Incubation begins as soon as the full complement of eggs is laid. The brooding instinct is not as strong in young females who have only just reached sexual maturity as it is in older females. Sometimes a person merely passing by the nest box is enough to cause the female to abandon the nest. Even the warning sounds made by other birds when one passes near ihcir nests in woodland can often cause a young female in an area quite a distance away to leave her nest. Brooding is normally a solitary and intensive occupation for the female, which is reluctant to leave the eggs. With most females, one needs to be very careful when taking down the boxes to make observations (including lifting up the bird to ring it, which in Britain may only be done by ringers licensed by the Nature Conservancy Council), otherwise the eggs will be abandoned.
The brooding tendency is initially weak, and birds of all species are sensitive to disturbance at this stage, so ringing of the females should only be performed 6 8 days after the start of incubation.
Temperature also influences brooding behaviour. On a warm, sunny day when the nest box is warm the female may leave the eggs for long periods, but brooding will be more intensive when it is cold.
The male feeds the female very meagrely at this time, usually only at hatching and when the chicks arc still very young.
The eggs hatch after an incubation period of 13 15 days and this often coincides with a day when the sun has warmed up the nest box a little. When hatching is completed, the female removes the eggshells from the nest box. Unlike birds in open nests, which are very careful to keep their nests hidden and take the egg shells far away, the pied flycatchcr drops the shells in the immediate vicinity of the nest box. This difference in behaviour is also reflected in the egg colour. Hole nesting birds often have white or pale coloured eggs (pale blue in the case of the pied flycatcher) because within the darkness of the nesi hole hiding the eggs from would be predators is not a problem. Birds which build nests in the open or lay their eggs on the ground however, usually have highly coloured eggs which are often elaborately patterned as part of their camouflage. Unhatched eggs and dead young are left in the nest for the remainder of the breeding period.
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