being among the first H5N1 infections detected in new locations.

Skuas often steal food from other seabirds as they travel away from their breeding sites, including when migrating to these areas. If skuas become infected en route, they can bring the disease to their breeding sites and beyond.

Frigatebirds are known for the red pouches on their male necks, which they inflate during the breeding season. But they have other remarkable characteristics, such as traveling tens of thousands of kilometers across oceans outside of the breeding season. These trips are often interspersed with “island hopping,” where they encounter and potentially infect other seabirds.

Frigatebirds and skuas have already suffered massive deaths from this strain of bird flu.

Although the virus is now present almost everywhere, it has not reached Australia, New Zealand, Oceania and parts of Antarctica and the sub-Antarctic. We can monitor skuas, frigatebirds and gulls for signs of disease to give us early warning of the virus’ arrival.

Avian influenza is a major threat to seabirds in its own right. But the outlook is even more dire when it is compounded by other human-caused threats. Identifying, managing and reducing these threats is essential for their conservation and the health of our islands and oceans.

This article is republished from The Conversation under a Creative Commons license. Read the original article.