The way in which a species of bird fights to maintain or regain its territory is referred to as “bird pressure”, and is defined as follows:
Birds on a heavy pressure site are committed to that site, and may well have hatched there, considering the area to be their home. Heavy pressure sites have easy access to food and water, and also provide safe, sheltered overnight roosts.
Birds on a medium pressure site are slightly less committed, and such sites are usually ones where the birds have found a feeding opportunity and/or simply choose the place to rest for short periods.
Birds on a light pressure site see that site as a take-it-or-leave-it area, and will use the site for short rests etc., particularly after feeding. There is an absence of food and water at such sites, and the birds will use them as casual daytime loitering areas.