As soon as the nest is completed the female begins to lay her eggs. This usually happens around about the end of May. The number of eggs varies between two and eight, with six being the most common number, and 5.5 eggs being the average clutch.
Egg laying behaviour begins five days before the first egg appears. The female's egg-regulating mechanism is fully ready from this moment, after which point external factors such as wind and weather come into play. If the average diurnal temperature falls below 10°C egg laying stops and only restarts when it becomes warmer. The female spends her nights outside the nest box during the egg laying period, and returns to overnight in the box when the full complement of eggs has been laid and incubation has begun.
She lays one egg each day, except in cold weather, in the few hours between sunrise and Sam.
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