not sure if it's a specific style, but it's adapted from some old Japanese watercolor paintings that I have (but instead of birds from Japan, they're birds that were/are always at the feeders at home in Upstate NY). Not sure how easy it is to tell from the pic, but each bird is done in a slightly different style: the inner arm looks like it was painted on, the top looks very sharp and direct, while the back of the arm is a little bit cartoon-y. The only "style" typical from the paintings that the artist didn't think would turn out nice was the very blotted style that you sometimes see - would have been tough to translate and incorporate into the other types of birds.
overall, I let the artist just do her thing. I had the painting references, and (pretty much) knew the types of birds to include, and that's just about all the input I wanted/needed to give.