June,
       Karen Darcy and I were also at Centennial and saw the gulls as they 
flew.  They were, as you said, in with a flock of Ring-billed Gulls.  Although 
Sibley's does say that Bonaparte's don't mix with other gulls, we didin't 
notice any black on the underside of primaries in flight which I believe would have 
stood out.  Also, the Ring-billed gull is 17" while the Black-headed is 16".  
Bonaparte's Gull is 13".  The small gulls were considerably smaller than the 
Ring-billed gulls and your photos support that.  They were too far from our 
viewpoint to notice bill or leg color.  Our conclusion was that they were 
Bonaparte's Gulls.  
       We are leading the bird walk tomorrow morning at Centennial.  Maybe 
we'll get lucky and they will have returned with the bad weather.


Kevin Heffernan
Ellicott City