The blackbird that built her nest in one of our bushes disappeared shortly after construction was completed and we assumed she was gone. Then, a couple of weeks later we spotted her going in and out, and careful closer inspection revealed that she was sitting on the nest most of the time. After about 4 weeks of nothing else happening we figured her eggs, if there even were any, weren't going to hatch.

Then I noted to Tina that I'd seen her change behaviour - she was collecting food and going in and out more regularly, although still spending most of her time on the nest. There was no sign of a chick, so maybe she'd got bored and hungry?