Seanetter Helen Rasmussen made an interesting find on her Portland, Maine beach this month. A dead male Common Eider turned up with a substantial wound on its neck and a peculiar stubby look to the wings. On closer examination, it’s clear that the flight feathers are all missing on both wings. This is a normal phenomenon in many sea ducks, including eider and scoters. In Common Eiders, the males and females separate and migrate to separate molting areas on the open water. There, they lose all their flight feathers at once, and spend an average of 36 days completely unable to fly. They continue to dive for food and swim around in a limited area.
Scientists are not entirely certain why this period of flightlessness evolved in these birds, but one theory suggests that the birds conserve energy they would otherwise spend flying, and use that energy to put toward the development of a new set of feathers. Molting is a very energy intensive activity, and it may be that these birds can’t afford to both fly and grow feathers at once.
An alternative theory points to the sea ducks’ heavy bodies as the reason–while many birds molt one or two flight feathers at a time and never become flightless during the molt, sea ducks may need every one of their feathers to support their heavy bodies in flight. Losing even one or two would make them so inefficient in flight, it may be more adaptive to simply molt all at once and endure a concentrated period of complete flightlessness.
As you might expect, this flightless period is one of heightened danger to the birds. They are, quite literally, sitting ducks out there on the water, able to escape predators only by diving, which often won’t save you from a determined seal, for instance. So, if we wildly speculate on the cause of the neck wound in Helen’s bird, we might spin out a yarn where a seal found a group of these flightless eiders and went on a killing spree, leaving some of the carcasses uneaten. Of course, the wound could also have inflicted postmortem by a scavenging gull on the beach, but that is much less cinematic.
We also had a report from Maggie Komosinski in Rhode Island shortly after Tropical Storm Irene passed through. Maggie had found a Black Scoter with similar, stubby-looking wings, still alive on the beach. Presumably, the bird was also in its flightless period and was pushed ashore by the winds and rains of the storm. It’s tough out there when 2 of your four limbs are seriously compromised!