Oh goodness, I haven't watched It's A Wonderful Life in years (but have been wanting to) so don't quote me but I think the crows and squirrels are symbolic - the house was abandoned and I imagine the same animals lived in it before Mary and uh, what's-his-face took it over and made it a home, so allowing those animals to stay (or at least visit) shows their compassion.

That said, I'll be googling it now as it's one of the few things from the movie I hadn't thought about before - if the answer turns out any different, I'll let you know.