Probably should ask anyone that has a bird at flying weight and being driven to the field where potential normal prey is seen along the way. The bird will use up all of its available energy bating at the passing prey. Thing about birds is that they are highly reactive to stimuli which is one of the reasons that they are generally readily trained compared to some other life forms. But, they do not normally have much ability to turn this reactivity off or tune it out, it isn't in their nature. You will thank them profusely for their reactive nature when you get out there and are hawking with one. Because trust me, when the conditions are uncomfortable for you and your bird is in a tree being calm and non-reactive to anything you are offering it...you'll be cursing profusely!
It can be a completely different story with some species when they are not at flying weight. Some will sit comfortably and ride across town through traffic etc., while other species (accipiters for example) would probably bate themselves to death in fear due to their very intolerant nature when above their flying weight and caught in slow traffic with full visual disclosure.