Lambs weren't in the manger. A "manger" is a food trough. I don't remember anything about lambs being at the birth either. The article explains that the timing

Lambs weren't in the manger. A "manger" is a food trough. I don't remember anything about lambs being at the birth either. The article explains that the timing of Jesus' birth was calculated from other events within the tradition and not just picked because there was a pre-existing winter holiday from some other religion(s).