You have cat carriers, that's very good. But I would not recommend offering water at rest stops. The birds will already be stressed and trying to get water into the carrier may allow someone to escape. Not a good thing. In a cool air conditioned car they can easily go for 8 hours without water. However, they could warm up a bit if their carriers are in the sun so to offer fruit, especially watermellon, would be a good idea. Especially during the last half of the drive. Congratulations on your new birds!