Heavy Pressure Birds are very committed to a high pressure site. Chances are they were born on this specific ledge, usually a protected overnight roost and may have raised their own young in the same place, thus they consider this area home. They react to products there much like you would if you found plywood nailed over your door; you d be determined to get past it. Heavy pressure sites always provide the birds shelter from the elements. Medium Pressure Birds are a little less committed to medium pressure sites. This is more likely an area where they observe food opportunities or rest in the afternoon there for many different reasons. They re fairly committed to the site, but they don t roost or nest there. Light Pressure Like a bench you occasionally sit on in your neighborhood park, light pressure sites are take-it-or-leave-it sites for pest birds. They may stop by after a meal to take in the warmth of the afternoon sun, like the parapet around an office building. The absence of food and shelter makes it a casual daytime loafing spot.