Many years ago
David Lack was addressing an audience of learned ornithologists on the subject of his
studies on Robins. He said that the average life expectancy of a Robin was a little over
one year. He was laughed off the stage. Everybody knew that they have had a Robin in their
garden for years and they were sure that it was the same one.
But he was right. And it takes only a moments thought for
anyone to work it out. No need for complicated modern maths - anybody can do it.
There are about as many Robins this year as there were last year
(populations usually fluctuate about an average); so if there was a pair in your garden
last year then there is a pair this year. But in the mean time that original pair has had,
say, two broods of five young - that is ten new Robins. But by the start of the next
breeding season there are only two. So:
So how many have died? Obviously, ten. So out of twelve (2+10)
birds, ten have died - thats more than 83%. Ringing studies have shown that about
60% of the adult Robins die each year, so of the original two adults only 0.8 of a bird
will be alive this year (yes, 0.8 of a bird is nonsense but we are talking about averages
here), so for us to have two birds again this year 1.2 (2 minus 0.8) birds must have
entered the population. In other words, those ten young have resulted in only 1.2 adults -
thats an 88% mortality.
Much of that infant mortality happens in the nest or
shortly after fledging. Ringing studies have shown that the first year mortality of young
once they have fledged is about 72%.
Robins are a familiar bird in
many British gardens.
So are we any nearer answering the question "How
long do birds live?". Well, Im afraid that we have to do a little bit of maths
now. The opposite of mortality is survival. Both can be written as a percentage or as a
fraction of one, for example a mortality of 83% can also be written as 0.83 and usually in
these kind of studies we use this fraction-of-one method. So if the mortality is 0.83 the
survival is 0.17 (17%) because if 83% died then 17% (100-83) must have lived. We can now answer the question, for Robins at least. There are two expressions
that are used for calculating life expectancy: the first one is more straightforward:
So if the mortality is 0.6 (as it is for adult Robins) then
the life expectancy is 1.2 years.
The other expression uses natural logarithms, so you
have to use a calculator (the natural log button is labelled Ln):
In the case of our Robins, if the mortality is 0.6 then the survival is
0.4 (i.e. 1-0.6) and the life expectancy is 1.1 years.
Which one is correct? Ornithologists usually use the second one
because it fits into several other equations that are used in studying bird populations.
But as you can see, the simple equation gives almost the same answer.
Most of our common small birds have similar survival rates and life
expectancies. Adult survival rates are usually between 0.4 and 0.6 with first-year
survival rates often being between 0.1 and 0.2 - so roughly a half of all the adult birds
and nearly ALL the baby birds you see will be dead in a years time. Its a sad
thought, but this has to happen if the population is to remain roughly constant.
Generally, large birds and seabirds live longer than small birds.
Albatrosses live so long that it is hard to calculate their survival rates - they can live
longer than the metal rings that identify them, and probably longer than most humans.
Even little seabirds like Storm Petrels live a remarkably long time
- about seven years on average for adults - and they take about four or five years to
But there is a very big difference between the average life
expectancy and the maximum one and the maximum known age for a Storm Petrel is over 31
years! And the oldest known Robin was 8½ years old. Even very small birds can live a
remarkably long time; for instance, the oldest Marsh Tit was more than 10 years old and
there has been a 21 year old Blue Tit! These are species for whom the average life
expectancy is only just more than one year, so you can see how very much better than
average these particular individuals are. Their contribution to the subsequent generations
is ten or twenty times as much as the average individuals so its easy to see
how natural selection could work.
|Although most adult Blue Tits live for only a little
over a year, at least one has been known to live for 21 years!
So, the answer to our question is that most adult
small birds in temperate regions such as ours live for between 1¼ and 1½ years, but that
only about 10-20% of young reach adulthood. Big birds, seabirds and tropical birds can
live much longer. But some individuals of any species can live as much as ten times as
long as the average - thats like an exceptional human living for about 800 years!
All of this information refers to
wild birds; birds in captivity can live to much greater ages - even exceeding the longest
lived wild birds.