We'll start with a well-known coloring book artist, Valentina Harper. Her book has bird drawings full of textures and patterns to fill in.
This one has a simpler patterns to color that might be good for the younger artist.
Here's a nice one where the birds are the main image. Not a lot of background but the birds are intricately detailed with patterns to color in.
Here's another one of birds with intricate patterns to color in.
Here's one with not only pattern-filled birds but also full-page intricately detailed feathers.
For more realistic coloring, for birds without patterns, here's an Audubon birds coloring book.
Another one without patterns that is suitable for younger artists:
Here's another one. This one's nice if you want more background scenery in your bird coloring pages.
This one is sort of a combination coloring book. Some of the bird are more realistic line art, some have more pattern. It also has a variety of other elements with the birds such as flowers, butterflies, and bird nests.
This coloring book is just about birds of prey for those who love eagles, hawks, and owls.
And, finally, one of Amazon's best sellers, a coloring book just about owls. This is a cute and quirky coloring book with owls in top hats, and owls riding rocking horses, owls surrounded by flowers, or fantasy landscapes. But all these other elements give you lots of things to color besides just the birds and will keep you busy for many hours.
All these will keep you busy until my coloring book is released in October. Of course, you can also pre-order it any time so you can be one of the first to get it when it's released!