Fledging

Open cup nests are particularly vulnerable to predators and so it's important for the parents to get their youngsters on the wing as soon as possible. The eggs in open nest clutches all tend to hatch at the same time and the babies all leave the nests at the same time.

Nests in holes, such as those of Bluetits, are not so vulnerable to predators and so the parents are not in a hurry. Synchronising everything is not important. A mother Bluetit normally starts to incubate when the last egg has been laid but some mothers start before the clutch is completed. The last egg or two may be laid after incubation starts. This often happens in a late nest when there is a danger that the food resources for the babies may become exhausted. Whether there are late eggs or not, clutches in hole nests tend to hatch over a longer period than those in cup nests. The effect of all this is that there may be babies of differing sizes and ages in a clutch. The 'runts', the weak late chicks, tend to lose out in the scramble for food and may be weaker and less well able to cope when the brood comes to fledge.

Bluetit babies are coaxed out of the nest hole by the parents who call to them from outside and wave juicy caterpillars at them. The babies take the plunge one by one. Occasionally, the runts are too weak or timid to leave the nest. They prefer to stay at home and go back to sleep. Unfortunately for them, it is a sleep from which they never wake. The parents continue to feed the birds which are on the wing but they don't return to feed the runts.

Richard Collins

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