Thursday, December 9, 2010

Red-breasted Merganser

The two birds awake on the ice are Red-breasted Mergansers.  Their field marks are basically opposite those of the Common Merganser I blogged about yesterday.  First, their brown heads fade into their grayish or whitish breasts--a sharp line of demarcation is not present.  Neither bird shows a distinct white throat patch.  Although a bit hard to observe at this distance, especially the bird to the left has a much narrower bill than sported by a Common Merganser.  These mergansers have slightly different colored heads.  I suspect the darker one on the right is a nonbreeding adult.  The paler bird on the left may be in juvenile plumage.

As I mentioned yesterday, Red-breasted Mergansers, although common during migration in Minnesota, are only casual away from Lake Superior in the winter (Janssen, Birds in Minnesota).  Most spend the winter off coastal North America.  Both species of mergansers mostly eat minnows (up to 70%, in the case of Red-breasted Mergansers) and, to a lesser degree, crustaceans. The Common Merganser tends to take larger prey than does the Red-breasted. As top predators on the food chain, mergansers are  adversely affected by pollutants and are therefore indicative of the ecological health of the lakes and streams where they feed. 

When Erika and I arrived at the Minnesota park opposite Prescott, Wisconsin, we were somewhat surprised that all the waterfowl were napping on the ice.  In this photo you can see Canada Geese and two male Common Goldeneyes.  Closer to shore, a dozen Trumpeter Swans dozed.  I will blog about the swans in my next post.

3 comments:

  1. Nice distinction of ID points between the two species, Dan. What about the forehead angle? I'm seeing a steeper one on the RBME and a flatter one on the COME -is that an ID clue or just an artifact of these particular birds/photos?

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  2. You might be right about the forehead angle. Sibley says that the Common Merganser has a more rounded head than the Red-breasted. I think that means the Red-breasted head is more square, which is what you are saying. Another field mark is that the Red-breasted's crest is longer and wilder, which I think you can almost make out on the left-hand bird.

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  3. Hi Dan, I checked my Advanced Birding guide from Ken Kaufman and he says the forehead angle is a reliable field mark, along with bill thickness, esp. at the base.

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