For those curious about birds (@llundgren, @jeanette-pirlo) – yes, they do have atlas and axis vertebrae. In fact, birds have more cervical vertebrae than most other animals; for example, a swan has 25 cervical vertebrae, while most mammals only have 7. If you’d like to see a fossilized bird atlas, I’m attaching a figure from Bertelli et al. (2006) showing the atlas of Paraptenodytes antarcticus (a Miocene penguin).
Also, just to clarify Lee’s comment: the majority of all vertebrates have modified first and second cervical vertebrae called the atlas and axis. Fish are known for lacking specialization of the atlas/axis, but all amphibians, reptiles, birds, and mammals have atlas and axis vertebrae. Obviously some groups have a more specialized atlas/axis than others, allowing for unique head movements (like those of owls).
Citation: Bertelli, S., Giannini, N. P., & Ksepka, D. T. (2006). Redescription and phylogenetic position of the early Miocene penguin Paraptenodytes antarcticus from Patagonia. American Museum Novitates, 1-36.