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Do hen harriers start incubating from the first egg laid?

Sent in by Dean MacAskill, Dornoch

Most songbirds, waders, wildfowl and game birds won't start incubating until the last egg has been laid. This means all the chicks should develop and hatch at the same time. With birds such as ducks that leave the nest soon after hatching and head for cover, it is important all the chicks can leave together. However, birds of prey often start incubating when the first egg has been laid.

Hen harriers usually lay between four and six eggs sometimes up to eight. There is normally a gap of one to three days between each egg being laid. Each egg will be incubated for 29-31 days and the clutch for 29-39 days in total.

The female hen harrier will start incubating after the first to the fourth egg has been laid. If they incubate when the first egg has been laid then the chicks' development will be staggered. The oldest chick could be 10 days further along in its development than the youngest.

The larger chick would obviously have the advantage but if food is abundant then all chicks have a good chance of surviving. If food is difficult to find then the younger chicks, particularly in large broods are less likely to survive but the older chicks still could. This harsh but effective method ensures that only the chicks that can be reared to an acceptable level will survive. In some rather gruesome cases, the older chicks may eat their unfortunate younger siblings.

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