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Blue Tits & Birth Control

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Cain and Abel in the nest

Family planning for blue tits happens after birth, according to Martyn Stenning of BIOLS. Sibling rivalry determines which and how many offspring survive and the odds are severely weighted against younger siblings.

Martyn's research into bird behaviour has been helped by new policies for environmental management on campus. It is a common problem that care for trees can lead to a shortage of nest sites for those species which nest in holes caused by damage or decay. The University has compensated for this by putting up nest boxes and these have proved popular with the local tit population. The handy nests have made it possible to observe closely what is going on and also to experiment by supplementing the diet in some nests.

The food supply seems to affect the time at which incubation starts and therefore how the hatching of the brood is spaced. Early hatchlings, if they survive, are bigger and get more than their fair share of the available food. If supplies are short, competition rules and the others rapidly go to the wall. This happens also in other species and in some the smaller hatchlings may even be eaten by their big brothers and sisters. It may seem cruel, but does ensure that some survive, reasonably well nourished, rather than all suffering equal deprivation. A rugged, but effective, method of matching family to food supply.

Only a very small proportion of the total number of hatchlings make it through their first winter to become parents themselves. Offspring from small, well nourished, families are more likely to be among the winners.

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Friday February 28th 1997