Update: Dave Irons has now answered the question so here is his method of sorting through species, and why this bird is a Bonaparte's Gull, followed by my photos.
Dave says: "Aside from Black Tern, which is essentially all black in Spring/Summer, there
is no tern species in North America that has an all black hood, thus
terns can be categorically eliminated from consideration.
There are just six species of small to medium-sized black-headed gray-backed
gulls that regularly occur in North America. All six (Bonaparte's,
Little, Black-headed, Sabine's, Laughing and Franklin's) have occurred
Three of these (Laughing, Little and Black-headed)are quite rare in the state at any season, so unless you saw field marksthat clearly point to one of those species, they can be quickly ruled
out. During the spring/summer breeding season both Laughing and Black-headed Gulls have red bills as adults (winter adults and young birds have white heads or dingy grayish heads). Your bird has a black
bill, which quickly reduces the pool of candidates down from six to four. Adult Little Gulls have short wingtips that are pale gray to white, not black like your bird, so now we are down to three potential
species. Sabine's Gull is most often seen offshore as a migrant, with a few inland records every year in the Fall and sometimes inshore during Spring. Sabine's has a distinctive pale yellow tip on the bill in its
adult plumage, plus its head is not jet black...more grayish or brownish-gray. After removing Sabine's Gull from consideration, we are down to two potential candidates.
The two most frequently encountered black-headed gull species in Oregon are Bonaparte's and
Franklin's. Franklin's is a prairie/Great Basin breeder that is mostly found east of the Cascades. A few stray west of the Cascades and/or move north and south along the outer coast in migration. Franklin's is larger than Bonaparte's and when in full black-headed alternate/breeding plumage it has a conspicuous dark red bill that is fairly thick. It's clear from your photos that your bird has thin, all-blackish bill, which takes Franklin's out of consideration.
By a fairly simple process of elimination, the cast of potential species that fit your bird
can be whittled down from six to one...Bonaparte's Gull. Not surprisingly, Bonaparte's is far and away the most expected small black-headed gull in western Oregon at any season. Bonaparte's are
common spring migrants along the coast. From late April well into May Bonaparte's can be seen passing near shore by the hundreds or even thousands (on rare occasions).
Bonapartes Gulls are noticeably smaller than even the smallest regularly-occurring white-headed gull
(Mew). In adult alternate plumage they have a solid or nearly solid black head, a thin, all-black bill, a medium to pale gray mantle and folded wings, and black wingtips that extend past the tail. These
primary field marks for Bonaparte's Gull can all be seen in your photos."
Mitch Ratzlaff offered this also helpful response: First summer Bonaparte gull in my humble opinion. Legs do look darker than normal, but everything else looks good. Bill is correct size, shape and color. Mantle is right color. Eye rings (partial) look right for Bonies.
Thanks to both Dave and Mitch. I know now that just because a gull-type bird is tiny doesn't mean it's a tern. I'm making progress!