In the long tract on "Gulls again", CORBEN at DELPHI.COM,
Re: Hybrid Western x Glaucous-winged Gulls, is quoted as saying

>between the primaries and mantle feathers make comparisons difficult. To
>illustrate this point, I recently saw a Gw which I initially thought had
>primaries the same color as the mantle (except for their tips, of course -
>this bird was in definitive Basic plumage). Later I saw this bird with its
>wings outstretched. Then I could see that there was indeed a contrast
>between the bases of the primaries and the darker, more distal portions,
>and it seemed then that the paler bases were closer to the mantle in color.
>The point is that this contrast was SUBTLE, only visible because we were

... and, ...

>Yes, but let's make sure we are talking about the same degree of contrast
>here. Dennis regards birds with primaries the same color as the mantle as
>rare, while I would say such birds are common. Is this because the
>contrasts which Dennis is talking about are subtle, such that I am likely
>to overlook them in the field? If so, is this the same level of contrast
>that Joe is referring to?

Many of us have read articles in Scientific American, and in other popular
magazines and more academic journals about how easily the eye is fooled
by the relative darkness/lightness/color/lighting of variously juxtaposed
patches of gray/color.

I wonder how much of the confusion in field observations, or even in
hand observations, such as described above by CORBEN can be contributed
to our human visual system's susceptibility to being "fooled" by
the situations...
Peter