The Canada Warbler has a distinctive NFC call. Pieplow (2017) notes three different types of calls: the tswit, tink, and chip call. The NFC call most easily identified is the tswit flight call, also called a pcht call by Evans and O'Brien (2002), or a zzee call by Reitsma et al. (2020).
The twsit call is given in flight both diurnally and nocturnally, making analysis relatively straightforward.
However, it is difficult to rule out other calls, especially for calls which aren't very clear or regular. At least one salient feature can be used to eliminate all other species - the initial dip in the inverted V - but multiple features will need to be taken into account simultaneously to come to a solid identification.
Pieplow (2017) identifies the twsit call as follows:
All year, often in flight. Quite variable in pitch and length.
This twsit call is described generically, along with the Wilson's Warbler, in the guide at the end of the book (pg. 499) like this:
Like Sreet, but with stronger consonant sound at start; generally lower, sharper, and more Chiplike. See Tseet, which is slightly longer.
Neither the Sreet or Tseet calls give a reference to the Canada Warbler, and their spectrograms are slightly different. The tink and chip calls for CAWA have different frequencies: tinks are between 8-10kHz, chips are much shorter and run from 3-10kHz. It's probably safe to assume that the tswit is the NFC given by Evans and O'Brien (2002) as the main examplar of a CAWA NFC.
Evans and O'Brien (2002) writes:
An abrupt, liquid "pcht". Sometimes a buzzier "pzch".
And describes it like this:
Measured calls (N=8) were 33.5-66.5 (42.2) mS in duration and in the 2.7-9.2 (3.2-7.8) kHz frequency range. The frequency track was single-banded and began with a steep descent from about 7-8 kHz to a low point of about 3-4 kHz (where the track was often faint or partially broken) followed by an equally steep rise back above 6 kHz where a brief and variable modulated section followed. The modulated section usually consisted of 1-3 humps with a spacing of 7.7-10.3 (9) mS and a depth of 1-1.8 (1.5) kHz.
He also notes that it sounds similar to Wilson's Warbler, and it can be similar to Common Yellowthroat.
Birds of the World (Reitsma et al. 2020) notes:
a high-pitched zzee in flight
They cite three different publications (Eaton (1914), Clement and Gunn (1957), and Curson, Quinn and Beadle (1994)) to back this up, but don't refer to the zzee again in the documentation, and never mention nocturnal flight calls or migration sounds.
Sibley (2016) notes:
Flight call a sharp, smacking, upslurred chwit.
Stephenson and Whittle (2013) note:
Clear; compressed; explosive; almost liquid; low.
Dunn and Garret (1997) note that "Flight Note said to be a high zzee."
A good call that involves a bird which was seen, recorded, and commented upon in the checklist is here: https://ebird.org/checklist/S29816506. This clip has multiple tswit calls in it.
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 most similar species is the sister species Cardellina pusilla, or Wilson's Warbler, and the more distantly related Common Yellowthroat.
Evans and O'Brien (2002) notes here are based on five samples, which may be a small sample size for analysis.
This rests on the assumption that some COYE calls lack modulation. These calls are generally short and rising, which can look like a short CAWA call without a clear downslope in the initial dip. Most COYE calls are easy to rule out; instead, this information should be used for calls which aren't particularly strong candidates for CAWA.