The Oporornis warblers, Mourning, Connecticut and MacGillivray's, can be difficult to separate in the field, given their similarity (especially first year birds) and the often incomplete or brief views that we get of them. If they call, though, identification becomes much more straightforward.
This morning I finally obtained a recording of the chip call of a Mourning Warbler, a first year bird that was in my backyard (I've been fortunate to have had Mourning Warblers in my suburban backyard in three out of the last four years; no doubt they appreciate the dense undergrowth and shrubbery).
Most of the Mournings I've seen in migration have been quiet, but this one began calling after about twenty minutes of on and off observation. In the fall I've actually found Connecticuts to be the more vocal of the two, especially at sunrise.
Mourning Warbler (Aug, PA).
Note the heavy insect noise, which makes August a difficult time to get decent recordings
The Connecticut Warbler chip is more musical or "sweeter". First year bird at dawn (Sep, NJ).
The chip of MacGillivray's Warbler is a sharper tik compared to the Mourning Warbler's huskier pwik (breeding grounds, July, CO).
There are a few calls that could be confused with the Mourning Warbler chip. In late summer Gray Catbirds start using a call that can be remarkably similar (July, PA).
Northern Waterthrushes are frequently heard in migration. They have a metallic twink (Oct, NJ).
The dry twik alarm call of Indigo Buntings is also a commonly heard sound that is fairly similar, but higher (Sep, NJ).