Swallow attacking Wren

[witham]

The Wrens are nesting again in our ponies stall in our small barn and yesterday I witnessed something I thought a bit strange, a couple of Swallows were flying in and out and when the Wren came in with food for her young one of the Swallows made a sudden dive for it and frightened it away, it then sat very quietly on a beam in the dark near the nest as if waiting and when the Wren assuming they had gone I suppose made another attempt to get to its young the Swallow attacked it again. This went on for a few minutes with Wren was getting very upset so I started rattling about doing jobs and the Swallows flew off and the Wren went back to feeding as they don't take too much notice of us.
There's no sign of the Swallows building a nest in there unless they are house hunting and want to clear out any sitting or rather nesting tenants first so why did they act in this way, would they harm the Wren chicks ? any clues. I've not seen them in there before.

[witham]

Just to say that the Wren is still feeding the young and I've not seen the Swallows in there again, although I've noticed the Wren is now using a different entrance in the back of the barn through a hole in a sheet of zinc