Meet the lovely and appropriately named Long-tailed duck, swimming happily through a recent snowstorm sporting beautiful non-breeding plumage. These incredible medium sized ducks will overwinter here in Maine starting in about October, but commute to the Arctic by May for the breeding portion of their life cycle. Their distinctive and unique call echos long distances off the surface of the ocean waters, often announcing their presence long before they are visible. They are also unique amongst ducks in their ability to dive more than 60 meters in search of prey! I was enjoying watching a small group of these ducks forage near the coast in southern Maine, and was somewhat suprised to see repeated episodes of aggressive fighting between the males.
This behaviour confused me, since wild animals most commonly "waste" energy only on things vital to them - and this usually centers around resources such as food...and sex. Some researches feel that Long-tailed ducks are monogamous (although many waterfowl have been shown in recent years to "cheat" on their partners) and these ducks were clearly foraging with steady partners. Besides which, they weren't appearing strongly "hormonal" - everyone was in nonbreeding plumage, and they are no where near the high arctic where they will be breeding in a few short months. Food resources really shouldn't be in short supply in the Gulf of Maine, either. So what was this all about?
Researching this question in Birds of the World didn't provide much in the way of answers, since they stated specifically that aggressive actions between birds is rare, other than during intense courtship when males will chase and bite at each other. In fact, it seems that it is more common for the males to drive the invading FEMALE off an established territory than the invading male. And yet I saw multiple very aggressive attacks, where there was significant attempts at biting and the pursued male was even driven off the foraging area, at least temporarily.
Now with my curiosity piqued, I decided to look a little further. Surprisingly, Long-tailed ducks really have not been extensively studied, despite their fairly wide distribution. I did, however, find an interesting study in other species of wintering diving ducks. The authors looked at aggressive behaviour in Canvasbacks, Ring-necked Ducks, Lesser Scaup, and Redheads. There were some very interesting facts which may help to explain what I witnessed. One, simply, is the competation associated with being within a flock. Many bird populations are regulated by food supply during the non breeding period - and when flocks are foraging together, they are more heavily draining the resources of the area than an individual pair would, triggering a natural source of aggression.
What I found perhaps most eye opening, however, was a statement that in terms of survival, the nonbreeding period is the more critical period of the annual cycle. Aggressive behaviour results in an unequal disposition of the resources, with dominant individuals having priority of access - and in some species, dominant individuals have greater survival probabilities than subordinates.
So in all, behavioral dominance in winter can result in the dispersal of birds into an "optimal habitat" in which dominant birds have a relatively high survivorship throughout the winter, and a sub-optimal "fringe habitat" inhabited by subordinate birds that have comparatively low survivorship through the winter.
This situation does seem to epitomize the idea of "survival of the fittest" - these birds may be continuing conflict even in the "off season" just to maintain their dominance and ability to maximize the resources available to them - and thereby, increase their reproductive fitness as well come the breeding season. And the cycle comes full circle - what is done out of breeding season ultimately also affects your offspring.
Fascinating!
Instagram quiz of the week -- who is this lovely little dabbler found in a sheltered cove? If you aren't sure -- click below to find out on my instagram page!
