"Swallow-tailed gulls hunt most often under a new moon, when fish come to the surface under the cover of darkness."
Will the gull leave just prior to the next quarter as the moon wanes, or as the clouds cover the moon with the change of weather mid-week? Since we're not sure when the gull made landfall in WA, perhaps it makes sense that it did (soon after being spotted at Carkeek Pk - Thank you Ryan!), that this was as the moon was waxing to full.
Note: The moon is full at 00:03am on the 6th but it is also starting to cloud over more lately too, not to mention the haze.
This note is for Gull watchers on the beach, please do watch out for the high-high tide. Don't get stranded and be forced to walk across the rocks (unless you're ok with that). The tide comes in super-fast several hours before the high tide mark 10' 9" (very high tide the next few days) at 5.55pm Tuesday. (It's advisable to start back from the point by 3pm to avoid problems if you don't like rock climbing, but some folks found a way along the railroad instead of the beach, but then this way involves scrambling down the sea-wall and some rock-climbing.) The best time to gull watch is in the morning at low tide (11am). I say this as I'm not too nimble on the rocks, and had to get rescued coming back Sunday, by a very nice gentleman!