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Gardens Are Great for Great Tits - Yes and No

Abstract from Journal of Animal Ecology
15th April 1996

More detailed research work on Great Tits at Wytham Woods (Oxford, England) has been reported recently in the Journal of Animal Ecology.

The first paper, by Andy Gosler (65, 1-17), looked into winter fat stores put on by the birds. It had been thought that those that could find the resources would always carry excess weight as insurance against the possibility of cold weather. However, recent theoretical work suggested that this would not be the case if the dominant birds were certain that their social status would mean that their access to food was guaranteed. Subdominant birds would not be so certain and would need to carry bigger fat reserves.

The Wytham Woods Great Tits are very modern birds and support the latest theory exactly. When food was abundant, in beech mast years, dominant birds remained lean and their fatness failed to predict their likelihood of surviving the winter. When food was scarce, the fatter low-ranking birds survived better than thin ones but dominant birds were still surviving better even though they remained lean.

This has implications for garden feeding. It shows that birds in the garden, if they trust you to continue to supply food if the weather turns nasty, will regulate their food intake and not become fatties liable to Sparrowhawk predation - or heart disease - just because the food is available!

The second paper, by Robin McCleery, Jean Clobert, Romain Juillard and Chris Perrins (65, 96-104), shows the value of long-term, detailed research. During the years from 1973 to 1976 the nestboxes in Wytham were replaced because the conventional boxes were so often predated by weasels - up to 50% of nests in some years. The new design chosen was made of 'wood-crete' by Schwegler GmbH, Germany, and the boxes were suspended away from the trunk of the tree to frustrate the weasels - with almost complete success. As a result, roughly 50% more of the recruits into the Wytham breeding population had themselves fledged from the study boxes. The local population of both Great and Blue Tits also increased.

But there were costs for the individual Great Tits. This is because there was an increased chance of their having to make the considerable effort of rearing young to fledging each year. The survival rate of both sexes, after reaching the age of five years old, went down. In evolutionary terms the birds were doing better because more of their offspring were themselves surviving to breed. This also has implications for our garden birds: if they are unable to raise so many young, the adult birds may be able to survive longer and have more breeding attempts.

Chris Mead

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