A Massachusetts Swainson's Hawk

Our Birdathon team and at least one expert have reached a consensus on the "mystery bird."  The bird was a light-morph adult Swainson's Hawk, rare in the east, and very rare in spring.

For perspective, the Massachusetts Avian Records Committee has seven attested sightings going back to 1978, but only one in summer, the rest in fall (when people are actually looking for these).  Veit and Peterson's Birds of Massachusetts lists ten attested sight records between 1955 and the early 90s, with a few in spring.  Before that we turn to Griscom and Snyder's Birds of Massachusetts, who list quite a few records of varying authenticity stretching back to the shotgun era in the 19th century.  Never common in the east, the Swainson's Hawk was neverthless not a rare bird in the era when much of Massachusetts was farmland.

Lacking a picture of our own, here is one by Brian K. Wheeler (1998) of an adult light-morph:

Image from Brian K. Wheeler (1998).  Hawks in Flight Gallery.  http://www.virtualbirder.com/vbirder/onLoc/onLocDirs/HAWK/gallery/bkwheeler/index.html