Chris, yes, I don't like the derogatory terms - even 'birds' is horrible- but when I first saw the clue I remembered the various people my teenage (and definitely not PC then) children referred to as 'old bats'.*
The clue doesn't have 'old' so the bats link is tenuous at best, and I expect it's down to setter error.
*Some of the people did deserve that term, I'm afraid - real ife does intrude on sensibilities at times!