When younger I lived next to a duck pond so my mum had her fair share of me and my siblings bringing ducklings back. We just used to give them bread and water, keep them in a box. They need heat so either stick them in airing cupboard or lay a hot water bottle on base of box with blanket over or a heat lamp will do. Sometimes if we got just one we would put a mirror in the box so they would get lonely. Unfortunately some cant be helped and we lost a few but helped alot too and they grew up and stayed in the garden. I have many home videos of me sat on couch with ducklings on my shoulders :-) Oooh they like banana too