请教各位英语大神一句游戏里的俗语！！！！！

ytq87349225

游戏一开始，师徒二人帮助锅盖头赶走狮鹫那段
锅盖头：You'd…You'd like a reward,I suppose?
白狼：You don't owe us anything.You were in need. We helpd.
锅盖头：And they call witchers heartless.Say they won't life a finger without pay.
白狼：They also say mice are born of rotting straw.
请问mice are born of rotting straw是什么意思呀？感觉像是俗语。网上也查不到。希望英语大神答疑！