Lots of info about BBWD vagrancy on the internets. Small sample:
"Black-bellied Whistling-Ducks have been expanding their range in the southern U.S., and the North American Breeding Bird Survey shows strong population growth, estimated at over 6% per year from 1966–2014.”
https://www.allaboutbirds.org/guide/Black-bellied_Whistling-Duck/lifehistory
"Known for wayward individuals wandering northward in spring and late summer, Wisconsin’s first record of BBWD came in 1998, according to the Wisconsin Society for Ornithology. After several additional records through 2002, the species has occurred more frequently over the past decade. Six records occurred from 2011–2014 and then in 2019, BBWD was documented in six Wisconsin counties, including an impressive group of eight birds in Outagamie.
In 2020, in addition to a long-staying Milwaukee bird, a single BBWD was reported in La Crosse first in early July and again in mid-August. Then, on September 11, Andrea Frisch spotted one at Myrick Marsh and spread the word to local birders. Gwyn Calvetti headed out hoping to find the bird, even though Andrea warned it had flown off. What Gwyn found was even more unexpected — an adult BBWD with at least 13 tiny chicks!"
https://ebird.org/atlaswi/news/first-confirmation-of-black-bellied-whistling-duck-nesting-in-wisconsin
"The New Brunswick Records Committee accepted the documentation in respect to the six Black-bellied Whistling-Ducks discovered last season at the Atholville Lagoons, Restigouche Co NB. This action resulted in the province gaining its first six records of that species. Accidental vagrants to Nova Scotia, the four Black-bellied Whistling-Ducks in the Shinimcas area, Cumberland Co 6–14 Sep (Daniel Penner, m. ob.) and later observed in the Village of East Apple River, Cumberland Co (ph. Kathleen Spicer) provided the first Fall records to that province.
The New Brunswick Records Committee accepted the documentation in respect to the six Black-bellied Whistling-Ducks discovered last season at the Atholville Lagoons, Restigouche Co NB. This action resulted in the province gaining its first six records of that species. Accidental vagrants to Nova Scotia, the four Black-bellied Whistling-Ducks in the Shinimcas area, Cumberland Co 6–14 Sep (Daniel Penner, m. ob.) and later observed in the Village of East Apple River, Cumberland Co (ph. Kathleen Spicer) provided the first Fall records to that province."
https://www.aba.org/atlantic-region-fall-2021/