Birds are especially good weather indicators because they also show the effect of a pressure drop in many ways. For example, some birds become irritable and quarrelsome and will fight over a piece of bread. Other birds chirp and sing just before a storm. It seems they know they won't get another chance for an hour or two. Birds also seek shelter before a storm. You will sometimes see birds roosting in trees or huddling together on a wire close to a building. This is a good storm signal. Pre-storm low pressure makes the air so thin that birds have difficulty flying, so they go to roost