As the Mallard flies

By Jonas Waldenström

Migratory animals are per definition mobile, performing regular movements between areas. Sometimes such movements are small, as in up or down a mountain. Other times they involve crossing 11,000 km over open sea, as the Bar-tailed Godwits do on their migration from Siberia to New Zealand.

Not exactly a rocket

Not exactly a rocket

Our model species is the Mallard. It is not exactly a rocket or a Godwit. No, it is a bulky and rather heavy bird, not designed for enduring intercontinental flight. But it does fly, and fairly decent distances. From band recoveries and analyses of stable isotope contents in feathers, we know that the breeding areas are for Mallards passing Ottenby in autumn can be roughly outlined as the Baltic States, Finland and parts of Eurasian Russia. Winter areas are more easily depicted, as a lot of ducks are harvested by hunters and the number of bands reported back during non-breeding is high.

But a dead duck is an endpoint, and doesn’t tell us much about its behavior before (or after) it was shot. As Mallards are an important reservoir host for influenza A viruses we want to know more about what movements actually mean for the epidemiology of disease. Does infection impair movements? Can infected birds transport viruses along migration to other sites? How does that affect local and global transmission?

A few years ago we started to collaborate with Martin Wikelski and his research group at Max Plank Institute of Ornithology in southern Germany. His group is a leading group on research in movement ecology, experts in animal movements. It is really a cutting-edge discipline, as new techniques for following animals are constantly developed. A lot of new cool gadgets!

Together with our German colleagues, we have carried out a number of studies with tagged Mallards, equipped either with satellite transmitters or with GPS loggers. There are a few articles in the tube, and Daniel Bengtsson, one of my PhD students, has Mallard movements as his subject area. The very first article on Ottenby Mallards appeared recently in Movement Ecology. Actually in the very first issue of the journal!

In this study, Kamran Safi gathered movement data from nine different species of birds (including our Mallards) and used it to analyze how the effect of wind support during migration best should be modeled. Completely still air is rare, and migrating birds need to adjust migration to wind strength and wind direction. A tail wind component can be extremely beneficial, and headwinds detrimental. With the modern tags birds can be followed at high sampling frequencies (at the scale of minutes and hours) during active flight, and their heading and speed can be examined in conjunction with global weather databases. But it is crucial that you used the right models, otherwise you may end up with the wrong conclusions.

2051-3933-1-4-1

Schematic representation of the calculated measures, where α represents the vector of a bird’s movement relative to the ground. Its length is vg. Wind support (ws) is the length of the wind vector in the direction of c and cross-wind (wc) the length of the perpendicular component. Finally, airspeed (va) is the speed of the bird relative to the wind and can be calculated as given above, or modeled as the intercept of a model with vg as a function of ws and wc.

Perhaps not surprising, Safi et al found that wind was a strong predictor of bird ground speed, but with variation among species. However: determining flight direction and speed from successive locations, even at short intervals, was inferior to using instantaneous GPS-based measures of speed and direction. Use of successive location data significantly underestimated the birds’ ground and airspeed, and also resulted in mistaken associations between cross-winds, wind support, and their interactive effects, in relation to the birds’ onward flight.

It is rather complex paper if you are not into the field, but it feels good that our flu-carrying little duckies can contribute with some pieces of the puzzle in the making of next generation migration models. We will return to Mallards and movements in this blog in the future, as the Mallard flies and the papers become published.

Links to the papers:

Safi et al 2013 Movement Ecology

Gunnarsson et al 2012 PLoS ONE

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