most herbivores will sometimes take animal protein when it's easily available. herbivores have a hard time getting enough protein in their diet at many times of the year; it doesn't make sense to turn down free protein when they run into it. rodents are pretty good at this, and all of the members of the squirrel family are relatively carnivorous/insectivorous compared to many other rodents. so yeah, they're gonna munch that chick if they find it unattended.
but so will tons of other things. pretty much every mammalian mesopredator in the area. a few snakes. lots of birds (birds are huge assholes). passerine nests don't have a terribly high success rate.
