Adults might have to wait their turn when the nest box gets crowded
The Royal Society for the Protection of Birds (RSPB) says it has received a surprising number of reports of different species sharing nest boxes.
It says barn owl nest boxes attract the most unusual tenants, with reports of kestrels and jackdaws moving in.
Sharing blue tits, great tits and pied flycatchers have also been spotted.
The charity is urging people to put up nest boxes now, ahead of the breeding season next spring, and to provide birds with winter roosting space.
The RSPB says its wildlife inquiries team has received reports of one species harassing another when trying to get into their shared home.
This, it says, makes it even more bizarre that birds should persist with the arrangements.
Experts believe there could be a number of reasons to explain why different species are cohabiting.
Multiple cavities in some nest boxes could lead to the birds fledging from one cavity and returning to roost in the 'wrong hole' that is already occupied.
Birds may also unwittingly lay their eggs in the active nest of another species.
Another reason could be a lack of nesting sites in some areas.
The charity says many larger birds which nest in holes in trees or in buildings are experiencing difficulties as older structures are knocked down or converted.
It says birds need to be given the maximum opportunity to find a suitable nesting site.
Richard James, RSPB wildlife adviser, said: "Now is a great time to put up new nest boxes or give old ones a clean ready for the next round of breeding in spring.
"Long before eggs are actually laid and chicks hatched, the adult birds will start to scope out possible nest sites."
He suggested that anyone finding two species sharing should try to put up another box as both species may wish to return to the same area the following year.
He added: "Different species have different nesting requirements, but the general rule of thumb is to position a nest box so it isn't easy for predators to get in, and try and face it away from strong sunlight and prevailing winds."