DC Audubon's chickadee field trip on Saturday, March 25, 2000, to Nolands Ferry, Hancock, and Little Orleans, each a beautiful location on the C&O Canal, was a delight. The idea of a trip devoted to chickadees occurred to President Peter Vankevich, not so much because chickadees are cute and familiar little songbirds, but because Maryland is favored with two species: the Carolina Chickadee (CACH) and the more northerly and westerly Black-Capped (BCCH). If you ascend the C&O Canal from the lower Piedmont toward the Ridge and Valley province, you pass through a transition zone, moving from the range of the CACH into the range of the BCCH, through a zone where hybridization between the two species occurs. For the observer armed with binoculars, tape recordings, and the willingness to look and listen carefully, evidence of gene flow and hybridization can be detected in birds along the towpath. The goal of our field trip was to learn the fine points of distinguishing Carolina, Black-capped, and hybrid chickadees.

Peter Vankevich, it should be said, loves the Canal enough to have walked the length from DC to Harper's Ferry (about 100K) many times, with compact binoculars or none at all! He has the T-shirts to prove it. (See his article on the Canal [0].) Our chickadee trip leader was environmental consultant and all-around naturalist Dave Smith of Carroll County, who is on the board of Audubon's Maryland-DC State Office. Fittingly, it was Dave who covered the four mile stretch just above Hancock in our January 23rd Second Annual C&O Canal National Historic Park Mid-Winter Bird Census. Hancock is near the center of the transition zone, and Dave conscientiously described the chickadees that he saw, before assigning birds to one species or the other. Equally fitting was the presence of our newsletter editor Beth Slikas, as her professional interests include the DNA evidence for origins and relationships of the world's chickadee and titmouse species.

Before the field trip, we did some research on distinguishing the two species of chickadee. CACHs and BCCHs are closely similar in appearance but differ consistently in some plumage characteristics. Most notably, BCCHs sport a white patch on the shoulder formed by white edgings on the greater wing coverts. The patch looks roughly like the white patch on the side of a Cape May Warbler but not as bold. CACHs lack the white side patch. In addition, BCCHs look decidedly longer-tailed than CACHs, and in BCCHs, the outer edge of the outer tail feathers has more white than in CACHs. Finally, in BCCHs, the lower edge of the black bib is more ragged. Hybrids, however, are tricky. Hybrid birds tend to be intermediate in plumage characteristics.

The two species of chickadee also differ in vocalizations. The "dee-dee-dee" call of the BCCH is slower and hoarser than that of the CACH. The typical song of the BCCH is a two-note "fee-beee," while the typical song of the CACH is a four-note "see-bee-see-bay." But song in both species is variable, and in areas where both chickadees are present, individual birds often sing both types of song.

Armed with this background information, we hit the towpath. Sure enough, at Nolands Ferry, the southern-most point of our trip, the chickadees all looked like typical CACHs in most traits. These birds also sang typical CACH breeding-season songs. At Hancock, in the middle of the transition/hybrid zone, most birds looked more like CACH*s and gave the faster, higher "chick-a-dee-dee" call typical of CACH*s. The song of the chickadees at Hancock, however, was usually three notes -- intermediate between the typical CACH and BCCH songs. At Little Orleans, just seventeen miles upstream, all chickadees looked mostly like BCCHs, and their "fee-bee" songs and "chick-a-dee-dee" calls had the right pitch and timbre for BCCHs, but some birds had shorter tails and less white in the greater wing coverts -- more like CACHs. We ended the trip feeling that much is yet to be learned about the chickadees along this stretch of the towpath.

Addendum: The April issue of The Auk (the publication of the American Ornithologists* Union) will include an article on hybridization between the Carolina and Black-capped Chickadees, based on the dissertation research of Gene Sattler at the University of Maryland, working with Mike Braun of the Smithsonian.