Highly pathogenic strains of avian influenza (HPAI) first evolved in intensive poultry farms and have repeatedly spread to wild birds through lax biosecurity. The H5N1 strain of HPAI was first identified in domestic waterfowl in China in 1996, and has since evolved to become progressively more destructive. During the winter of 2020/2021 a new H5N1 virus started to be detected in Europe. Compared to other variants, this one is particularly well adapted to infecting wild birds, resulting in much higher levels of circulation within wild bird populations (James et al. 2023). There has also been a shift in the seasonality of the virus. Historically, outbreaks of HPAI have been mostly confined to the winter months, whereas the new H5N1 appears to have become endemic (present year-round) in many areas (Pohlmann et al. 2022, Harvey et al. 2023). Since its detection in Europe, the new H5N1 has spread worldwide along migratory flyways, resulting in an unprecedented epizootic that, at the time of writing, has reached every continent except Oceania. This global outbreak has led to the death or destruction of c.0.5 billion poultry (Klaassen & Wille 2023) and has impacted more than 400 bird species (CMS FAO Co-convened Scientific Task Force on Avian Influenza and Wild Birds 2023), in some cases resulting in significant population declines.

Examples of species impacted by avian influenza during 2021–2023

Photo credits: Great Skua by Claudia/Flickr (CC BY-NC-SA 2.0); Dalmatian Pelican by anastasija26/Shutterstock; Common Crane by Piotr Krzeslak/Shutterstock; Hooded Crane by Josh More/Flickr (CC BY-NC-ND 2.0); Cape Cormorant by Alandmanson (CC BY-SA 4.0); Great White Pelican by GRID-Arendal/Flickr (CC BY-NC-SA 2.0); Brown Skua by MZPHOTO.CZ/Shutterstock; Peruvian Booby by Vladislav T. Jirousek/Shutterstock; California Condor by kojihirano/Shutterstock.

Eurasia
Some of the first reports of mass mortality from HPAI H5N1 in wild birds came from the coastlines of Scotland, UK. Towards the end of the 2021 breeding season, large numbers of dead and dying Great Skua Catharacta skua were reported on Scottish islands (Banyard et al. 2022). Over the next year, more than 2,200 Great Skua deaths were reported, representing 7% of the global population (Falchieri et al. 2022). Just a few months after the first cases were detected in Great Skua, in the winter of 2021/22, more than 13,000 Barnacle Geese Branta leucopsis (one-third of the Svalbard-breeding population) perished in southwest Scotland’s Solway Firth (NatureScot 2023). While previous outbreaks of HPAI in wild birds have tended to be seasonal, this time occurrences continued into the summer. For example, in June 2022, HPAI was detected in Northern Gannet Morus bassanus breeding on Bass Rock off the east coast of Scotland, resulting in a c.25% population decline at the world’s largest breeding colony of the species (Scottish Seabird Centre 2023).

Meanwhile, outbreaks were also occurring throughout the rest of Eurasia. In Greece, an outbreak of HPAI in Dalmatian Pelican Pelecanus crispus breeding at Prespa Lake (the world’s largest breeding site) during February–April 2022 wiped out c.60% of the colony (Alexandrou et al. 2022), while in the Hula region of Israel, more than 5,000 Common Crane Grus grus died after HPAI struck in December 2021 (Reuters 2022). The following winter, HPAI was detected at the congregation of 12,000 Hooded Crane G. monacha and White-naped Crane G. vipio overwintering at Izumi, Japan, resulting in the deaths of more than a thousand of the former and dozens of the latter (EAAFP 2022). Outbreaks throughout Eurasia continued into the summer, with more than 20,500 Sandwich Terns Thalasseus sandvicensis found dead around the coasts of northwest Europe during the 2022 breeding season, representing more than 17% of the total breeding population in this region (Knief et al. 2024).

Africa
From Europe, the virus soon spread to Africa (Lo et al. 2022). Data on confirmed outbreaks are sparser in this region, but there have been several reports of mass mortality. The first reports came from countries in West Africa lying along the East Atlantic Flyway. For example, in Senegal 750 Great White Pelican Pelecanus onocrotalus were found dead at the Djoudj National Bird Sanctuary in January 2021 (Lo et al. 2022). Further south, extensive mortality of Cape Cormorant Phalacrocorax capensis was recorded during 2021–2022 in South Africa’s Western Cape (>20,000 deaths recorded; CapeNature 2021) and on the west coast of Namibia (>6,500 carcasses retrieved from Bird Island; Molini et al. 2023). In early 2023, thousands of dead seabirds were recorded around the coasts of Senegal, Gambia and Guinea-Bissau, with the main casualties including Caspian Tern Hydroprogne caspia, Royal Tern Thalasseus maximus and Grey-headed Gull Larus cirrocephalus (BirdLife International 2023).

Americas
In December 2021, HPAI H5N1 was detected in poultry in Newfoundland and Labrador, Canada. Genetic analyses confirmed that the virus had originated from Northwest Europe, having most likely been carried by migratory birds travelling along the East Atlantic Flyway (Caliendo et al. 2022). Within months the virus had spread throughout North America, with more than 230 outbreaks recorded in wild animals during 2021 and the first half of 2022 (World Organisation for Animal Health 2022). Waterbirds and seabirds were particularly hard hit—the carcasses of more than 850 Common Eider Somateria mollissima were found along the St Lawrence waterway during the 2022 breeding season (Larouche 2022); more than 1,000 dead Snow Geese Anser caerulescens were recorded along the waterways of Colorado (Colorado Parks & Wildlife 2023); and the colony of Northern Gannet nesting at Rocher aux Oiseaux, Quebec had declined by 58% when counts were carried out in July 2022 (Fauteux 2023). Raptors also fared poorly—more than 300 Bald Eagle Haliaeetus leucocephalus tested positive for HPAI in 2022 (Sidik 2023), resulting in high rates of nest failure and mortality (Nemeth et al. 2023); and at least 20 Critically Endangered California Condor Gymnogyps californianus (7% of the wild population) died from the virus in early 2023 (Wetzel 2023).

It was only a matter of time until the virus was transmitted to Central and South America by birds travelling south along the Americas flyways. The first cases were detected in October 2022, with a subsequent rapid spread of the virus throughout Latin America. In Peru, more than 100,000 wild bird deaths due to the virus were recorded in the country’s marine and coastal protected areas during November 2022–March 2023, with mortality across the whole country estimated to be more than double this figure (Gamarra-Toledo et al. 2023a). This included mass mortality of economically important “guano birds” such as Peruvian Booby Sula variegata (>47,500 deaths), Guanay Cormorant Leucocarbo bougainvilliorum (>28,900 deaths) and Peruvian Pelican Pelecanus thagus (>21,100 deaths).

Antarctica
In October 2023, the first ever known cases of HPAI were confirmed in the Antarctic region. Brown Skua Catharacta antarctica on Bird Island, South Georgia, tested positive for the virus, having likely been exposed during their migration to South America (British Antarctic Survey 2023). There have since been a number of other cases confirmed among Brown Skua and Kelp Gull Larus dominicanus at sites across South Georgia, while Southern Fulmar Fulmarus glacialoides and Black-browed Albatross Thalassarche melanophris in the Falkland Islands have also tested positive (Scientific Committee on Antarctic Research 2023). Birds aren’t the only taxa at threat—symptoms of avian flu and widespread mortality have also been reported among Antarctica’s Elephant Seal Mirounga leonina colonies (Scientific Committee on Antarctic Research 2023), mirroring the spread to marine mammals seen in other parts of the world (e.g. Gamarra-Toledo et al. 2023b). Given the high density of birds and seals breeding in this region, and the lack of previous exposure to the virus, HPAI may have devastating impacts on Antarctica’s wildlife.

While avian influenza is undoubtedly having significant impacts on many wild bird populations, there are some signs that populations can recover following an outbreak. For example, Northern Gannet populations on Bass Rock appeared to be recovering in 2023, with research suggesting that surviving individuals may develop immunity (Lane et al. 2023).

There is no quick solution to halt the virus, but the BirdLife Partnership is working around the globe to prevent and halt outbreaks. Preventative measures include the removal of dead birds from colonies (where appropriate); suspension of practices which result in an unnaturally high concentration of susceptible wild birds, such as supplementary feeding (unless essential for their conservation); and restriction of public access to vulnerable sites. Monitoring of the virus through frequent and widespread testing, outbreak reporting and monitoring of background mortality is crucial to improving our understanding of which species are affected and the effects of the virus on their populations. Further research is also needed into the spread and evolution of the virus, the development of immunity in wild birds, and to test practical measures to prevent or minimise outbreaks. In the long-term, reformations of the poultry industry are needed to prevent transmission of diseases to wild birds.