Master birder/fieldguidist/artist David Sibley has recently picked up the discussion of "how many rare birds to we miss?" on his personal blog, Sibley Guides Notebook. Since I had interviewed John of A DC Birding Blog about that very topic back in March, I left a comment with the link in Sibley's comments. On Thursday Mr. Sibley posted his own answer to the "how many do we miss?" question, suggesting that the number is somewhere around 3 to 10 percent.
NY - one or two Short-eared Owls
2 hours ago