A customer living in a newly built condo was hearing lots of noises coming from the venting system in the attic. At about 5 am he noticed about 4 birds flying in and out of the vents. These types of vents are easy access to birds and squirrels because of the louver doors on them.
A bird can easily open the louvers with its beak.
After removal of the birds and nesting material, cleaning and disinfecting installing a vent guard is always a smart thing to do to prevent these things from happening.