Acadian Flycatcher (Empidonax virescens)
Above: A nesting Acadian Flycatcher near Smithsburg in Washington Co., Maryland (6/10/2006).
Below: An Acadian Flycatcher along the C&O Canal at Noland's Ferry, Frederick Co., Maryland (5/25/2008).
Below: An Acadian Flycatcher in Charles Co., Maryland (9/7/2008). This bird was first identified in the field as a Least Flycatcher after hearing its whit-type calls, but review of the photos proved it to be an Acadian. It seems the most dangerous trap in birding, confirmation bias, snared me again. My first impression of the call note, which is quite similar between Acadian, Least, and Yellow-bellied, blinded me to plumage details that are seen here, particularly in overall structure, shape of the eye ring, and tail shape.
Below: An Acadian Flycatcher in the Nassawango area of Wicomico Co., Maryland (5/11/2011).
Animated U.S. range map from eBird (click to read more)