Because Princeton Books has a suggestion!
Here's a sample for Parasitic Jaeger...
One of the best things about this book is that while overall the format is somewhat like Sibley (with photos instead of watercolors) with an intro plate showing small images of all the species in a grouping and then going into each individually with its own page more or less, he changes things up where necessary. If he needs to depart from that format to do a specific comparison of a given set of birds he does. Here's an example
I would recommend this book to anyone who is going to be spending time on the ocean. The photographs are excellent (apologies for the low quality reproductions here) and I think should give a person a reasonable gestault foundation from which to build in the specific points that the authors highlight.
And full disclosure, I got a free copy for writing a review! (though I've never seen any evidence that Princeton reviews the reviews).