Went to see the Black-headed Gull in Gloucester, MA yesterday and lucked out; it was there. Here are some digiscoped photos. It's mainly found in Eurasia. A few breed in Newfoundland and they're common there in winter. They are uncommonly found here in the Northeast in winter. So it's exciting to see one.
The red bill (with some dark on the tip) on this bird really stood out. Note the red legs. This adult bird is in winter plumage, so it does not have the dark hood of breeding.
Here it is with some Mallards.
There were some Bonaparte's Gulls, (the front five, the back two are Ring-billed Gulls) out in the water. Bonaparte's look similar to the Black-headed Gull but are smaller and have dark bills. Black-headed Gulls are know to hang out with Bonaparte's, so look closely through any Bonaparte's you see for a larger gull with a longer, slightly down-curved reddish bill.