House Martins and Swifts together.

There's not a lot you can do, nesting birds are protected by the Wildlife and Countryside Act 1981 and it wil lbe down to the birds who nests where. If a nesting site is suitable, then it will be a first come first served basis, though there are times when nests are challenged and in some cases, birds will evict the occupier so they can have it themselves.

Nature can be harsh at times, but in the end, left to its own devices, it wil lsort itself out.

For more information on the Wildlife and Countryside Act 1981 click on the link below.

https://www.legislation.gov.uk/ukpga/1981/69

Thanks for your replies.
I might try putting up a swift box and see what happens - although it's an expensive mistake if the sparrows take over!
Recently we get more trouble from woodpeckers breaking into the mud nests than we do from sparrows taking over. Basically they are out-numbered, we have about 20 house martins nesting!

It might not work out the way we'd like but as I see it more nesting sites is always only a good thing and the rest will sort itself out. If it helps at all I have crows and pigeons feeding right alongside songbirds in my garden and they barely even argue so peace can reign!

That's great that you have swifts in your nest box. I'd be thrilled with that! Hope you get some young.

We now have 12 nests occupied by house martins. There are 2 male sparrows hanging around, they go in and out of one broken nest, but don't seem to be a bother to the martins this year.