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In the past six months we have all learned the word ‘coronavirus’ and observed the incredible research effort that has been poured into a single coronavirus species: SARS-CoV-2, the causative agent of COVID-19.

This virus species is a member of betacoronaviruses; the family Coronaviridae is divided into four genera: alpha-, beta-, gamma- and deltacoronavirus.

While there is currently considerable discussion about the important of bats (and pangolins) as hosts for viruses that may one day spill over in humans, birds are also important hosts for many viruses.

Furthermore, birds, through their amazing migrations, have the capacity to connect really distant parts of the planet. For example, Australia went into lock down in March, and shortly there after approximately two million wild birds left the Australian continent, and with stops all along the South East Asian and Chinese coast lines, made their migration to Siberia to breed. With them, they potentially moved viruses, bacteria, parasites.

Birds have a long history as hosts for members of the Coronaviridae. The first coronavirus ever described in the 1930’s, was infectious bronchitis virus, which remains an economically important respiratory virus of chickens and turkeys. Today, there are 11 coronavirus species found in birds: four gammacoronaviruses and seven deltacoronaviruses.

Based on available studies, we know that coronaviruses have been detected in 108 wild bird species. Specifically, members of the Anseriiformes (ducks, geese, swans) are important hosts for gammacoronaviruses, Charadriiformes (gulls and shorebirds) are hosts for both gamma- and deltacoroanviruses, and Pelecaniiformes (specifically herons and egrets) are important hosts for deltacoronaviruses.

Further, coronaviruses have been found in wild birds on every continent, including in penguins on Antarctica. Data from Australia, which I undertook in collaboration with Deakin University and GCEID suggested similar trends – we found gammacoronaviruses in an array of Australian duck and shorebird species, and deltacoronaviruses in shorebirds and herons from across Australia.

Unfortunately, despite 10 years of research on wild birds, we find there are many biases in the methods being used, so a thorough assessment of the ecology of these viruses is very challenging. Despite the challenges, there are a number of important observations.

First, the gammacoronavirus Duck Coronavirus 2714 (DCoV) is very common in wild birds, especially ducks – both wild ducks and domestic ducks raised for food consumption.

This is important – we know domestic ducks are an amplifying host and reservoir for avian influenza viruses, and it would be possible for viruses to go from domestic ducks into poultry, especially in countries where these animals are raised together. Overall, it would be detrimental to the poultry industry if another viral species were introduced to the system.

Second, there is limited evidence that wild birds are hosts for avian coronavirus, the virus species found in chickens which includes infectious bronchitis virus and turkey coronavirus. However spill over infections have been recorded – but always connected to poultry production. This is highly concerning – in 2014 and 2016 we saw the spillover of highly pathogenic H5N8/H5N6 from poultry into wild birds. Wild birds spread these viruses across Asia, to Europe and North America, at which point they were introduced into poultry production on these continents with devastating consequences.

And last, coronavirus HKU15, a deltacoronavirus, has been found in birds (sparrows, quails) and is an important virus of swine (called porcine deltacoronavirus). Data suggest that this virus spilled over from birds into pigs. Experiments show that this virus is able to grow on human cell lines, suggesting the barriers to zoonotic-spill over are limited.

This virus species has also been found in mammals in wet markets, although in 2005/2006. Of concern is systematic under sampling of deltacoronaviruses, and therefore we do not know the true extent of cross species transmission and circulation in mammals beyond pigs.

We are beginning to appreciate that many birds have a high abundance and diversity of viruses without observable disease. The coronaviruses are no exception. The first coronaviruses in birds – infectious bronchitis virus and turkey coronavirus continue to cause losses in poultry production systems due to disease. That records of disease by coronaviruses in wild birds are few suggests that disease due to coronavirus infection is the exception, not the rule.

Despite the fact that these viruses cause no disease, it is imperative for us to study viruses of wildlife as they may pose a threat to animal production systems (eg Duck Coronavirus) or be of zoonotic concern (eg Coronavirus HKU15).

So what about SARS-CoV-2 you may ask? To date, birds are the main hosts of gamma and deltacoronaviruses. Mammals, including bats, are the main hosts of alpha and betacoronaviruses. There are records of SARS-CoV-2 reverse zoonosis (transmission from humans to animals): disease in domestic cats and dogs, zoo felids such as tigers, and minks. However, so far there are no records of this virus spilling over into birds and in experimental studies SARS-CoV-2 did not successfully infect chickens. Taken together, current data suggest spill over into birds is unlikely.

Read the full review at FEMS Microbiological Reviews.