I could say from my short visit to Seattle this last New Year's that Mew Gulls hang out in the parking lot at Sayer's Park along the southwest side of Lake Washington. Michael Donahue and I had a mixed flock of the following on 01 January 2012:
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24 Mew Gulls
35+ Ring-billeds
3 California Gulls
6 Glaucous-wingeds
1 Herring Gull
2 Thayer's Gulls
3 Glaucous-winged x Westerns
1 Glaucous-winged x Herring
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I can add that the Mews?were much more reluctant to stay in the parking lot once human traffic increased.
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Best wishes,
Amar Ayyash
Frankfort IL