Maybe something scared them, and they all dropped dead?
I remember that in California, the public health department would occasionally send notices to watch out for large numbers of dead birds, and not to touch them if you found them. They could have an encephalitis virus that is also deadly to humans.
But in Boston, I don’t know... it does not seem like a disease would cause a group of birds to fall out of the sky, dead. But suddenly startling them, or a whiff of poison could do it.
Seems like there was fear of “bird fall” with West Nile, if I recall correctly.