Field Journal: Kleptoparasitism
Today I walked over Foster Island to Union Bay where I expected to see ducks – and see ducks I did, along with coots. There were a hundred coots, if not more, among other waterfowl in the bay near the boardwalk which connects Foster and Marsh Islands. The vast majority of birds were American Coots which were busy diving for aquatic vegetation, but mixed in were a fair number of American Wigeon and a few Gadwall and Ring-necked Ducks. I’ve had a fascination with the dynamics between the coots and other ducks for a few years because they’re a regular winter presence in Lake Washington and I see them often. In the winter the coots will come together creating rafts hundreds of birds strong which is rather curious. The fact that wigeons are always found with the coots is also curious. I’ve read in the past that wigeons are kleptoparasites of the coots, meaning they let the coots do the hard work of diving for food and then move in to steal their meal.
In fact, Gadwall are also known to participate in this behavior. There’s some evidence that coots have learned to take evasive action, but otherwise they tolerate the presence of these thieving ducks. Today I watched the coots diving and the wigeons fighting over the coot’s vegetation. In fact the wigeons fight amongst themselves as well, engaging in serious games of tug-o-war. It’s not easy being a thief among thieves.
While I watched the interactions between the ducks and coots, something scared the waterfowl and they all took off, flapping low on the water, creating a massive fan shape. I couldn’t tell what had startled them, but I’ve seen this behavior frequently and my theory is that it’s an otter in the water. The coots and ducks resettled and returned to feeding, but a short time later they scattered again but this time it was obvious what had scared them, a soaring immature Bald Eagle coming their way. The eagle landed in a tree overlooking the bay and as I watched, a second eagle soared low over the water and the birds spread out again. The eagles flew together right over the floating boardwalk where I stood creating chaos in their path until they landed in trees on the far side of the island.
I walked further along the boardwalk and studied the water where the birds had been before being initially scared, and noticed some bubbles coming to the surface. I followed them along the path, back and forth but couldn’t see anything in the depths of the water. At one point a huge amount of bubbles surfaced, creating a pathway in the water. I never saw anything surface, but I’m pretty confident there was an otter in the water. River Otters are commonly seen around Marsh Island. There’s something interesting going on between otters and waterfowl that I suspect is not widely researched or observed. They live together in the same habitat and I’ve read that otters have been known to eat waterfowl. This is not topic I’ve found much information in but something I’m interested in researching further. I feel there’s a missing story here.