The birdies may be in your vent. Like the swallows to Capistrano, in the spring, birds head for vents in homes. Even though the vents have flaps, these birds are able to fly in and build a nest in the ductwork. Starlings are so agile that they appear to fly at full speed towards the vent, flip up the flap with their beak and disappear inside without seeming to slow down.
Stove exhaust and bathroom fan vents are the ones most commonly used. Nests are sometimes made in dryer vents but this usually indicates a break somewhere along the duct. Roof vents can also be used if the screening inside the vent is damaged. Starlings are so invasive that they will sometimes come down the furnace chimney and get into the house covered in soot. They will even go down the downspout of eaves troughs. I had one of my dogs pull the downspout right off the side of our garage trying to get at the starling that he could hear inside. Usually when birds go into these pipes they are not able to get out and will die, sometimes plugging the spout.
Nesting usually begins in April. Starlings typically lay 4 to 6 eggs and incubate them for 11 to 13 days. The young birds can take their first flight in 19 to 22 days. If the eggs are removed or the young die, then they will lay another batch of eggs. Sparrows lay an average of 5 eggs and take their first flight around 15 days. It usually takes 5 to 6 weeks from when the eggs are laid until the birds are ready to leave.
Nests consist of a mass of grass, weeds, twigs and other debris. Large amounts of material can build up in a nest especially if it has been used for several years. I have filled half of a garbage bag from a single vent. Nests can be constructed anywhere along the duct, sometimes right up against the fan. New nests will be built in front of older nests.
The problems associated with birds in vents are noise, odour, bird parasites, droppings defacing the exterior of the house and impairment of fan function. Noise usually becomes a problem once the young hatch out and the cacophony starts. This will continue until they leave the nest. Odour results from the accumulation of droppings and dead young, which the adults do not remove from the nest. Bird parasites including mites migrate away from empty nests or dead birds and may drop from the fan into the house. Occasionally, maggots and flies may appear indicating dead birds in the duct or chimney.
Prevention of these problems involves screening of vent openings to prevent entry. Ready-made screens that are relatively easy to install can be purchased at most hardware stores. Guards are available to fit into the tops of downspouts. It is usually necessary to screen all vents, as the birds will move from one vent to another. Nest material should be removed. A long piece of wire with a hook on the end can be used. Ducts should be treated after all the nesting material has been removed in order to eliminate possible parasites. Now, all your vents are screened, no birds can enter the house but wait did you just see something fly into your mailbox?