As well as "mama" and "da" and "bye bye", Helen has spoken approximations to "duck", "up", "ball", "cat", "dog", "bird", "banana" and (at nursery) "nappy". "Duck" was one of her first words and she used it of toys (the first use I observed was of a blue toy duck in the Wesley Memorial Church) and of pictorial representations (notably in the book Toddle Waddle) before extending it to real birds. That and "bird" are currently her favourite words, though she's still a bit confused - if not positively put out - by not all birds being ducks.
Here's a video of her chasing a pigeon, going "du, du, du" as she toddles after it:
But nested categories are hard! (We're thinking of going to the Wildlife and Wetland Trust centre at Slimbridge this weekend if the weather improves, where many other kinds of birds may complicate things further. The Linnaean binomials can wait a few more years, though.)
Helen's passive vocabulary must be many times bigger, possibly as many as a hundred words, though many of those would only be vaguely understood. She understands the difference between "splash" and "slosh", along with words like "tummy", "nose", and so forth.
Update: We took her to Slimbridge, where there were many, many ducks - especially since we decided that coots and geese would count as ducks for the day - but we tried to distinguish swans and other "birds". Helen also liked the otters.