I watched the nest for about 90 minutes yesterday morning. There were four noisy flypasts in that time, two with the briefest of stops on the reeds opposite, but nothing coming in or out of the nesthole. So if there is a sitting bird the shift changeovers are longer than 90 minutes. I wondered if my presence might be affecting things, but it seems logical that there would be territorial flybys without a stop, even if the nest is empty. I last saw a pair on the wooded bank about four days ago, so who knows? Incubation seems to be about 30 days, followed by fledging of 25 so plenty of time to watch and wonder!