Wild great tits live into old age
12 May 2009, by Sara Coelho
Great tits live long enough in the wild to suffer the burden of old age, says new research. As the years pile up, the birds become less able to raise healthy chicks and produce fewer grandchildren.
Nest-boxes with great tit chicks in the Wytham Woods.
The findings come as a surprise because it was thought that wild birds die well before the onset of ageing.
The results are part of a long-term research project on the great tits of Wytham Woods, a mixed deciduous forest near Oxford.
The project is managed by the Edward Grey Institute, part of the Department of Zoology, Oxford University, and has been compiling data on the behaviour of great tits since 1947.
Sandra Bouwhuis, a PhD student at the University of Groningen in the Netherlands, used the Wytham Woods dataset to find out how short-lived species, such as great tits, lose their ability to produce offspring as they age. 'We also wanted to see what phases of the reproductive cycle are especially prone to ageing,' Bouwhuis says.
The dataset records 49 years of clutch size (egg count), brood size (hatched chick count), number of fledglings and parentage. But Bouwhuis measured reproductive success in terms of recruits, the number of offspring that return to the population to breed, because looking only at eggs laid, for instance, would be too early in the process of reproduction to gauge the full effects of ageing.
'Our most important finding is that female great tits living beyond 2 years of age show reduced offspring production,' says Bouwhuis. This means that at any time 1 in every 5 breeding females is affected by ageing.
The analysis, recently published in the Proceedings of the Royal Society B, shows that great tits lay the same number of eggs as when they were younger, but fewer chicks hatch, fewer offspring survive to leave the nest, and fewer fledglings survive to breed themselves in the next spring.
The findings suggest that ageing is 'much more prevalent in wild populations because it was previously thought that wild animals generally do not live long enough to show any signs of ageing,' says Bouwhuis. The exact mechanism behind the findings is not yet known, but 'the results suggest that egg fertility, incubation behaviour and the provisioning of offspring with food could play a role,' she adds.
The role of males is also not fully understood. Male great tits provide sperm for egg fertilisation, food for the female during incubation and for the nestlings. But since about 14 percent of chicks are not fathered by the male who raises them, age-specific parentage is not accurate. 'Future research will have to disentangle the role of the male and female in this process,' says Bouwhuis.
The paper: S. Bouwhuis, B.C. Sheldon, S. Verhulst and A. Charmantier. Great tits growing old: selective disappearance and the partitioning of senescence to stages within the breeding cycle. Proc. R. Soc. B. published online 29 April 2009. doi: 10.1098/rspb.2009.0457
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