Birding field guides show definitive maps of the breeding and winter ranges of every species. Or do they?
Two centuries of specimen collecting and bird banding have uncovered the distributions of North America’s birds, and modern tracking technologies are even showing us the fine details of once-mysterious migration routes.
But there is one species of bird that has become quite common in Virginia for which we don’t know the breeding range. Nor do we know how or why this species invaded the United States, increasing dramatically in abundance in the late 1990s. The bird is the under-appreciated Lesser Black-Backed Gull, abundant in Virginia Beach and not uncommon up and down the rest of the mid-Atlantic coast.
First seen in America in the 1930s, this common European relative of the Herring Gull remained rare in the U.S. until the 1980s. Many older birders still do not recognize the Lesser Black-Backed Gull because they did not grow up with it. It’s actually easy to identify with a unique slate-gray color on its upper parts, combined with striking yellow legs and feet and a colorful bill.
Immature birds are less obvious, but still straightforward to identify because they have a darker back and lighter underparts than the similarly sized Herring Gull. They also are much smaller than a Greater Black-Backed Gull.
So where did they come from?
For years people suspected there was an undiscovered breeding colony in the far north. But only two individuals have ever been found breeding in the New World, one in Alaska and one in Maine; both were mated to Herring Gulls.
On the other hand, two wintering Lesser Black-Backed Gulls spotted in the United States were wearing leg bands placed on them by researchers in Europe. Our wintering population is the result of a new migration route that has evolved since the 1930s. Lesser Black-Backed Gulls nesting in Europe may have gotten blown across the Atlantic in a storm and accidentally discovered a fertile and warm wintering area.
Why the big increase in wintering population since the 1990s? This part of the mystery remains unsolved. It could be that climate change has made the winter weather here more favorable. It could be that changes in fishing methods have depleted winter food sources in Europe. Or, the warming of Greenland may have led to an increase in the breeding population there, with the east coast of the United States as a natural wintering destination.
Whatever the cause, this new addition to our avifauna still thrills me whenever I see one, even though this immigrant is now quite common.
Get out and find some now because they will head to Europe in a few weeks. Bird populations are more dynamic than we often realize, serving as sensitive indicators of slight environmental change.
Cristol teaches in the Biology Department at the College of William and Mary and can be contacted at dacris@wm.edu. To discover local birding opportunities visit williamsburgbirdclub.org.
