Great tits also have age-related defects

Bouwhuis researched aging under female great tits (Parus major) in Wytham Woods (Oxfordshire, UK) and on the island of Vlieland (NL). She investigated the number of eggs and offspring and the reproductive success of this offspring, among other things.

Old mothers

Although great tits can live for nine years, breeding success declines rapidly after the age of two. Nevertheless, older great tits keep on breeding every year, says Bouwhuis: 'They carry on to the bitter end'. What is remarkable is that at the start of the breeding period there's very little difference between the nests of older and younger females. Bouwhuis discovered, however, that massive mortality occurs just after the young leave the nest. 'The parents still have to guide their young in the first crucial weeks after leaving the nest. Perhaps the older mothers have more trouble with that guidance; their young often fall prey to sparrowhawks, for example. Or maybe the older mothers have only been able to find less suitable places in the woods.'

Disposable body

Bouwhuis also demonstrated that it is beneficial for great tits to invest as much as possible in generating offspring during their first two years of life. Such hardworking great tits do age more swiftly than tits that get going at a more relaxed pace, but despite this early aging being costly, the early breeders end up with the most descendants overall. 'The costs and the benefits thus balance', says Bouwhuis. 'It's the theory of the "disposable body" -- investing in offspring instead of body maintenance. But why all great tits don't follow this tactic is not clear.'

Aging characteristics

Although the decline of the great tits is measureable by their breeding success, it's not easy to spot the older ones by eye, says Bouwhuis: 'They don't have any clear aging characteristics like wrinkles or grey feathers. What is being investigated at the moment is whether the yellow breast feathers change as the great tits get older. It's perfectly possible that elderly great tits have aging characteristics that we just don't notice but which are very clear to other great tits.'

Male great tits

Bouwhuis does not yet know what's going on with the male great tits. 'This research concentrated on the females -- males are also more difficult to catch during the breeding season.' Male great tits are not always faithful, either, says Bouwhuis: 'Males can have offspring with their permanent partner, but also with extramarital partners. That makes it very difficult to determine the exact number of offspring.'