murder at my doorstep

I have a small tree by that I mean a short tree,beside my front door and directly in front of a living room window.For years I've seen this routine of birds,every mornining and again in the afternoon,they congregate here.I don't know why they'd meet here,I didn't feed them,there wasn't much shelter the tree goes bare in the winter,yet still they come and go and always in sync,they comunicate somehow cause they'd all fly in together or leave together.Recently my son had to make some project at school ,it ended up being a small birdhouse,we figured why not put it in the tree,give the birds a little shelter.And so it was..We enjoy seeing those little critters every morning,and I like to hear them and it gives our indoor cat something to look at.Well I guess it's time for nesting.I've watched birds going in and out of the little hole,I have no idea what it looks like inside.And I don't know if it's the same bird going in or not.Quite often I've noticed one bird waits outside,like standing guard,while the other is inside,seams pleasant enough.I thought he must be a nice husband and proud soon to be daddy!Then I heard awful screaching and looked to see the one (nice guy?) holding some poor other soul with his beak,and dangling the other bird, he had it pinched at it's neck,while it was hanging from his mouth I thought for sure he'd continue to it's death.So I banged on the window and the wee one flew off to freedom again.Well it's happened again.I now realize those cute little sparrows are awful mean birds and now I've read that people should do whatever they can to avoid allowing them to nest anywhere,since they'll kill other birds to take over their nests.....So that's nature,I'll keep an eye on them.