The hawk's heart trembled against my hands as I clasped it as gently as possible. It was a full-grown sharp-shin, a small bird that seemed made of nothing but air and feathers.

Minutes before, a team of biologists had lured it into a net on a ridge in the Goshute Mountains in Nevada, USA. With efficiency born of experience, they had examined it, measured it, and tagged it. Now, it was my privilege to release it back to the wild.

"Toss him up, so he has time to fly before he falls," said my guide, a wildlife biologist named Liz Moseman. "A couple of feet is plenty." I complied and the bird exploded into motion, carrying with it a brightly coloured tag telling future biologists how to access the data about its visit to the Goshutes.

Traditional animal tagging is like betting on long-shot racehorses. The dream is that someone will recapture the same animal, giving a snapshot of where it's been and how its size and weight have changed in the interim. These tags can also be used on migratory species to allow researchers to identify individual animals on their return. But even these can produce exotic, long-shot results.

Trans-global migration

Such a tag made international headlines this week when it was found in the stomach of a sooty shearwater chick in southern New Zealand. The tag, comprised of an electronic device similar to those used for identifying pets, had originally been implanted on a steelhead, to track its migration through fish ladders on North America's Columbia River. Apparently, the chick's parent had been to the U.S., eaten the steelhead and, years later, regurgitated the tag when feeding its chick.

But all too often, the tags are never seen again. No longer though, with a new breed of electronic satellite tag. Unlike traditional tags, or even the pill-like device eaten by the sooty shearwater, these tags communicate directly with orbiting satellites, allowing researchers to track the animal's progress, wherever it may be.

The simplest are small enough to be carried by a big bird – although battery weight is a limiting factor. "Three per cent of the bird's weight is the maximum we can go," says Jeff Smith, science director of Moseman's group, HawkWatch based in New Mexico, U.S., which has been using satellite tags since 1999.

The tags cost thousands of U.S. dollars, making it impossible to track many birds. But the ability to track even a few gives new insight into their behavior. Redtailed hawks, for example, don't like to cross big mountain ranges. They pick one side and pretty much stick to it. Not so with the eagles. "Crossing a mountain range isn't a big thing for them," Smith says.

The life of birds

Then there's the occasional hawk that adds new meaning to the term 'bird-brain'. "Some are amazing," Smith says. In one case, a redtail tagged in New Mexico made two round trips to Mexico before coming back to settle down near the original site. "Did it forget its toothbrush, or what?" Smith jokes.

Even dead birds reveal information. The satellite tags are expensive enough to be worth retrieving, so the biologists go in search of them when a bird quits moving. On one occasion, Moseman says, they found an eagle that had been killed by a falling rock – not normally thought of as a hazard for birds. "It must have been chasing a rabbit or something and loosened the rock," she says. "We found it with its skull crushed."

Other researchers are studying marine animals using tags. In California, a multi-university consortium called TOPP, for Tagging of Pacific Pelagics, has been tracking everything from sharks, tuna, and giant squid to elephant seals and albatrosses.

Some work has used satellite tags, but recent experiments have used extremely lightweight data loggers that record a great deal more information. Weighing less than 15 g, these tags can be carried by marine birds such as sooty shearwaters, says Scott Shaffer of the University of California in Santa Cruz.

The tags reveal that the birds travel nearly 65,000 km per year, flying in figure-of-eight patterns that take advantage of the Pacific Ocean's prevailing winds and give them an opportunity to eat North American steelhead fish.

More bang for your buck

The tags used in this study don't need heavy batteries because they don't send radio pulses to orbiting satellites. Rather, they kept track of the birds' locations via sensors that detected the times of sunrise and sunset, information that can be converted into latitude and longitude. Additional sensors recorded the water temperature whenever the birds settled onto the ocean. This could then be compared to satellite maps of sea-surface temperatures to further fine-tune each tag's flight track.

Since none of this information is beamed to a satellite, it's necessary to recover the tag in order to retrieve the data. But in the case of the sooty shearwaters, that was easy. The researchers tagged the birds at a rookery in New Zealand, and year later, most of them came back. "We got tracks from 80 per cent of the birds we deployed," says Daniel Costa, also of the University of California.

In other research, the TOPP team has tagged tuna, hoping that if the fish are later caught, fishermen will return the data-logger to claim a reward.

Along with the daylight sensors used for sooty shearwaters, the tuna tags measure the depths at which the fish swim. One new finding is that as the fish feed in cold, deep waters, they periodically return to warmer surface waters to keep from becoming too chilled.

Marine animal tags can also be equipped with sensors that record salinity, depth, temperature, and light levels – in effect, converting the animals into mobile oceanography laboratories. "Oceanographers like that," Costa says. "We're getting lots of data from places where we either had no data or it's very difficult to get."

Suck-on tags

Researchers at Woods Hole Oceanographic Institution, in Massachusetts, have developed yet another type of tag. Called a digital tag, or D-tag, it can be attached to a whale via a suction-cup.

The mobile-phone-sized device monitors the whale's depth and body motions through one or more dives, then releases and floats to the surface. A radio beacon allows it to be retrieved. "These are very elegant," says Ari Shapiro, a Woods Hole Ph.D. student.

In addition to measuring the whale's movements, the devices record any sounds the animals make or hear – a feature Shapiro has used to eavesdrop on the vocalisations of Arctic narwhals; small whales with a unicorn-like tusk.

Other researchers use the tags to determine how whales emit sonar-like pulses to locate their prey. "These are used like an acoustic strobe light to give an image of the prey," Shapiro says. "We can hear the echo off of a particular prey item and watch as the whale approaches."

Another major use is in monitoring the effects of offshore oil exploration on whales. Such exploration involves repeatedly firing underwater air guns designed to create loud pulses for sonar-like mapping of seabed rock strata. It's an effective way to hunt for oil, but biologists are concerned that the racket may be detrimental to marine mammals.

To study the effect, Peter Tyack of Woods Hole and Patrick Miller of the University of St. Andrews in Scotland tagged eight sperm whales in the Gulf of Mexico. They discovered that even though air guns are designed to beam sound waves downward, not sideways, the whales encountered substantial decibel levels at distances as far as 12 km away.

Fat floaters and skinny sinkers

The animals didn't attempt to flee, but that doesn't mean they weren't disturbed. Rather, the noise appeared to make them more lackadaisical and apparently less successful in their efforts to find prey – revealed both by the vigor of their motions and the frequency with which they emitted the echolocation noises sperm whales make when closing in on their prey.

With only eight whales, however, the study is by no means conclusive. "It would have been really nice to have gotten even three or four more tests," Miller said.

He thinks D-tags could be used to track the general health of an entire population of whales. That's because the tags measure the whales' motions so precisely it's possible to count their fluke strokes. This allows Miller to divide the animals into two types, based on whether they have to work harder to descend or ascend.

One group, which he calls fat floaters, has to work hard on the descent, but can glide on the way back up, getting a free ride from their buoyancy. The opposite are "skinny sinkers" which descend easily, but require more fluke strokes to ascend.

Unlike humans, fat whales are healthy whales. Miller suggests that ecologists could use the proportions of the two as a measure of the health of the population as a whole.

Even D-tags aren't the ultimate in animal tracking. Already, Costa's group is working with high-precision GPS-based tags. They're also trying to develop a pill-like transponder that can be used to monitor an animal's stomach temperature – something that changes, for example, whenever the animal ingests a large, cold meal, such as a fish.

Beyond that? Who knows. But the days of old-fashioned animal banding are on the wane.

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