According to my nest recording book (see here- well worth getting a copy), the eggs are laid daily. I had 2 eggs on the 14th, so if she's laid 4 eggs, as she did last year, the full clutch should have been finished today. Apparently incubating begins with the penultimate egg i.e. yesterday's, and with he bird sitting tight on the nest, this seems to be a good indication that incubation has begun. My check only lasted 30 seconds, and as incubation usually lasts about 12 days (range of 11 - 15 days) days, I'll leave well alone now until the chicks have hatched. Hopefully there will be four healthy chicks by the end of the month!.
As always, this observation has been entered into the BTO's Nestbox Challenge Survey.