The great tit lays one egg each day and starts the incubation after having laid the last egg. Usually there are about ten eggs in a clutch. While the great tit covers the eggs with nest material during the egg-laying period  it does not do so when it leaves the nest during the incubation period.

Those who have followed the nest life of the great tit in the webcam have evidently noticed that the female started to sit on the eggs some days before the laying of the last egg. Already after laying the third egg the great tit did not cover the eggs and starting after the laying of the fifth egg, the female could be seen constantly in the nest. Altogether the great tit laid eight eggs in the nest. Does this mean that the female started incubating the eggs before the clutch was complete and that the chicks will hatch at different times?

Marko Mägi, researcher at the bird ecology department of the University of Tartu explains the behaviour of the great tit.

”Sitting on the eggs does not necessarily mean the beginning of incubation. The great tits start as a rule to incubate the eggs after the last egg has been laid, which ensures that all chicks will hatch at the same time. Sitting on the eggs before the clutch is full may indicate a need to warm the eggs from time to time. The weather in the meantime was rather rainy and cool and so the female may have been in the nest not to incubate the eggs but to protect them against cooling too much. In eggs that have become too cold the foetus may perish. Also, the brood patch that develops already at the start of the breeding period is not necessarily pressed against the eggs while warming them and so the incubation does not start.“

Brood patch of great tit. Photo Margus Ots

„The incubation period of great tits – and also of many other similar-sized passerines – is 12 days as a minimum. Since the last, eighth egg was laid on June 16th, the first hatching of chicks may be expected on June 28th, in unfavourable conditions however a little later.“

The direct stream image from the great tit webcam is organised by the Estonian Ornithological Society in co-operation with Looduskalender, transmission is by EENet and Tele2, the forum is managed by Looduskalender. News from the nest in the tree trunk are forwarded in the Linnuvaatleja.

Margus Ots
info/at/linnuvaatleja.ee