A reed warbler can pick one strange egg out of its nest, then spend weeks feeding a cuckoo chick far bigger than itself. It looks like a lapse, and it may be a calculation. In 1993 Arnon Lotem argued that a bird learning the look of its young from its first brood would be taking a terrible risk: if that first nest held a cuckoo, it would learn the wrong picture, and go on to reject its own chicks for the rest of its life. Better, on that argument, never to learn at all. Some warblers do eventually walk out, but apparently without recognising anything. Their own young take 11 or 12 days to raise and a cuckoo takes about 18, and the cue seems to be the calendar. The bird cannot tell that the child is not its own. It can only tell that the job has gone on too long.