I have a birdie "travel bag" myself, that I actually custom ordered just for the purpose. I really just custom ordered the embroidery on it, but it is still strictly for my boys. The main part of the bag is a thermal, so that if I am carrying water and it is warm out, I can throw an ice pack in it and keep it cool for them during the day because who doesn't appreciate a cold drink? Or if we are traveling to visit family for the weekend, it can keep a couple days worth of chop cold on the drive. It also is big enough to carry their dishes, harnesses, toys for their travel cages since I don't actually travel with toys in their cages, food, spare perches, and other odds and ends. I always carry a pack of baby wipes for the inevitable "accidents" that always happen.
I also have their first aid kit which is a separate bag, but it can attach to their travel bag with a clip to make things simpler. It has things such as blood stop, a small container of hand feeding formula (in case of illness where nutritional supplementation becomes necessary) as well as other hand feeding necessities, antibiotic ointment, gauze pads, vet wrap, cipro tablets (prescribed by my vet as a just in case; since we travel quite a bit, he prescribed me 2 tablets that I can use to make an antibiotic that can be given to the birds if I am on the road and we decide they are necessary IF I TALK TO HIM FIRST). I also have band-aids in there for me. They are by far the most frequently used item in that kit. There is more, but that's all I can think of just off the top of my head.