When it comes to thread painting it seems to me that most designs/kits are floral-oriented. I would venture a guess that the second most common theme is birds.
I have stitched a handful of birds over the years and I definitely have more planned for the future. I find that birds can be a bit more tricky to stitch than flowers, but once you embroider a few birds they become easier. I wouldn't, however, start with a bird as my first thread painting project, a simple flower is probably a better choice.
The first bird I stitched was
Robin on Blossom Branch which is
a kit of an American robin designed by Tanja Berlin. I had thought about starting with a bluebird by Trish Burr, but didn't think I was quite ready. I wanted a simpler bird.