I am learning Chinese with Rosetta Stone software and it is based on guessing the sense of words by pictures.
And I can't guess the difference between various types of "eggs".
When I see normal, probably hen's egg, they write
一个鸡蛋
but when I see some other types of eggs, they can write
一个鸟蛋
In these cases eggs are probably from different birds, not hens.
So what is the difference? Only bird genus or something else?