Wednesday, December 4, 2013

Identification: Red-breasted and Common Mergansers in Flight

Adult Males in Breeding Plumage

Eric writes: Adult male Red-breasted and Common Mergansers have very distinct patterns thus they are quite easy to identify even at large distances.  At close range, Red-breasted Mergansers appear grayish overall with a dark head, white neck patch, and reddish breast.  Their wings have large white patches that include most of the secondaries and greater and lesser coverts.  Common Mergansers have similarly dark heads but have an overall white-bodied look as opposed to the gray-bodied look of Red-breasted.  Commons also have similar white wing patches but they are larger with more contrast then Red-breasted. Even at a distance these patterns are easily noticed.

Red Breasted Merganser in Flight
P2. Breeding plumaged male Red-breasted Merganser-Notice the overall gray-bodied look, dark green head, reddish breast and white wing patches.  

川秋沙 Common Merganser (雄成鳥)
P4. Breeding plumaged male Common Mergansers-notice the overall white-bodied look, distinct and contrasting dark head, and large white wing patches.  Commons also have whiter underwings which lead to a more white overall appearance.

Females and Non-breeding Males

I'll treat females and non-breeding males (including first year birds) the same, as the only major difference between the two is the amount of white in the wing.  The adult males have white median and lesser coverts while the females and first year birds only have white secondaries and greater coverts.  

From a distance, it is important to pay attention to overall patterns as well as shape.  Red-breasted Mergansers appear mostly gray with a brownish head and white wing patches (amount of white in wing patches dependent on sex).  Commons have a much colder appearance; overall they are white and pale gray with very similar white wing patches to Red-breasted.

Red-breasted Merganser (Mergus serrator) in flight
P1. Nonbreeding plumaged male Red-breasted Mergansers.  Notice the overall grayish-brown appearance, low contrast between head and neck, and slightly upturned bills (doesn't always appear this way).
CAB03880a
P6. Female Common Mergansers.  Notice the overall cold gray/white appearance, high contrast between the head and neck, thick/straight neck, and the straight appearance to the bill.
It takes some practice to identify these two species by shape but it can be done with some practice.  If the birds are close it is easy to see the differences in bill shape.  Red-breasted Mergansers have thin-based bills while Common Mergansers have thick-based bills.  This is tough to see at a distance but gives the impression that Red-breasted bills are curved slightly upwards while the Common has a straight-billed appearance.  The difference in the thickness of the neck of these two species is apparent at a distance.  Common Mergansers have thick, straight necks, while Red-breasted have thin, spindly necks.

Red-breasted Mergansers f.
P3. Female Red-breasted Mergansers-even at this angle the thin, spindly-necked appearance is noticeable.

Common Merganser
P5. Female Common Merganser-notice the thick, straight necked appearance

P1-Photographer's Photostream on Flickr
P2-Photographer's Photostream on Flickr
P3-Photographer's Photostream on Flickr
P4-Photographer's Photostream on Flickr
P5-Photographer's Photostream on Flickr
P6-Photographer's Photostream on Flickr

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