The Connecticut Warbler is an elusive and much sought-after fall migrant, occurring mostly from mid-September to early October in the Delaware Valley. It is generally considered to be a rather rare find, but there is a method of discovering this bird that I have had success with at sites alongside the Delaware River in New Jersey.
Connecticut Warblers have a unique chip call, a squeaky sweet “pwik”. They regularly call at daybreak, sometimes incessantly, but almost exclusively at that time or shortly thereafter. Later in the day they are generally silent and cannot easily be found. They seem to be more vocal when there are two or three together, which is often the case. Calls are most often made from low branches and then may allow excellent views of the birds.
If well heard the Connecticut Warbler chip is unmistakable, but there are other birds in the same habitat that make somewhat similar calls, such as Northern Cardinal, Indigo Bunting, Gray Catbird and Northern Waterthrush.
Cardinals have a variety of calls that could be confused with the Connecticut Warbler chip if not well heard.
The dry alarm call of Indigo Buntings is also a commonly heard sound that is fairly similar.
Juvenile Indigo Buntings sound more metallic and are often heard into September (Sep, NJ).
Northern Waterthrushes also have a metallic chip call.
In late summer Gray Catbirds make calls that can be confused with other species.
The Mourning Warbler chip is very similar to some Gray Catbird calls, but quite easily separable from the Connecticut chip.
Surprisingly, the hoarse chip of the Magnolia Warbler can sometimes sound a bit like a Connecticut Warbler.
I've also located a number of Connecticuts by hearing their Blackpoll/Yellow Warbler-like buzzy flight calls low down in vegetation. Other warblers with this type of call (including Magnolia, Chestnut-sided, Blackburnian) tend to be more active and higher up.
Swamp Sparrows have a similar but longer and finer buzzy flight call.
Indigo Bunting flight calls are also longer.
Note also that fledgling Gray Catbirds make similar buzzy calls into September.
The Connecticut Warbler’s preferred habitat is areas of weeds near woodland edges, and they may stay in the same small area for several days.
Typical Connecticut Warbler habitat in fall migration