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Some birds may well live for over 80 years. |
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George Mackenzie Dunnet
was a well-known Scottish ornithologist who passed away in 1995, aged
67. The photograph that accompanies his obituary shows a timeworn face,
mostly bald head and grey locks clinging to temples. However, his sense
of fun survives - literally. Throughout his life George studied fulmars
on Eynhallow in the Orkney Islands.He
had his picture taken in 1951 holding a ringed fulmar when he was a young
man and, by great coincidence, had it taken again 30 years later with
the same bird. Personal vanity was brushed aside in his admiration for
the fulmar's unchanged looks. Meanwhile, the bird lives on. It is a sobering
thought that, when ringed, it was already a mature breeding adult making
its true age a mystery.
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Unlike humans, birds do not go grey or develop wrinkles. If their outward appearance does not
change, how can we tell their age? In most cases ringing provides data
on lifespan and longevity. Natural clues to the precise age of a bird
are few and only serve to chart survival into adulthood - not beyond.
Small birds replace distinctive juvenile plumage with that of an adult
within months of being born. After this, their looks do not alter. Larger
birds take several years to reach maturity during which time they grow
progressively more adult-like feathers. Brown juvenile gannets acquire
white plumage resembling their parents' through a series of moults over
a five-year period.
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The only known change in appearance linked to old age occurs in captive drake wildfowl. For some bizarre reason geriatric males develop female features, a trait never noticed in free populations. In the wild, because of the overriding mortality from shooting, it is difficult to estimate the natural lifespan of ducks and geese. For example, based on ringing recoveries, most shot teal are between one and three years of age. Comparatively few reach ten years or more: an age structure shown by non-quarry species such as barnacle geese (with 30 per cent of individuals in protected populations aged ten and over).
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Safeguarded from predators, disease and other life-threatening factors, birds in captivity can outlive human owners. So far, the record goes to a sulphur-crested cockatoo in London Zoo, which was over 80 years old when it died in 1982. In the real world it is a different story. By and large, the smaller the bird the more eggs it lays and broods it raises in an attempt to reproduce itself. After all, each breeding pair has only to replace itself with another during its lifetime to ensure that the species survives. High reproductive rates insure against heavy losses. Sadly, mortality can be enormous among first generations. Adult robins reach three, four or five years of age (exceptionally up to 12) but about 75 per cent of juveniles die during their first winter. Those that make it through to their first birthday can look forward to a drop in annual mortality rate - but only to 60 per cent. Life is cheap for small songbirds. Even so, some have made it to the ripe old age of 15 for great tits, 16 for swallows and 20 for starlings and blackbirds.
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Strange to say, the rigours of long distance migration do not seem to unduly shorten life expectancy. An arctic tern has reached 34 years of age and a ringed common tern is currently in its mid-thirties and still going strong. Seabirds are particularly long-lived, yet many species do not begin breeding until they are several years old and lay only single-egg clutches. They have few natural enemies, which partly explains their relatively high reproductive success. Care of offspring is a further significant factor. Young shearwaters are reared in burrows and leave the nest chamber under the cover of darkness. Most adult auks accompany young at sea for several months and continue supplemental feeding. Juvenile auks are assimilated into auk society, which provides a range of life-prolonging benefits such as communal fishing and flock navigation to feeding grounds and safe nesting sites. Little wonder, therefore, that with an annual survival rate among adults of 90 per cent, many auks are long-lived. Razorbills, guillemots and puffins have all reached more than 20 years of age.
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Of course, since proof of age relies upon the chance recovery of a ringed bird, examples of longevity in seabirds are probably far from true upper limits. Current records are 46 for a royal albatross and 44 for a manx shearwater. In both cases the birds were ringed as adults and are still alive. Instead of relying on luck to provide statistics, data based on large numbers of marked adults monitored at breeding colonies over successive years provides all sorts of interesting information, including annual mortality rates. From this, life expectation can be estimatedSpecies with around a three per cent annual mortality have a further expectation of life of about 30 years, and approximately 5 per 1000 could theoretically reach more than 80 years of age. If a low mortality rate signifies long life then skuas, albatrosses and fulmars come out on top. George Dunnet would approve of the fact that adult fulmars may have the lowest annual mortality of all birds, at a mere two or three per cent. How then, do seabirds die?
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They are supremely adapted to life on the oceans but evolution has not prepared them for the contribution of man to their world. Introduced predators take eggs and young at colonies, drowning in fishing nets kills all age classes including breeding adults and contamination of the seas by oil and other pollutants has eradicated seabirds in great numbers. What about natural casualties? The commonest causes of death are starvation and bad weather, often in concert. Another fatal hazard may come as a surprise: landing accidents. The long wings of gannets, shearwaters and albatrosses are built for gliding, not for touching down. In busy colonies collisions are not infrequent and at large gannetries dozens of adults kill themselves every year in mishaps during landing and take-off. Maybe, due to old age, they need glasses?
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