For those contemplating a trip to see the Slaty-backed, it might be worth mentioning that it was absent most of today. From what I gathered from speaking to another birder, the gull arrived only shortly before 3:30pm. Apparently, few gulls where present during the low tide in the morning and early afternoon. There was, however, a large group of gulls on the distant mud flats on the Washington side, near the mouth of the Cowlitz river. When I arrived nearer high tide most of the gulls including the SBGU where back on the docks. I’m not sure if this is a daily pattern for this bird but it’s worth considering.
Also, if you have bread to spare the Slaty may thank you with good photo views.