I've had a pair of robins regularly picking up food and going back to a nest they've made in some ivy in my garden (presumably to feed their fledglings). 2 days ago I came home to see that they were very distressed, calling to each other and darting around my garden. I've not seen either of them since. Do you think that something had attacked their nest? Or is there any chance that the fledglings have flown the nest? I fear the worst, but am surprised that I've not seen the parental robins at all.