Sparrows v House Martins (1 Viewer)

I have three House Martin nests at my house. They all appear to have been doing well until about a week ago.

I noticed about a weeks ago that 2 featherless chicks had left one the nests somehow and died.

A couple of days later a further two similarly immature chicks had somehow left a nest and died (All four were dead underneath the nests).

On keeping an eye on this I noticed a male and female sparrow watching the three nests and randomly attempting to enter them.

I therefore assume these birds have at some point entered the nests and forced the chicks out.

All three nests are still being visited and by the House Martins and they seem to be continuing to feed chicks within.

Why would the sparrows be doing this? Has anyone heard of this happening before?

Thanks

Craig (this is my first post)

House Sparrows are notorious in the States for this kind of behavior, particularly in the case of bluebird and Tree Swallow nests around human habitations. Nest cavities/boxes tend to be in short supply in such places, leading to lots of no-holds-barred competition within and between species. Destruction of eggs and the death of nestlings often result.