Take a look at the range maps in your field guide for Blackburnian, Bay-breasted, and Canada Warblers, and you can plainly see that they just don't occur here on the West Coast. But all three of these eastern warblers, in fact, have been seen in our county this October, and these rare, eastern warblers are what compel many birders to scour their favorite patch of willows this time of year.
Vagrant warblers — and no, I don't mean some disreputable bird in shaggy plumage hanging on the street corner begging caterpillars — are rare, out of place birds that somehow are on a misguided migration pattern. Some of these vagrants are more "common" than others. American Redstarts, and Chestnut-sided and Black and White Warblers, for example, are generally seen every fall in our area.
Others, like Cerulean, Golden-winged, and Worm-eating Warblers are extremely rare, with only a handful of records in this county for all three combined. There are several theories why one group of birds occurs more than others, and why they even show up at all.
One theory why some birds are rarer vagrants than others is the logical argument that there are just less of them. That certainly rings true for two of the above listed birds. Both the Cerulean and Golden-winged Warblers are declining dramatically in their eastern woodland homes, due mainly to habitat loss. The Cerulean is currently being considered for an endangered species designation.
There are other factors considered too, such as the species' normal length of migration, and the "angle of deviation between the species' normal migration route and the route that would bring it to California." (These migration theories are from the paper "Rare Migrants in California: The Determinants of their Frequency" by Steve Hampton.
Another theory is that these wayward birds suffer from "mirror image misorientation." In other words, birds that normally migrate southeasterly will migrate southwesterly instead. One study of caged Blackpoll Warblers captured on the Farallon Islands — a famous "vagrant trap" off the coast of San Francisco — found this to be the case, and most were immature birds. If you continue the line between their breeding grounds in northern Canada through the Farallon Islands, one might conclude that the next leg of their journey will take them to the deep Pacific Ocean and their doom, but the Blackpoll Warbler in its "normal" migration will travel non-stop for 88 hours, and 1800 miles, over the Atlantic. It's hard to say where these mis-oriented birds might end up.
And then there are the outliers in any study — the Common Cuckoo found in Watsonville making it's second appearance in the lower 48 states, or the Ivory Gull found on Pismo Beach instead of its usual arctic ice floe, or the Little Curlew making its first North American showing in a farm field in Santa Maria. Every birder dreams of finding those kinds of rarities.
Mr. Hampton, in the paper cited above, admits that there are many factors involved, including the fact that some birds are just difficult to identify, especially some of the notoriously difficult flycatchers, or warblers in dull fall plumages. He acknowledges that many birds go unnoticed due to stealthy behavior, or that it is simply impossible to see every bird migrating through our county. I would bet that we see a small fraction of the birds in this county, and I often wonder just how many Cerulean Warblers are here right now, maybe even in your backyard.
