https://phys.org/news/2018-05-birds-dont-teeth.html

Compared to an incubation period of several months for dinosaur eggs, modern birds hatch after just a few days or weeks.

This is because there is no need to wait for the embryo to develop teeth—a process that can consume 60 percent of egg incubation time, said researchers Tzu-Ruei Yang and Martin Sander from the University of Bonn.