Since 2021, the global wildlife community has been grappling with a wave of mass bird mortalities caused by the highly pathogenic avian influenza virus (HPAIV), particularly the H5N1 strain. This pandemic-like spread has severely impacted seabird populations, resulting in unprecedented losses, particularly in regions such as Europe, South America, and most recently, even Antarctica. The virus’s rapid transmission through wild bird populations has prompted scientists to investigate every possible transmission pathway, including some that were previously overlooked. One such behavior, kleptoparasitism—or the act of stealing food from other birds—has emerged as a potential conduit for the spread of the virus.
Kleptoparasitism is a behavior where certain birds, known as kleptoparasites, force other birds to regurgitate their food, which is then consumed by the thief. This regurgitated meal can potentially harbor viral particles from an infected bird, providing a direct and efficient transmission route for pathogens like HPAIV. While kleptoparasitism has long been documented among seabird species such as frigatebirds and skuas, its role in disease transmission is only now beginning to receive serious attention from scientists. In this article, we will explore the potential impact of kleptoparasitic behavior on the spread of avian influenza and how it may exacerbate the ongoing global crisis of H5N1.
Kleptoparasitism and Its Role in Seabird Behavior
Kleptoparasitism is not a rare phenomenon in the avian world. It is a highly evolved and efficient foraging strategy practiced by a variety of bird species, particularly seabirds. Among the most notorious kleptoparasites are two families: Fregatidae (frigatebirds) and Stercorariidae (skuas). Both groups are widely distributed across the world’s oceans and frequently engage in food theft, particularly during the breeding season when competition for resources is high.
Frigatebirds, for instance, are known to harass other seabirds—such as boobies, terns, and tropicbirds—until they regurgitate their catch, which the frigatebird promptly snatches in mid-air. Skuas, on the other hand, are equally ruthless, targeting a wide range of seabirds including gulls, petrels, and puffins. These kleptoparasitic acts are not limited to isolated incidents but are integral parts of the feeding strategy of these species. Importantly, both frigatebirds and skuas are long-distance travelers, often migrating across entire oceans or hemispheres, which significantly amplifies their potential as vectors for disease spread.
The Threat of Avian Influenza in Kleptoparasitic Species
High-pathogenicity avian influenza viruses (HPAIV), particularly the H5N1 lineage, pose a severe threat to wild bird populations. HPAIV primarily spreads through direct contact with infected birds or through exposure to contaminated environments, such as water sources or nesting areas. However, the kleptoparasitic behavior of seabirds presents a novel and potentially underappreciated route for the virus’s transmission.
The act of kleptoparasitism involves close physical contact between the parasite and its victim, often leading to the immediate ingestion of regurgitated food. If the targeted bird is infected with HPAIV, the regurgitated meal may contain viral particles from its digestive or respiratory tracts. The kleptoparasite, by consuming the regurgitated material, risks direct exposure to the virus. Given the wide range of species targeted by kleptoparasitic birds and the sheer frequency of these interactions, this form of transmission could play a significant role in the spread of avian influenza, especially in densely populated seabird colonies where such interactions are common.
In 2021, the H5N1 strain began its rapid global spread, causing mass mortality events in seabird populations. Research has identified kleptoparasitism as a potential contributor to this pandemic. Both frigatebirds and skuas have been documented contracting HPAIV, with significant mortality events recorded in species like the Great Skua (Stercorarius skua) and various frigatebirds. For instance, during the 2021/2022 outbreak in the United Kingdom, Great Skua populations declined by a staggering 76% due to HPAIV infections, representing a catastrophic loss for a species that comprises approximately 65% of the global population. Similarly, in South America, where H5N1 recently reached, thousands of frigatebirds have perished, raising alarms about the future survival of these already vulnerable species.
Geographical Spread and Migration of Kleptoparasites
One of the most concerning aspects of kleptoparasitism as a transmission pathway is the wide geographic range of kleptoparasitic seabirds. Frigatebirds and skuas are not confined to one region; rather, they are highly migratory and can travel vast distances, sometimes across entire oceans. For example, frigatebirds are known to migrate thousands of kilometers during their non-breeding season, often island-hopping among roost sites in the Indian and Pacific Oceans. These long migrations can potentially spread avian influenza across continents and oceans, exposing new populations of birds to the virus in regions that have previously been unaffected.
Skuas, particularly the South Polar Skua (Stercorarius maccormicki), migrate between Antarctica and the northern hemisphere, including the North Atlantic and Pacific Oceans. During these migrations, skuas may pass through regions where avian influenza outbreaks have occurred, potentially contracting the virus and spreading it to other populations as they return to their breeding grounds. This long-range mobility makes kleptoparasitic seabirds ideal vectors for the broad-scale geographic dissemination of HPAIV.
Moreover, kleptoparasitism is not limited to the breeding season. Both frigatebirds and skuas engage in kleptoparasitic behavior year-round, albeit to varying degrees. Frigatebirds, for example, continue to steal food from other seabirds even when they are not breeding, and they often roost in mixed-species colonies during the non-breeding season. This behavior increases the likelihood of viral transmission even outside the breeding period, potentially spreading the virus to new locations as the birds migrate or disperse to different roost sites.
Implications for Seabird Conservation
The implications of kleptoparasitism as a transmission pathway for HPAIV are profound, particularly for seabird conservation. Many kleptoparasitic species, such as the Christmas Frigatebird (Fregata andrewsi) and the Ascension Frigatebird (Fregata aquila), are already listed as vulnerable or endangered due to their limited breeding ranges and small population sizes. If these species were to experience a significant HPAIV outbreak, the consequences could be disastrous, potentially pushing them closer to extinction.
Furthermore, kleptoparasitic species often breed in dense colonies, sometimes among or near other seabird species. These breeding colonies provide an ideal environment for the rapid spread of avian influenza. The close proximity of individuals, combined with the frequent regurgitation of food, creates ample opportunities for viral transmission. This is particularly concerning during the breeding season when seabirds are central-place foragers, meaning they repeatedly return to the same location to feed their young. Infected kleptoparasites could introduce the virus into the colony, where it could spread rapidly through both kleptoparasitic and non-kleptoparasitic species.
The vulnerability of seabird populations to avian influenza is further exacerbated by their already declining numbers due to other threats, such as habitat loss, overfishing, and climate change. The addition of HPAIV to this list of challenges could have devastating effects on global seabird biodiversity, particularly in regions where kleptoparasitic seabirds are abundant.
Surveillance and Conservation Strategies
Given the potential role of kleptoparasitism in the spread of avian influenza, it is crucial that wildlife health monitoring programs include kleptoparasitic species in their surveillance efforts. Regular testing of kleptoparasitic seabirds for HPAIV, particularly during migration and breeding periods, could provide early warning signs of outbreaks and help mitigate the spread of the virus to other populations.
In addition to monitoring, conservation strategies should prioritize the protection of seabird breeding colonies that are at high risk of HPAIV transmission. This could include implementing measures to reduce human disturbance in these areas, as human activity can exacerbate stress and increase the likelihood of disease outbreaks. Conservationists should also focus on restoring and protecting key habitats, such as island breeding sites, which are critical for the survival of many kleptoparasitic species.
Kleptoparasitism is a fascinating yet ruthless survival strategy in the avian world, one that has likely existed for millennia. However, as the world faces an ongoing pandemic of highly pathogenic avian influenza, this behavior has taken on new and deadly significance. By forcing other birds to regurgitate their meals, kleptoparasites may inadvertently expose themselves—and the broader seabird community—to HPAIV. The wide geographic range and migratory habits of kleptoparasitic seabirds like frigatebirds and skuas make them potential vectors for the global spread of the virus, with potentially catastrophic consequences for seabird populations.
To address this emerging threat, researchers must continue to investigate the role of kleptoparasitism in disease transmission, while conservationists work to protect vulnerable seabird species from the dual threats of habitat loss and infectious disease. Only through a combination of scientific research, vigilant monitoring, and targeted conservation efforts can we hope to mitigate the impact of avian influenza on the world’s seabirds and safeguard their future in an increasingly challenging environment.
More information: Kleptoparasitism in seabirds—A potential pathway for global avian influenza virus spread, Conservation Letters (2024). conbio.onlinelibrary.wiley.com … l/10.1111/conl.13052
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