Chickadee Invasion?

A week ago, Nuthatch wrote about an invasion of black-capped chickadees in southern Ontario and elsewhere around the Great Lakes. There the southward movement has been counted in the thousands at some locations. While this species is not usually associated with migration, it does move southward occasionally.

Since then, reports have been appearing of black-capped chickadees in Maryland. In the last few days, reports on the local birding listserve have indicated black-capped chickadees in Harford, Rockville, possibly Violette's Lock, and possibly Washington, D.C. as well. This means it is time to start paying closer attention to the chickadees in the area. (There was also a suggestion of looking out for boreal chickadees as well.) The black-capped chickadee is not on the Maryland review list (pdf), nor on the DC review list (pdf).

A sighting of a black-capped chickadee in eastern Maryland or the District of Columbia would be an exciting event. While black-capped chickadees are common up north, they are rare down here, where the local chickadee species is the Carolina chickadee. The problem is that black-capped and Carolina chickadees look superficially the same, and can be difficult to distinguish in the field.

Most guides tend to focus upon several identification points:

• Voice: black-capped sings a fee-bee or fee-bee-ee compared to the Carolina's fee-bee-fee-bay; the black-capped's chick-a-dee-dee-dee is slower and deeper than the Carolina's
• Head shape: the black-capped appears to have a proportionally larger head
• Wings: the black-capped has more white edging on the flight feathers, and (more importantly) have substantial white edges on the greater coverts
• Bib: the lower edge of the bib is more ragged on the black-capped than on the Carolina
• Tail: black-capped may have white edging on tail feathers, and the tail appears longer
  

The important thing is not to rely on one field mark alone, especially in areas of contact between the two species. In those areas, there is substantial hybridization, and chickadees have been known to learn the "wrong" song. In Washington, at least, we are below the dividing line, which runs through Central Jersey and southeastern Pennsylvania.

Here are some online resources for solving the identification problem:

• Notes on black-capped versus Carolina chickadees from Project Feeder Watch.
• A general article on chickadees from Birdscope.
• Species accounts of the Carolina chickadee from CLO and Patuxent.
• Species accounts of the black-capped chickadee from CLO and Patuxent.
• Some additional id points from the local listserve.