The Spotted 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 Solitary 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 two calls given during migration: the pee-pwee-pwee and the shorter pee call. The pee-pwee-pwee has:
2 upslurred whistles, second slightly lower. ... Longer versoin: First 1-3 notes shorter, higher.
He notes:
Given all year, in alarm and in flight, including nocturnal migration; 2-note version most common, especially from ground. Longer version sometimes very long, sound like (and prehaps grading into) Song. Lower and slower than Solitary Sandpiper Pwee-pwee-pwee, with series almost always downslurred.
The pee series, he notes:
2-4 high short monotone whistles.. Longer than Pips and usually given in shorter series... Given by birds in flight at any time of year, including during sustained migration (dya or night).
Sibley (2016) describes Solitary as "higher and more urgent" than Spotted:
Flight call a high, clear, whistled series peet weet weet lazier and lower-pitched than Solitary.
Birds of the World (Reed 2020) doesn't cover nocturnal flight calls in depth:
Consist of weet weet and peet peet peet and are given on the ground or in flight.
Dunne (2006) notes:
Utters a short, three-penny whistle of a call, "pee, pee, pee," that is similar to Solitary Sandpiper but not so sharp, loud, or emphatic.
Clarfield has also heard Solitary described as "tinnier" or "hollower". (Clarfield, personal communications)
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.
The eBird pee-pwee-pwee call example is here, which is the two-noted call. A good example of the longer flight call is this one, which shows more of a variable nature for the calls.
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 Spotted Sandpiper has a three-parted call, and he gives examples of other birds with the same format, noting Verdin, Olive-sided Flycatcher song, American Oystercatcher Pittiweew, and the Killdeer Dee-dit-dit. Verdin and Olive-sided Flycatchers aren't worth going into here, due to range and lack of night song during migration, respectively. American Oystercatcher can be disambiguated easily: It has a long overslurred whistle as its third call. For the upslurred call, he also lists Baird's Sandpiper, Olive-sided Flycatcher, and Prothonotary Warbler song - all of which can be discarded as based on a typical Spotted 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 Killdeer and the Solitary Sandpiper as main contenders for confusion.
I think another confusing bird would be Upland Sandpiper, which has a similar, if lower, call.
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).
This information is eye-balled from the one example in Pieplow (2017). More sample sizes and analysis would be beneficial.
The dee-dit-dit call is relatively easy to disambiguate; there's a long deet, followed by one to five notes, each shorter and lower than before. (Pieplow 2017)