If you had a bird which was large enough to ride, how fast could it fly? I need this information to work out the scale for my maps as i know how long it takes for one of the birds to fly from A to B.
If you read the excellent answers to this questions: Could a large bird be used as transportation? you'll learn there are a lot of interesting physical problems with a human riding on a large flying animal. But since that never stopped the determined worldbuilder let's discuss humans riding on birds in terms of what would strain suspension of belief. Because let's face it, riding birds (or dragons, or a pterosaur) is just too cool to let physics get in the way.
Correct or not there are some common intuitions about flying that I think most people are going to want satisfied by your world.
First let's talk about some real birds. A Falcon can dive at 90 mph, so there is an upper limit of sorts. We can say that our magical bird's body and bones can withstand up to 90mph. Geese can cruise at 40 mph. Winds up high can be pretty significant. If you told me your magical bird that carries humans cruised a little faster than geese and dove like a falcon I'd keep reading and hope for some cool aerial combat scenes! If you want your bird to go 200mph or mach 2 you are talking more about modern aircraft and I would have a hard time believing that something can get there by flapping.
If I'm riding a bicycle very fast it becomes risky to have my eyes unprotected. I need sunglasses or something to keep bugs and dust from getting in my eyes around 15-20 mph. If I'm riding a motorcycle, they are essential unless there is a wind screen, and then I can go up to maybe 60-70mph. Racing motorcycles can go a lot faster with the rider protected by substantial clothing and a helmet and somewhat by the bike itself. Open cockpit airplanes can travel over a hundred miles per hour and particularly crazy people have been known to exit the cockpit while in flight without being immediately blown off the airplane. https://en.wikipedia.org/wiki/Wing_walking This suggests to me that for speeds under 100mph I would expect the pilot could get away with protective clothing, goggles and a helmet. Some kind of saddle/harness would be advisable to keep said pilot in touch with his flying friend.
You're already using magic to make your bird fly while violating the squared/cubed law, bone density, and power to weight ratios so the truth is I can't give you a physics based answer. But hopefully I've given you a little perspective on what a nerd would think reasonable upon hearing about your world of giant bird riders.