Final answer:
Both fish and bird embryos exhibit pharyngeal slits in their early developmental stages, which in fish contribute to gill development and in birds morph into structures of the ear and throat.
Step-by-step explanation:
A similarity between a fish embryo and a bird embryo in the early stages of development is the presence of pharyngeal slits, often referred to colloquially as "gill slits." These structures are part of the pharyngeal arches and connect the throat to the outside early in development. In fish, they contribute to the development of gills, while in birds and other terrestrial animals, they typically develop into other structures, such as parts of the ear and throat.