Kids should be given more credit. When I was no older than that I would read encyclopedic books about guns and fighter jets. Not birds, but I don’t see how reading non-fiction early is anything but healthy.
From just thumbing through it a few times, I started recognizing birds outside which made me thumb through it more. I didn't have TV or video games back then so most of my after school time turned into thumbing through the book. I had alot of it memorized within a year it seemed.
From then on, any family trip to a new territory, I could name any birds that I hadn't seen in person before, or if nothing else, could quickly look it up.