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How old are birds? We're not counting birthday candles on a cake of seed and suet. Rather, how long have our commonly seen birds been part of this world?

For the Eastern bluebird, the answer is at least a couple of million years.

Fossil evidence is rare. Small, delicate creatures like birds are far less likely than large animals to have their remains survive in any form.

A few Eastern bluebird fossils have been discovered. Three leg bones found in an abandoned lime quarry in Florida were the first. They were among other remains of late-Pleistocene animals, creatures of the Ice Age.

A cave in Illinois contained bluebird fossils dating from 8,000 to 10,000 years ago. Caves in Wyoming and New Mexico also have yielded fossils, these perhaps as old as 25,000 years.

The best evidence of the age of these birds comes not from old bones, however, but from the DNA of today's bluebirds. Scientists can track the history of a species and its relationship to other species by examination of mitochondrial DNA (mtDNA).

This particular DNA is present in large numbers in each cell of a bluebird or any other organism. It can be used to distinguish one group from another.

MtDNA is inherited from one's mother, so any maternally related individuals might be expected to share a similar mtDNA sequence.

Thus, a study of two species could be made to determine if at some point in the distant past two now distinct species once had what amounts to a common mother.

Studies of mtDNA published in 1997 suggested that Eastern bluebirds evolved into a species as long as 2.5 million years ago. They split from an ancestor they share with today's Western bluebird, the two being, in effect, sister species.

Those studies were co-authored by ornithologist Robert Zink, formerly at the University of Minnesota, now at the University of Nebraska.

The split could have happened when two populations of one species, the common ancestor, were isolated from each other.

Perhaps an inland sea or a mountain range was formed, some kind of major geophysical occurrence. This would be a long-term event, millions of years.

Eventually, that could have split the single population into group A over here and group B over there.

A new species then evolved as one of the isolated populations, A or B, changed to meet its new living conditions. That is one way we get new species.

Scott Lanyon, former director of the University of Minnesota's Bell Museum of Natural History and current vice provost and dean of graduate education at the school, is another ornithologist who has worked with mtDNA.

One of his specialties is the oriole family. The Baltimore oriole you see in your neighborhood is related to many other oriole species scattered throughout the Western Hemisphere.

Working with oriole DNA samples, Lanyon and his colleagues have established that all 33 living species of orioles share a single common ancestor.

This isn't new populations being discovered, he explained. Instead, they found that several species had populations that were not in fact all that closely related. Those have been split into separately named species.

Based on the research, he and his teammates will be able to illustrate — in shape and colors — what that grandmother oriole looked like.

We can take these trips back in time, to determine the beginnings of a species, even to know the appearance of an individual species so far removed.

We know how to discover birds' history. We do not yet know how to guarantee their future.

Lifelong birder Jim Williams can be reached at