The flight call of the Indigo Bunting is a fine buzz, and is commonly heard in the fall from foraging birds hidden by vegetation (Sep, PA).
It is longer than the short buzzy calls of warblers, and lower than the call of the Swamp Sparrow (see Sparrow calls I : flight calls for comparison of the three calls).
Presumed Indigo Bunting nocturnal flight call (1.24am, Oct, PA)
The Blue Grosbeak flight call is lower (perched immature, Oct, PA).
The Dickcissel flight call has the lowest buzz (Blue Grosbeak is in between Indigo and Dickcissel).
This bird was in flight at Supawna Meadows NWR, NJ in November (with Carolina Wren calling).
Dickcissel in flight at night (Aug, PA).
The Rose-breasted Grosbeak is quite vocal during nocturnal migration, having a variety of flight calls, some of which sound like the nocturnal calls of thrushes, see Thrush calls I : flight calls. They call throughout the night, but without the increase towards sunrise that is heard from the thrushes. The calls are quite variable and are rarely heard after dawn, when the birds switch to the familiar squeaky call. There are three main categories, off-key whistles (which can sound like Swainson's or Hermit Thrush), distinctive hoarse calls, and buzzy calls (which sound similar to Wood Thrush, and are presumptive: they have apparently never been confirmed by birds calling during the day). Many Rose-breasted Grosbeak calls are "polyphonic", consisting of two simultaneous sounds made by the use of both syrinxes. All nocturnal calls are presumed identifications.
This recording was made 55 minutes before dawn, and features whistles and hoarse calls of Rose-breasted Grosbeak; note that the call at 3s is a presumed Swainson's Thrush (end of Sep, PA).
Whistle and hoarse call (90 mins before sunrise, Sep, PA).
Hoarse call and whistle after sunset (8.00pm, Sep, ME)
(5.30am, Sep, PA)
Whistle, which could be confused with Swainson's Thrush (2.47am, Sep, PA)
Hoarse call (1.05am, Sep, PA)
This call is Hermit Thrush-like (1.30am, Sep, PA)
Short call (1.01am, Sep, PA)
More presumed Rose-breasted Grosbeak calls (Sept, PA):
At first I posted this as possibly an odd pure-toned Gray-cheeked Thrush, but now I'm fairly certain it's yet another Rose-breasted Grosbeak variant (6.09am Oct, PA)
Rose-breasted Grosbeak buzzy call (9.21pm, early Oct, PA). It sounds lazier than the finer buzz of the Wood Thrush, but the calls are similar. Note that this call is very unusual in that it has never been confirmed by a sight recording during the day, and is a call presumed to be made by migrating Rose-breasted Grosbeaks based on its association with other known calls (Evans and O'Brien).
Another presumed Rose-breasted Grosbeak buzzy call (10.59pm, early Oct, PA)
Calls by perched birds shortly after dawn (Sep, NJ). Calls often sound "off-key". Similar calls by Swainson's Thrushes tend to rise, and may have a burriness.
The commonly heard nocturnal "hoarse" call (and other flight calls) are only occasionally heard after sunrise, perhaps when several birds are found together. This bird called for over 40 minutes (7.45am, Sep, PA).
A variety of calls by a perched bird, some vireo-like (with Tufted Titmouse, American Robin, Sep, PA).
The Scarlet Tanager flight call is a clear chu-wee. Similar Veery calls are burrier (perched bird, Sep, PA).
The flight call seems to be less frequently heard at night than those of the thrushes (11.06pm, early Oct, PA)