What’s a Duck Sentinel?

A 21st-century canary in a coal mine.

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On Monday, Cabinet officials released a five-point plan for detecting the H5N1 bird-flu virus. According to the document, the U.S. government will test sick and dead wild birds, asymptomatic wild birds, hunter-killed birds, and bird sentinels. How do you test for the flu with a bird sentinel?

Clip its wings, stick it in a lake, and wait to see if it gets sick. In general, an “animal sentinel” refers to any fauna used as an indicator for a disease, and especially one that could affect humans. In this case, the Department of Agriculture plans to set up flocks of sentinel ducks in marshland and wetland areas where the bird flu has already been identified, at the times of year when you’d expect the highest rates of infection. (Strains of the virus have been discovered in about 30 North American locations over the last few decades.)

To establish a sentinel flock, you first need to build a pen or fenced-in enclosure big enough to house between 10 and 20 birds. Then you fill the enclosure with influenza-free ducks that have been raised in captivity. Each duck is tagged with an aluminum ID band, and its wings are clipped so it can’t escape the enclosure.

The Department of Agriculture will try to make it as easy as possible for infected migratory waterfowl to interact with their sentinels. Enclosures will be set up in such a way that the sentinel ducks and the wild birds share a single source of water and may even be able to interact through the holes in the fence. Sentinels can be tested by picking up samples of their feces from inside the pen. Some ducks can also be designated as “messenger” birds and released from the enclosure into the wild population for more intense intermingling.

Why bother with sentinels when you can just test the wild birds? It’s cheaper and easier. You can test an entire flock of sentinel ducks in an enclosure without too much effort, whereas it might be very difficult to trap wild animals. Even the messenger ducks would be easier to catch than a wild bird, since they’re more used to human contact.

Ducks aren’t the only birds used to screen for dangerous viruses. Many states keep flocks of sentinel chickens to identify West Nile virus or other forms of encephalitis. Chickens can pick up the virus from mosquitoes without contracting the full-blown disease, which makes them safe for veterinarians to handle in the field.

Sentinel animals are used in many other settings as well. For many years, canaries served as sentinels to detect carbon monoxide and other dangerous gases in the mines. (Some birds are still used for these purposes.) Laboratory scientists often place disease-free sentinel rodents in with their experimental animals. Dirty bedding from other cages is transferred into the sentinels’ cage; after a little while they’re tested for an array of diseases. Household pets can also serve as useful sentinels for environmental hazards  like lead or asbestos poisoning, since they show the health effects sooner than humans would.

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Explainer thanks Christopher Brand of the USGS-National Wildlife Health Center, David Griswold of the Pennsylvania Department of Agriculture, Gail Keirn of the U.S. Department of Agriculture, and Ted Toppin of the Mosquito and Vector Control Association of California.