What month was the photo taken? Gulls' predeliction for chronic
molt makes day of year highly significant. The bird at 4:30 is a Slaty-
back--note the characteristic streaking on the back lower neck, eye
smudge, narrow bill. There's a bird at 9:30, full profile but head turned
away that's presumably a Western, as are the other dark mantled birds.
In November and December the only white headed gulls in nw Oregon
are Westerns. But by March and April, when plenty of gulls are sometimes
still around Portland, almost all of them have white heads.
Dan Gleason explained not long ago the mechanism involved. It surprised
me as I had long assumed either dark feathers were molted, or dark tips
wore off. In fact doesn't some pigment simply fade away?lpn
