Birds putting rocks in ground feeder

I have a ground feeder which is frequented mainly by house sparrows, jackdaws, rooks and pigeons. I have noticed that when it is empty or low, the jackdaws/rooks will place rocks in it. It's a rock garden, so the feeder is surrounded by them. I have tried searching for why they might be doing this but so far haven't found anything. Is there a reason? Are they 'trading' rocks for food possibly?

  • i'd guess they're trying to raise the water level up.

    i have crows which are always putting bread they've got from other gardens in my water bowl. then they come back later and take it away again, presumably after it's been softened up

    [edit... sorry, i. misread that and thought you were talking about a water bowl on the ground. I haven’t got a clue why they would do it for food!]

  • Do they use the stones to stand on if the feed is low instead of reaching over the edge of the feed tray ?

    Pete

    Birding is for everyone no matter how good or bad we are at it,enjoy it while you can

  • In reply to Seaman:

    Not that I have seen, they just leave the stones there shortly before flying off. I think you might be right though, although I haven't noticed them having any issues reaching over the edge of the feeder, I struggle to think what else it could be! I have seen house sparrows standing in the feeder, but the stones are far too big for them to pick up and I'm not sure that the rooks/jackdaws would do that for their benefit.