Mystery bird?

Many a young birder has been mystified by one of these at about this time of year. It's a robin, still in its juvenile plumage.

No-one quite knows why some young birds have plumage so different from their parents. But it seems to occur in aggressive species, so my theory is that it's a signal to older birds to say 'give this one a break - he's only little - leave him alone'. By the time they're big enough to fight for themselves, they're moulting into the familiar adult feathers.