Does anyone know what this little bird is please?

Not a very good picture I am afraid but this little bird is tiny and slender, smaller than a robin or a sparrow - but similar colours to a sparrow, it doesn't sit still long enough for us to have a good look. It darts around like a bat, turning direction quickly. It dives into low level bushes and perches momentarily on very thin plant branches. It may have a nest under my front porch as it seems to be feeding babies as it often has food in its beak, but it won't go under the porch roof if we are sitting there until we move, but I don't hear babies tweeting when it comes back with food. Could the babies be dead and it doesn't know, any ideas what it might be please? Thank you