Giant Bats are worse than Giant Cave Swallows.
What? I dare you to say that again!
Just 20 minutes ago from now I lost 5 good dwarves to Cave Swallows. 2 others were gravely wounded with mangled body parts.
During the entire length of my fortress I've lost 7 dwarves to Cave Swallows.. 2 isolated incidents, and 5 in the massacre that just happened a few minutes ago.
One time a lone woodcutter was attacked, but he got in a lucky slash and survived unharmed.
However...
Shortly after this, a group of immigrants arrived from the north, and as such they had to walk around the chasm. Usually all activity is from the south side.
All looked good and well... what I didn't notice was that 3 Giant Cave Swallows had seen the migrants arrive and trailed them towards my fortress, or, more importantly, towards my more experienced dwarves that were busy collecting lumber that the woodcutters left behind a little to the west of my fort entrance.
Suddenly I noticed one of the swallows chasing a migrant away from the group, and I immediately set my army to go outside and help the guy. While they were running outside, another migrant got torn apart by a Giant Cave Swallow more to the north that I hadn't even noticed.
A group of archers arrived, but it was too late.. Just as they arrived the migrant got torn apart as well. The first (unbelievably agile) archer arrived first, and took stance, but the Swallow swooped high, dodging his shots and swooped down again, breaking the archers arm instantly.
A legendary miner was nearby, and I activated him to help the archer. Together they killed the swallow, but not before the archers arm was mangled.
Meanwhile more to the north, the other swallows were swooping down on two other workers busy carrying lumber. I set my other archers on a detour to help them out, but once again they arrived too late.
One archer loosened all his bolts and ran towards the swallow, which was still busy tearing an experienced leatherworker/weaver apart. Just as he arrived there, the Swallow was done ripping limbs off, and turned his attention to the incoming archer. The swallow charged, but the archer dodged... Right into a chasm. Nothing was ever seen or heard from him again.
The left-over swallow was shot down quickly, as was the other, that had just started hitting on the last migrant (mangling his arm before being shot down).
I had to use up half of all my new-come migrants to replace the casualties.