Why has a new egg has appeared in the nest chamber?

At 5.30pm yesterday a stock dove visited the nest box and laid an egg. This is not unusual in barn owl nests but they rarely succeed.
Stock doves lay two eggs and incubate them from the first so it is unlikely she will return to lay the second. She isn’t far away as her daylight purring can be heard in the tunnel to the nest chamber.
The adult owls have been hunting quite late these past few nights as the strong winds has been hampering them. The female has also started hunting as the chicks don’t need brooding. They are four weeks old and can regulate their own temperature.
Sometimes viewers can get the impression the adult owls have not been seen very frequently. As the chicks grow the parents roost away from the box during daylight hours in a nearby tree or barn. Rest assured they return at night with food as the hissing calls of the chicks travel a long way and spur the adults to keep hunting whatever the weather.
There is no doubt this is a very poor year for their main prey the short tailed vole. So they are dependant on field mice, small rats and shrews. However, the forthcoming harvest should provide a glut of mice for a few weeks which will help our chicks at the time they will need this food the most.