Ducks may like water, but they don’t use it to navigate

It would make sense for waterfowl to use waterways as navigational aids while they migrate south for the winter. And biologists thought that North American ducks used rivers as leading lines for migration. Scientists in the mid-20th century even reported seeing ducks do so.

But as is often the case in science, what we think should be true does not always turn out to be so: Researchers tracking ducks in Illinois found that the birds completely ignored a river that should be useful for their navigation. Benjamin J. O’Neal of the University of Illinois in Champaign and colleagues report their findings August 20 in Ibis.

The problem with relying on the earlier scientists’ findings is that ducks are nocturnal. That makes tracking their migration a bit more difficult than simply looking up into the sky. So O’Neal and colleagues used weather surveillance radar from 1995 to 2009 to detect the direction of departure of ducks leaving a major stopover point along the Illinois River during the fall.

Most of the waterfowl that use that spot are dabbling ducks — mallards, American black ducks, northern pintails, green-winged teals, American widgeons, gadwalls and northern shovelers. They are generally headed southeast, to winter in Kentucky, Tennessee, the lower Mississippi Valley and southeastern U.S. wetlands.

The Illinois River does head south-ish through Illinois, so it would seem to be an option for migrating ducks. But it veers southwest, and the radar revealed that the birds don’t head that way. In each of the 15 years of the study, the birds immediately flew southeast, away from the river, upon departure.

The birds aren’t paying any attention to the river for navigation, the researchers conclude.

Just what method the ducks are using to navigate isn’t yet clear. One possibility is geomagnetism, which would have the advantage of working even at night and during periods of cloud cover. Plus, the birds were consistent in timing when they departed, which, the researchers note, “provides evidence for a navigation mechanism that is precise and perhaps socially related.”