The Dusky-capped Flycatcher is most similar to the other flycatchers in the genus Myiarchus: Great Crested, Brown-crested and Ash-throated. Great Crested and Brown-crested are larger with larger, thicker bills. Great Crested has a darker gray throat and breast. The Ash-throated has paler yellow underparts. Dusky-capped has a browner, less rusty tail than Great Crested and Ash-throated. The calls of these very similar species are an excellent way to distinguish them. Some species of kingbirds are similar but have paler heads and lack rusty in the tail.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.