Birds of a feather

Want to know where birds come from? Go rent ''Jurassic Park."

Dinosaurs, most paleontologists agree, are the ancient ancestors of the robin in the tree branch outside your window. And the more researchers learn about dinosaurs, the more they are understanding the evolutionary path of birds.

''Dinosaurs are all around us today in a very real sense," said Patrick O'Connor, a paleontologist at Ohio University in Athens, Ohio, speaking from Tanzania where he had been digging up dinosaur fossils. This ongoing search for new information on dinosaurs gives us a historical perspective about how birds came to be who they are, he said.

Such things as a bird's feathers, breathing system and light bones -- thought to have evolved for flight -- are also showing up in fossils of dinosaurs that never took to the sky.

''Feathers are for flight, right?" asked Farish A. Jenkins, a paleontologist at Harvard University. ''Wrong."

Some small dinosaurs that did not fly had feathers -- probably to keep them warm, rather than to fly, Jenkins said. ''It is another example that things are not always as they seem," he said.

As evolution progresses, features that arose for a particular purpose in one species often end up serving a different purpose in another species.

The relationship between birds and theropods, the group of two-legged ground-walking dinosaurs that preceded them, ''really illustrates well how evolution works. You have characteristics that accumulate" over a time span that eventually become what we think of as the animal," said Chris Organ, a researcher in the department of organismic and evolutionary biology at Harvard. There is a ''common sense that fossils are about dead things and biology is about living things, but we really need more of an integrated sense of where things come from."

O'Connor and Leon Claessens, a professor at the College of the Holy Cross in Worcester, published a study in Nature last month suggesting that the breathing systems of birds -- once thought to have evolved to help birds fly -- are quite similar to those of the Tyrannosaurus rex, a theropod that obviously did not get airborne.

The breathing system of birds consists of lungs that do not move -- unlike those in humans and most other mammals -- and air sacs in the bones that act like tiny bellows drawing air through the lungs. This is a system that allows fresh air to constantly move through the bird, in contrast to humans and other vertebrates that have no fresh air coming inside the body during exhalation. In this way, a bird's breathing is very efficient, said Claessens.

Finding that dinosaurs appeared to have a very similar type of breathing system is ''a major step forward in deducing where the supposedly unique respiratory system of birds arose," said Jenkins, a former academic advisor to Claessens. ''It's the first really significant line of evidence that the avian, or bird, pulmonary system actually began to evolve in non-flying avian ancestors. It is interesting that [a system] always thought to be associated with flight is not, [that it] probably evolved for other purposes that we don't know."

One possibility, said researchers, is that this efficient system allowed an elevated metabolism so that predatory dinosaurs could be active for long periods of time during the day. This warm-blooded behavior is in contrast to cold-blooded animals such as the crocodile which are relatively inactive as they store energy for short periods of activity. It is not known whether dinosaurs were warm- or cold-blooded.

''We are saying it appears that T rex had a breathing system capacity much more [in keeping with a warm-blooded animal] than the lizard view of cold-blooded," said Claessens. ''Maybe it was in-between: 'dinothermic.' Not cold-blooded or warm-blooded."

Birds are warm-blooded, and so a link between pulmonary systems in the extinct animals to that of modern birds may offer an additional way to fill in some of the activity over the millions of years of evolution between the Jurassic period some 205 million years ago and now. One conclusion is that birds' light bones evolved not for the purpose of flight but for thermoregulation.

Evolutionary changes ''weren't happening so dinosaurs could give rise to birds," stressed O'Connor. Rather, the changes ''gave an animal a particular advantage in day-to-day existence."

While looking at dinosaurs and comparing them with birds reveals similarities, even similar systems could produce markedly different results in the two very different types of animals.

For example, the researchers said that dinosaurs and birds both have air sacs running through their sinuses in the skulls. Still, dinosaurs most likely would not have sounded like ''Tweety Bird," said Claessens. Instead, because of the size of some of the dinosaurs' heads, they may have sounded more like elephants, he speculated, the air pushed through the sinuses and emerging in an ''impressive and scary loud roar." Theropod dinosaurs with a body size similar to today's emus or ostriches may have shared a similar loud shriek, he said.

O'Connor says his research is part of an ongoing story being told connecting dinosaurs with modern birds and helping scientists learn about both sets of animals. ''This [new observation] is another piece in the puzzle," he said. And unlike solving a crime on a forensic show, this examination of extinct animals is ''put out there as a hypothesis, not a fact, [this is] not a done story."