I've read to leave them if they're feathered and not injured. They are likely fledglings who are just not great fliers yet. It's totally normal for baby birds to go through a few-day period where they spend time on the ground. The parents are probably located nearby waiting. If it's your cat, maybe you can run interference for a few hours?
I can't find the article that a wildlife rehabber sent me, but this might be helpful: http://audubonportland.org/wcc/urban/babybirds