Exhaust vents move stale air from the interior to the exterior of a house. They are usually made of metal, are located on the roof of the home, and connect to some sort of fan inside of the home. These vents may run for quite a few feet and are not very easily accessible when installed. If they do not have a screen installed to keep out critters, birds may hide inside them because the air that comes out of them is usually warm.
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Go to your attic -- or the roof if you don't have an attic -- and find the vent. Bang on it with a solid object and the noise will make most birds leave.
Disconnect the beginning, or interior, end of the vent from the fan.
Hook up a leaf blower hose or the exhaust of a vacuum cleaner to the beginning part of the vent and blow air at a high speed through the vent to get the more stubborn birds out.