The Wilson's Warbler has a distinctive looking flight call, given both diurnally and nocturnally. It is most similar to the Canada Warbler, and unlikely to be confused with many other birds.
David Troupes, the author of Buttercup Festival, named the "yellow Warblers wearing black caps" as "the Baubles," noting that there were "little yellow Baubles, hung around the woods!" in his book, Renaming the Birds. He must have seen them in migration in New England, as they are barely residents here. While there is some breeding in the NEK (Renfrew 2013), this is not the most common of birds here, except in dense undergrowth in the fall and spring. It may be better to look for them migrating in the fall (Renfrew 2013).
Stephenson and Whittle (2013) note the flight call among their rising pitch section, noting "clear, dense, similar to Canada, but slower, more explosive." Sibley (2016) calls the flight call "a clear, abrupt tilk." Curson et al. (1994) vary this by calling it a "hard tlik or a downslurred tsip, given in flight." Dunn and Garrett (1997) add some behavioural notes with "a sharper, slurred chip or tsip when flushed and as flight note (and occasionally when perched); this note recalls a soft Lark Sparrow (Chondestes grammacus) note and is sometimes given in a rapid series." Dunne also notes a tch or tchimp, but it's hard to tell if he is talking about the flight call or the alarm call.
Pieplow (2017) calls the flight call a Tswit, and notes that it is "given all year, often in flight, including by night migrants." He notes it as "high, quick, complex", which goes against Stephenson and Whittle (TK). He gives only one species that also gives a tswit - the Canada Warbler. Here, he notes it as "lower, sharper, and more Chiplike," and compares it to the longer tseet.
Evans and O'Brien (2002) have this to say about the subject: "Sounds similar to Canada Warbler but softer and less buzzy. Similar to Black-throated Blue Warbler but softer and more complex." They note both diurnal and nocturnal usage, calling the call an "abrupt, liquid spiv."
Birds of the World only mentions the flight song, and one chip note, probably the alarm call (Ammon and Gilbert 2020).
All notes for species are coming from Evans and O'Brien (2002), unless where noted.
Evans and O'Brien (2002) notes:
Measured calls (N=5) were 15.9-37.5 (27.5) mS in duration and in the 4.4-8.8 (5.4-7.9) kHz frequency range. The frequency track was single-banded or with a partial second band and had an initial steeply rising section followed by an equally steep descending section creating an inverted "V". In some cases there was an additional variable rising section. Presumed Wilson's nocturnal calls were typically longer and more often showed a rising section at the end of the call.
Ryan Terrill managed to get many recordings of Wilson's Warblers on a single checklist, during migration in California. His audio recordings, here, show good diurnal examples. Unfortunately, many of eBird's examples are NFCs themselves, which makes reliable identification difficult, as those are all presumed.
Other calls:
This section may not be helpful. The idea is to make it easier to rule out other species more easily without going through the whole list, which may not be possible.
The following species were noted as being similar by the cited experts. Any notes underneath the species can be used to rule out the other species for particular calls.
When describing this call in comments on an NFC checklist, try and add as many salient notes as possible and refer to this page for justification. If there are any species which seem similar, please get in touch so we can add them here. The intent here is to have a full list of differences for each possible similar call.
All notes for species are coming from Evans and O'Brien (2002), unless where noted.
The main differentiator is, of course, the shape: the Canada has a normal "V", while the Wilson's goes the other way. Some poorly recorded calls may seem similar. Note the flat modular section on the end of the Canada as a good indicator.
The Black-throated Blue Warbler can occasionally have a sharp descender, and can sometimes have a squiggle mid-call that may make the inverted V shape of a Wilson's. To distinguish, note that it goes from 6kHz to 10kHz, and that the descender has little variation. This is most likely an uncommon call. (Evans and O'Brien 2002)