There's more to birdwatching than the simple sensory pleasures of bird song, colorful plumage, balletic soaring and swooping, and so forth. There's also statistics. If you want to identify a bird, you're likely to check the distribution map in your field guide. Is the bird normally in this region at this time of year? Usually, probable location is an excellent clue. But sometimes birds surprise us. Suddenly they appear in new places far from where we expect them. These birds are the outliers of ornithological data, statistical anomalies. We tend to call them rare birds.
"Vagrants" is a more descriptive term for rare birds, most of whom are migratory. Avian vagrancy raises tantalizing questions. What leads a bird to indulge in that very human trait, wanderlust? Migration is perilous under the best of circumstances. Why abandon a familiar route and risk the dangers of unknown territory? How does a corncrake (crex crex) known to breed in northwest Europe and winter in sub-Saharan Africa wind up in Suffolk County, NY? Did he lose his way?
A new book, Rare Birds of North America by Steve N.G. Howell, Ian Lewington, and Will Russell (Princeton University Press), addresses these questions and a host of others. This is, in fact, the first book to deal exclusively with vagrant birds in North America. Using data collected between 1950 (when avian census taking began) and 2011, the authors have compiled a formidable guide to species recorded in the U.S. or Canada fewer than 5 times annually. Five or fewer individuals of a given species, that's very rare indeed.
Or so it seems. As Howell and Russell point out in their comprehensive, information rich Introduction, sparse reports of a species may reflect a scarcity of birds -- or a scarcity of birdwatchers. The data are inherently skewed in the direction of human activity. If no one sees the bird, for the record, it's not there. (Isn't this another reason to encourage citizen science?)
Most of us are probably resigned to never seeing rare birds, unless we're the ones who've made the migration. So Ian Lewington's handsome drawings and the accompanying text detailing geographic distribution and field identification (including molt plumage) are the stuff of armchair reveries.
Yet everybody gets lucky sometime. One fine spring day, you or I might come upon a bird native to Europe who caught the trade winds over the Atlantic, those same winds that brought the Spanish and Portuguese explorers to America. A birder has to dream; it comes with the territory.