I just want to answer a few questions I have been getting.
 
In regards to Robert Royce's gull photo at http://www.rarebird.org/forum/forum_posts.asp?TID=2643, I have seen numerous birds like this through the years and have tried to turn them into something.  I have come to the conclusion that they are just herring gulls.  Herring gulls show a tremendous amount of variation throughout North America.  The wingtip patterns of the West Coast birds are different from our birds.  There is a difference between birds around the Great Lakes and the Northeastern part of North America.  When I saw these pale-tipped birds I initially wondered whether they were the Scandinavian race of the European herring gull, as these birds show a considerable of white in their wingtip.  They however are said to have a darker mantle color.  In the northeast part of North America some of the herring gulls are said to have similar wingtips to these Scandinavian birds.  Maybe this is one of those Northeast herring gulls.   If anything, I think these birds fall within the range for herring gull.  I really don't see anything that appears to make it a hybrid or leucistic.
 
Next time you're at somewhere like East 72nd Street where there are a lot of herring gulls, look at the variation in 1st winter birds.  Some are so dark they look black from a distance while others are fairly pale.  Some birds have all dark bills and others have pale bases.  In adult birds look at the wingtips.  Some have two white spots at the tips and others have one.  There is a variable amount of black.  Some have a considerable amount of white at the bases of the tongues of the wingtips giving them a string of pearls look as in slaty-backed gull.  The one year we had several with yellow legs!  There is a tremendous amount of variation in the herring gulls.  
 
The Nelson's gull is merely a name for the hybrid between glaucous gull and herring gull.  As Brewster's warbler is what we call the hybrid between blue-winged and golden-winged warbler, someone has come up with the name Nelson's gull for this hybrid.  It is a good hybrid to be aware of, because some of them look amazingly like huge Thayer's gulls.  They are pictured in some of the field guides.
 
People have asked me what are the best gull guides to use.  The best two are the new Dunn an Howells (2007), "Gulls of the Americas", and the slightly older "Gulls of Europe, Asia, and North America" by  Olsen and Larsson (2004).  The Dunn guide is all photographs and covers North America.  The other guide has photos and drawings and covers the Northern Hemisphere.  Both are great guides with lots of information.
 
Gulls are very challenging with lots of variation which leads to lots of confusion, but that's what makes them fun.  Just remember, unlike warblers they are sitting right out in the open giving you plenty of time to get a good look.  Learn the common birds first (all the different age plumages) and rest will come easier.
 
John Pogacnik
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