Eggs are an interesting and slightly more complicated case for food taxonomy. Most of the time by “egg” we mean chicken egg, or more precisely “hen egg“. But eggs from other birds, fish, turtles etc. can be consumed.
Currently FoodOn doesn’t list egg directly under the “vertebrate (animal) food source” facet as it isn’t an animal directly, but rather a product of one (a similar case exists for milk). Instead various kinds of eggs are mentioned under the “foodon product type > vertebrate animal food product > egg food product category”, which encompasses fish, bird, reptile etc. eggs.