Adult Northern Harrier males display a very curious and interesting behavior during the mating season. The male courts the female by flying high in the air and then dives down twirling and spinning. They are very territorial during the breeding season and will attack anything and anyone that gets near their nest site.
Relevant Bird Species Information Page (includes more stunning photos of this bird species)
Photographer: Greg from Vancouver, Canada - http://www.flickr.com/people/greg7/