Wild Neighbors: It’s a Bird’s Life

By Joe Eaton
Thursday January 28, 2010 - 08:58:00 AM

How long do birds live? That question is surprisingly hard to answer, in part because of the range of lifespans involved and in part because of data limitations. Apart from captive birds, what we know about avian longevity is based on recovery of wild individuals that have been banded. And recovery of banded birds is so rare that there’s not really enough information to calculate meaningful averages. What we have to work with are the extremes. 

Given all that, a recently published study by Cornell biologists Daniel Wasser and Paul Sherman makes some interesting generalizations about which kinds of birds live longest, and why. They worked with maximum longevity records and body-mass data for 936 bird species. For a subset of 470 birds, Wasser and Sherman also looked at other variables that might affect mortality, including diet, sociality, migratory behavior, breeding latitude, breeding habitat, and the tendency to nest on islands.  

Their results show the parrot, flamingo, and petrel/shearwater/albatross orders as avian Methuselahs, with average maximum lifespans exceeding 30 years. Perching birds, woodpeckers, and grebes had the shortest maxima, averaging less than 10 years. Within the perching-bird order, corvids had the longest maximum lifespans (over 17 years), wood warblers and tyrant flycatchers the shortest (6 years.) 

Wasser and Sherman also did a multivariate analysis for the 470-species subset to determine how much each of the selected variables influenced longevity. Their conclusions: large species live longer than small species, vegetarians longer than carnivores, social species longer than solitary, and island-nesters longer than mainland-nesters. Factors like migration and breeding latitude had no significant effects. 

Although I haven’t seen the actual paper—just the abstract and press release—the authors’ interpretation seems to be that bird species whose lives are less risky undergo natural selection for traits that delay aging, not just extending their lifespans but also maximizing breeding opportunities. For a species with a high probability of dying young, the optimum evolutionary strategy would be to breed early and often. Lower the risk of early mortality and you can get by with laying one egg every other year for a couple of decades, or longer. 

That’s plausible, but I have to wonder about a couple of things. Why wouldn’t migration—an enormously dangerous project, which many individual birds fail to survive—have more of a negative influence on longevity? How are Wasser and Sherman defining sociality? Do they mean the sort of complex behavior that involves extended families and nest helpers, or just the tendency to form flocks? Those are very different phenomena. 

Out of curiosity, I took a look at the records for maximum lifespans of banded birds compiled by the U.S. Geological Survey’s Bird Banding Laboratory in Laurel, Maryland. Those records don’t overlap completely with the data set used by the Cornell scientists (no penguins, no parrots), but it’s large enough to show some interesting patterns. (Incidentally, the parrot record, 80-plus years, was held by a captive sulphur-crested cockatoo.) 

The BBL data seem to show that size does matter. Based on maximum records, bald eagles live longer than buteos (red-tailed hawks and their relatives), which live longer than falcons. Within the falcon family, peregrines and prairie falcons live longer than American kestrels. Great horned owls and spotted owls live longer than burrowing owls and screech-owls. 

There’s also support for the notion that social birds live longer. Among North American woodpeckers, the two highly social species, our own acorn woodpecker and the red-cockaded woodpecker of the Southeast, have the highest maximum age records (17 and 16 years, respectively.) The oldest recorded pileated woodpecker, the largest surviving species, was only 12 years old. Does sociality trump size? 

The three North American scrub-jays are an interesting case. The Florida scrub-jay is a cooperative breeder, with older siblings acting as helpers at the nest; the island scrub-jay is confined to Santa Cruz Island but doesn’t breed cooperatively; the western scrub-jay is a mainland species with no nest helpers. The BBL database has maximum ages of 15 years 9 months for the western scrub-jay and 15 years for the Florida scrub-jay. I had to look elsewhere for the island scrub-jay; the Birds of North America account for that species cites a maximum documented longevity of 15 years 3 months. Pretty darn close. 

As another approach to the island-nesting factor, I compared the Hawai’ian honeycreepers with their presumed closest relatives, the North American finches. The 5 honeycreepers in the BBL database have a range of 6 years 5 months to 12 years and an average maximum age of about 9 years 3 months. The 14 mainland finches have a range of 5 years 8 months to 16 years 3 months and an average maximum of 9 years 5 months. Where’s the island effect? 

I suspect nesting on an island is actually a proxy variable for being a large seabird. More on that next week.