Sitting ducks: Starving waterfowl and the freeze of 2014

There’s been a disturbing trend on the Chicago lakefront these past few weeks: ducks have been dying, their carcasses littering the rare bits of open water on the mostly frozen shoreline. The speculation around why they were dying was focused on starvation. Lake Michigan—indeed over 90% of the surface of the Great Lakes—was frozen solid. The ducks simply did not have enough area of open water in which to find the fish and molluscs that they eat.

We in the Bird Division wanted to be sure. So on March 9 & 10, Field Museum staff and interns set out along the Chicago Lakefront to pick up as many dead ducks as possible and bring them back to the museum to see if we could determine the cause of the birds’ demise. In total we collected over 30 dead ducks (and were given a few more by community members), mostly Red-breasted Mergansers. We estimated there were at least that many dead individuals that we couldn’t reach. In total we collected four different species (and saw a fifth that was out of reach), all highly migratory diving ducks that are exclusively in this area from late fall to early spring.

The easiest way to gauge their health was to weigh them and compare those weights to typical weights published in the ornithological literature. But even before this, just by feeling the birds’ breast muscles, it was clear that they were not just skinny, but starving, emaciated. Weighing them confirmed this (see Table below): the dead birds' average weights were 23.4-46.7% below published averages. What is particularly distressing is that early March is the time of year when these ducks should be nearing their maximum weight, gorging themselves to ready their bodies for their arduous migration back to their Arctic breeding grounds.

The last two days has brought the first thaw of the spring. Lake Michigan has opened, the water is crystal clear. Hopefully this means that these ducks will be able to spread out and find the food necessary to fuel their return north.

| Species | Sex | Published average (grams) | Chicago average (grams) | Difference | | White-winged Scoter | Male | 1588 (n=7) | 906 (n=2) | Anchor:

"n" denotes sample size. Difference indicates percent difference between recently collected Chicago samples and published winter averages. Sources for published weights: Birds of North America series and the CRC Handbook of Avian Masses, 2nd Ed., edited by John B. Dunning, Jr. (CRC Press, 2008).