Best bet is to pull a ventilated, enclosed trailer. Make sure the bird(s) has food & water enroute on trips lasting more than a day. Stop every couple of hours (depending on heat & humidity) to open the door for fresh air. Avoid car trunks, especially in hot weather. If there is no other choice than to carry the bird(s) in the van or car with you, try using a shipping container with the membranes over the air holes, and maybe use a bit of barn lime under the wood shavings to cut down on odor.