Why Don't Birds Have Teeth?
Adam Rifkin stashed this in Birds!
The first response is simple:
Beaks are heavier than teeth.
But the real answer is more complex:
Having teeth means additional associated features, including bone lining the sockets and the subdental platform associated with a tooth row on any given portion of jaw. In comparison, a beaky rostrum isn't just an airy, modern bird's beak: early birds with beaks had the same internal makeup as early birds with teeth. An enantiornithine with a beak didn't look very different from one with teeth.
But beaks have additional tissues associated with them than rostra without teeth, forcing a necessary comparison to the weights of these distinct structures. A beak will also cover more space in a skull than a tooth occupies, as it wraps around the rostrum and mandible. Following Lautenschlager et al., the weight of keratin is higher than the weight of a tooth, but not merely because how much is required. Keratin only forming a plate on a limited extension of the outside of the jaw has greater weight than a tooth along the same section of jaw.
Thus, a beak cannot be due to "weight saving" in a bird. Birds also seem to have diversified and even developed the full modern flight system with a full set of teeth (e.g., _Ichthyornis_), leading one to wonder how the idea of weight saving came about. Minimization of the early bird lineage doesn't necessarily correlate to loss of teeth, and multiple losses of teeth occurred in enantiornithines as well as standard non-neoavian carinates without concordant shifts in body sizes.