Ducks of the corn

Our research shows that there are many scary things lurking in the cornfields. (The Mallard head source, under a CC BY 2.0 license; rearranged by M. Wille).

Our research shows that there are many scary things lurking in the cornfields. (The Mallard head source under a CC BY 2.0 license; rearranged by M. Wille).

By Jonas Waldenström

The Mallard is the most widespread and abundant duck in the world. It inhabits almost any type of water body, from the shores of great open lakes, to the smallest ponds. Most people don’t really notice them; they are just there. (One exception here on the eco-evolutionary dynamics’ blog).

Our negligence of Mallards should not be taken as they are unimportant. On the contrary, they are important parts of the food web, as consumers of invertebrates and seeds, and providers of juicy dinners for aerial raptors such as Peregrine falcons and White-tailed Sea-eagles. They make good food for mammals too – many a fox has dined on Mallards eggs, and we ourselves shoot them in large numbers. Very large numbers.

In our research we are interested in Mallards as hosts for diseases, especially the influenza A virus. We have studied virus carriage in great detail over the last 12 years and are now trying to connect the various pieces to a coherent epidemiological picture. However, one part of the puzzle has been largely neglected until now: their stopover ecology.

Yes, Mallards are migratory, especially the populations that live on northern latitudes . The ducks we trap at our field site originate from the Baltic states, Finland and Russia, and migrate predominately to southern Denmark and northern Germany. Daniel Bengtsson in my lab works on Mallard ecology, focusing on what the ducks do when they are on stopovers during migration.

Mallards are relatively heavy birds, with an average body mass of just below one kilogram. Migration is taxing and requires a lot of energy, and with the heavy bulk the Mallards don’t fly the whole distance at once; rather they make several bouts of migration interspersed with longer stays at suitable stopover sites.

So what do the ducks do all day, and all night? A seemingly straightforward question. During daytime we can watch them with binoculars and note their behaviors. But it turns out they don’t do very much. A typical Mallard in an autumn day in November does pretty much nothing more than sleeps, poops, and preens its plumage. Pretty dull. Occasionally it may dabble a little in the murky water, but on average they are just chilling.

This makes sense from a prey perspective: if you are a juicy meal you should stay in groups and divide the predator scouting among individuals. The talons, beaks and jaws are always ready to take a piece of you. However, at the same time a duck needs to replenish the energy stores in order to finish migration. So if you don’t feed much at day, you have to do it at night.

Together with colleagues in Germany and Sweden we have started to tap into the whereabouts and behaviors of Mallards during migration. At our aid we have modern telemetry gadgets that help us track the birds remotely. Last week we published a paper in PLOS ONE with the latest results.

A Mallard ready for departure (Photo D. Bengtsson)

A Mallard ready for departure (Photo D. Bengtsson)

We equipped wild-caught Mallards with small GPS-transmitters, fastened on the back of the birds as little rucksacks. These devices took a GPS fix every 15 minutes, thereby allowing us to see where the birds moved, and when. However, in order to get the data we first needed to find the birds in the field and then download the data with a receiver on a radio link. The best way of doing this was to make daily flights in a light airplane, gently soaring amidst the clouds until the receiver made a ‘beep’, signaling that it located a duck. On days with bad weather the plane could not be used; instead we trudged through kilometers of shoreline on foot.

In the paper we analyzed movement patterns and habitat use of 16 individuals followed across a couple of weeks. During daytime the ducks behaved just as I said before – they didn’t move much at all. But when dusk fell all birds got on their wings and flew inland, sometimes quite long distances. Each duck seemed to follow its own nightly routines, but it was also evident that some ducks followed other ducks around. A typical night would consist of a first flight from the coast to an agricultural field, most often harvested cornfields. In the fields they would settle for a short while, often less than an hour, likely stuffing their crops full of leftover corn, before embarking on another flight out to various inland water bodies. In these ponds and wetlands they spent the reminder of the night. Just before dawn, the flight would go in the opposite direction, including a short stay in the cornfield, before settling down on the coast again.

An airial view of the study site. Ducks chill by the coast at day and fly out in the agricultural landscape at night (Photo D. Bengtsson)

An airial view of the study site. Ducks chill by the coast at day and fly out in the agricultural landscape at night (Photo D. Bengtsson)

Example of typical mallard movements between frequently used sites on southeast Öland, Sweden, October – December 2010. Inset A: Orange ovals = coastal meadows; yellow ovals = maize fields; red ovals = flooded areas; blue ovals = coastal day-roosts; green ovals = coastal reefs used as day-roosts; grey circle = duck trap location. Inset B: Yellow oval (1) = maize field visited during dawn and dusk; red ovals (2) = various small (flooded) wetlands on alvar steppe (the upper one reaching into a maize field), visited at night; green oval (3) = coastal reef used as day-roost. Inset C: Yellow oval (1) = two maize fields frequently visited, mostly during dawn and dusk; red oval (2) = flooded area (stream) visited most nights; light purple oval (3) = flooded pasture visited during two consecutive nights (probably for feeding); blue oval (4) = most frequented day-roost. From the article in PLOS ONE, under CC Attribution License.

Example of typical mallard movements between frequently used sites on southeast Öland, Sweden, October – December 2010. Inset A: Orange ovals = coastal meadows; yellow ovals = maize fields; red ovals = flooded areas; blue ovals = coastal day-roosts; green ovals = coastal reefs used as day-roosts; grey circle = duck trap location. Inset B: Yellow oval (1) = maize field visited during dawn and dusk; red ovals (2) = various small (flooded) wetlands on alvar steppe (the upper one reaching into a maize field), visited at night; green oval (3) = coastal reef used as day-roost. Inset C: Yellow oval (1) = two maize fields frequently visited, mostly during dawn and dusk; red oval (2) = flooded area (stream) visited most nights; light purple oval (3) = flooded pasture visited during two consecutive nights (probably for feeding); blue oval (4) = most frequented day-roost. From the article in PLOS ONE, under CC Attribution License.

A postdoc in our lab summarized this study as “ducks like water, and food”. Although correct, I think we could say a number of other things. Firstly, we could estimate how large home ranges the ducks were utilizing, and that they very selectively chose cornfields (which comprised only a minority of the arable land in the study area). Secondly, we could show that the omnivorous Mallard is flexible in diet selection, as corn (intended for winter fodder for cattle) is a new crop in the area, and that this happens already in inexperienced juveniles during first migration – in fact, corn isn’t a much grown crop in the recruitment areas. Thirdly, our data suggests that the ducks balanced intake of corn with predation risk, trying to minimize the time spent in open fields. Knowing these things adds to our understanding of the connection between wildlife and agriculture, especially important for the Mallard,  a prime reservoir for influenza A virus that can infect domestic poultry.

Link to the article:

Bengtsson, D., Avril, A., Gunnarsson, G., Elmberg, J., Söderquist, P., Norevik, G., Tolf, C., Safi, K., Fiedler, W., Wikelski, M., Olsen, B. & Waldenström, J. 2014. Movements, Home-Range Size and Habitat Selection of Mallards during Autumn Migration. PLOS ONE 10.1371/journal.pone.0100764

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