Scott Downes wrote:

>As I was waiting at the Keystone ferry on Sunday afternoon I observed
>a adult glacous-winged gull chase and attack a male red-breasted
>merganser. The gull would jump on top of the merganser causing the
>merganser to flee under the water, but when the merganser came back
>up the gull ewould repeat the action. This continued for 3-4 minutes
>until the merganser turned on the gull and snapped at the gull. The
>merganser did not have a fish or anything else in its mouth does
>anybody know what would cause this action?

I don't have a definitive answer for you, Scott, but have observed these
gulls harassing several species of diving duck, usually one-on-one, though
not usually with the intensity you describe. I even witnessed a mild case
of this when I twitched the King Eider -- a glaucous-wing was following him
around and lunged at him a couple times as he surfaced. My assumption has
been that they are behaving as gulls often do when involved in the
beginning of a feeding aggregation of seabirds. They recognize diving birds
(including alcids) as potential sources of a meal. The strategy often works
well for them. Try watching the process sometime. Bonaparte's gulls will
follow Rhinoceros Auklets, Heermann's gulls are notorious for following
Brown Pelicans around, etc. A large feeding aggregation can include a
couple alcid species, cormorants, loons, (even seals or sea lions), and a
couple gull species, presumably depending on who's around to get excited
about it. Last summer I watched one aggregation in which the gulls (GWGU)
were dipping up suirming beaksful of baitfish (sandlance??) which had been
rounded-up/scared to the surface by diving birds, particularly rhinos.

Happy birding!

-- Janet Hardin
Port Townsend, WA
wings at olympus.net