The Solitary Sandpiper is a common shorebird in Vermont, both in the Champlain basin and further inland. It is also a commonly nocturnal migrant, being quite vocal in flight and easily identifiable as a peep species. Disambiguating it from Spotted Sandpiper is the main challenge for identifying this species. Many birds would be best left unidentified. Luckily, the night calls are almost identical to the day-time flight calls for this species.
Pieplow (2017) notes one call given as an NFC: the pwee-pwee-pwee.
Commonly heard from migrants. Given in alarm or upon flushing; also during nocturnal flight. Almost never more than three notes.
He notes an infrequently heard version on breeding grounds, where the notes are nearly monotone. This is useful, because the pwee-pwee-pwee example he gives is not monotone - the notes have a depth from 4.5kHz to 6kHz, not including the ascenders going from 3-7kHz. The Spotted Sandpiper gives a more monotone call as it's pee-pwee-pwee call, and lower, averaging around 4khz. He goes on:
Series of 2-3 upslurred whistles... usually 3 notes, all on same pitch.
Sibley (2016) describes Solitary:
Flight call a clear, high, rising whistle peet weet weet; higher and more urgent than Spotted Sandpiper.
Birds of the World (Reed 2020) covers nocturnal flight calls in some depth:
When flushed and in migratory flight, gives a 3-note wheet-wheet-wheet call similar to that of Spotted Sandpiper but more evenly pitched (Paulson 1993). Call also similar to that of Green Sandpiper but softer (Webber 1968). When call has 3 syllables, bird is either leaving an area or briefly passing through; when 2 syllables, it is more likely simply changing position at existing site (Nichols 1920a). Nichols (Nichols 1920a) describes an unusual kikikiki and also says there may be 2 types of pip or pit notes.
Clarfield has also heard Solitary described as "tinnier" or "hollower". (Clarfield, personal communications)
Dunne (2006) notes:
Most often heard call is a high, sharp, clear, whistled "Pee-Peet" or "Pit-Weet" (sometimes trebled) that recalls Spotted Sandpiper but is louder, sharper, and more assertive.
The pwee-pwee-pwee call is identified as an NFC call by Pieplow (2017). Note that this information has a sample size of 2. Call assessments eyeballed from Pieplow (2017).
A few good examples of the calls are here and here.
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.
Pieplow (2017) notes at the back that the Solitary Sandpiper has an upslurred call, where he lists Spotted Sandpiper as well as Baird's Sandpiper, Olive-sided Flycatcher, and Prothonotary Warbler song - all of which can be discarded as based on a typical Solitary Sandpiper call comparison by looking at the length of the calls, for Baird's, and the time of night and location, for the other two. This leaves the the Solitary Sandpiper as main contenders for confusion.
I think another confusing bird would be Upland Sandpiper, which has a similar, if lower, call. Green Sandpiper (à la Webber 1968) isn't useful to go through, here, as it is unlikely to be recorded migrating in North America.
Call assessments eyeballed from Pieplow (2017). There are several different types of NFC call, also given diurnally. The pee series are flat, or form small inverted u's, and are not easily confused. However, the pee-pwee-pwee series is shaped similarly, with up-ticks. This is used for the information below. Note that this information has a sample size of 2.
This information is eye-balled from the one example in Pieplow (2017). More sample sizes and analysis would be beneficial.