THE SMALL GROUP of visitors walked up to the nesting bird and stood over it. The bird, showing no fear, didn't move. Then, almost as if on cue, the bird raised its body so that the visitors could see its powder-blue feet -- the trademark of the blue-footed booby. A few moments later, the booby moved its webbed feet and exposed the eggs it had been sitting on.

"When will the eggs hatch?" a visitor asked.

"Oh, the eggs are infertile," replied Pepe, the guide. She's been sitting on them for 2 1/2 months -- but she won't give up."

This little scene on a tiny island out in the Pacific Ocean captured the essence of life on the Galapagos Islands -- friendly, in trouble, but hopeful. The Galapagos are a score of volcanic islands that straddle the equator about 600 miles west of Ecuador. They are a nature lover's dream. You can walk among the animals and birds, which show no fear. But perhaps because of their tranquil existence, many are endangered. Only within the last two decades has an effort begun to save the distinctive species that inhabit the islands.

Because of the islands' out-of-the-way location, only 12,000 persons visit the Galapagos each year. There are two air-flights to the islands each week, with the planes flown by Ecuadorian military personnel. The American tourists tend to be experienced travelers. But while they may have visited Paris or London first, many say that the visit to the Galapagos is the culmination of a long-time dream.

Tourist facilities are limited. The largest and most modern accommodation is the Hotel Delfin on Santa Cruz Island, but it has only 16 rooms and hot water won't be provided until next year. Hotel visitors spend nights ashore and make day cruises to nearby islands. Several small boats accommodate passengers and, because they sail during the night, can cover a wider area of the islands. A new 90-passenger cruise ship, the Santa Cruz, will start to sail around the Galapagos next month and provide the most convenient way to view the islands.

The islands were discovered in the 16th century and later became a haven for pirates, whalers and sealers who found an abundance of fresh water, firewood, and giant tortoises. They would take the tortoises aboard ship, stack them live in the hold and slaughter them for fresh meat as required during long voyages. It was reported that the giant tortoises could live for a year without food or water under these conditions.

The Galapagos (Spanish for tortoise) first came to the attention of the scientific world in 1835, when Charles Darwin visited the island for five weeks aboard a British ship, the Beagle. He found that animals had developed differently on the various islands because of different living conditions. This provided the evidence for Darwin's book, "The Origin of Species by Natural Selection," in which he established his theory that species were not unchangeable, but were subject to evolution.

In almost a macabre illustration of Darwin's theory, the survival of the Galapagos species has been threatened by the intrusion of new animals, new plants -- and humans. Only since the establishment of the Darwin Research Station in 1962, and the activation of the Ecuadorian National Park Service in 1968, has the camapaign to preserve the native flora and fauna begun.

Now Ecuadorian law forbids visitors from going to 90 percent of the Galapagos Islands that constitute a national park without being accompanied by one of 20 members of its specially trained, multilingual international guide force.

The visitors generally arrive on tours because the facilities have been booked months in advance by tour sponsors. Several operators have a regular schedule of tours in which the Galapagos is a stop (one is a two-week trip that also features a visit to Machu Picchu, the lost city of the Incas in Peru). The various trips, including all air fares, accommodations and most meals, range from $960 to $2,900 a person.

A recent Maupintour trip had 23 participants -- eight from Long Island, eight from California and the rest from points between. There were 14 women and eight men, ranging from a 12-year-old boy to an 83-year-old retired lawyer now living in the Virgin Islands.

During the visit, the guide duties were shared by Pepe of Ecuador, Dolph of the Netherlands, and Steve, an Irish zoology graduate. All three shared the "status" trait of the Galapagos guides. They walked barefoot on the lava rock islands. (Walking on the sharp rocks isn't a problem," Steve said. "But it takes a couple of months to harden your feet so that they can handle the heat.")

As Pepe walked along the shore of tiny Santa Fe Island, where sea lions sunned themselves and a Galapagos hawk perched on a cactus not 10 feet from onlookers, he described the many ways that Galapagos animals have adapted themselves to live harmoniously with one another.

"There is a nice relationship between the short-eared owl and the Galapagos hawk," Pepe said. "They eat the same food, but the owl is nocturnal and the hawk diurnal. So they don't compete."

None of the snakes is poisonous. And while the scorpions are poisonous, they are not deadly.

The finches and the tortoises have a close working arrangement. When the finch taps the tortoise shell, the tortoise raises its long neck. Then the finch eats the ticks off the tortoise's neck. Mockingbirds peck insects off the hawks. "On the Galapagos," Pepe added, "the mockingbirds are so tame, they often walk rather than fly."

The whole area is so peaceful that Pepe and the visitors swim in the waters off Sante Fe Island amid frolicking sea lions and large swimming manta rays. "What a place," sighed a Long Island woman. "It's so peaceful. The bees don't sting and neither do the stingrays. It's all love and goodwill."

Unfortunately, the love and good will exhibited by the animals endemic to the Galapagos (animals found only there) and animals indigenous to the Galapagos (animals found there naturally but also found in other parts of the earth) are not shared by the animals "introduced" into the Galapagos.

This peaceful paradise where the lack of predators had made the native animals tame and unfearful, has been threatened by the introduction by humans of non-native animals -- goats, dogs, cats, pigs, etc. "On Pinta Island in 1957," Steve related, "some fisherman put ashore three goats for us to use for milk and food. The goats escaped and became wild. By 1968, there were 20,000 goats on Pinta."

The goats ate the same vegetation as the native land iguanas, but did it more efficiently than the ferocious-looking but slow-moving iguanas. The advent of the goats nearly wiped out the iguana population on Pinta by starvation.

In the past decade, the Ecuadorian Park Service has begun hunting expeditions to kill the wild goats. They now number 2,000 on Pinta, and the Iguana population is slowly coming back.

"But on James Island there are still 100,000 wild goats," Steve said. "It's much more difficult terrain, and shooting them in large numbers is impossible. Killing the goats is not something we want to do. It's something we have to do. You have goats all over the world, but we have iguanas that you can't replace.

But the land iguanas seem to be doing well on South Plaza Island, where the visitors stopped for a walk among sunning sea lions and a colony of land iguanas. The visit to South Plaza was typical of the pace of each island visit -- a leisurely two-hour walk along clearly marked trails with frequent pauses.

The conversations among the guides and visitors frequently drift back to the ecological problems of the area, perhaps because the animal existence on the islands seemed so fragile and perhaps because Galapagos visitors tend to be ecologically oriented.

The visitors were particularly curious about how dogs and cats, brought to the islands by settlers as pets, had gone wild and threatened the harmony of the Galapagos. Pepe said that the wild dogs had adapted themselves to run very fast on the sharp rocks. Huge numbers run in packs on most of the islands. About 500 have been counted on the south coast of Isabela Island. These wild dogs attack and eat land iguanas, marine iguanas, sea birds, sea lions, fur seals and baby tortoises.

A scientist from the Darwin Research Station recently spent almost half a year studying the activities of the wild dogs. He has recommended that meat impregnated with poison having a short life be buried in the areas frequented by the dogs.

"We have no carrion-eating animals or birds in the Galapagos, so we think there is no danger that the poison would be transmitted to other animals," Steve said. "But naturally, we will undertake small tests before any full-scale program."

The untold numbers of wild cats eat small lizards, and birds and finches. The park service is currently considering the possibility of infecting them with a disease that only affects cats.

Wild pigs dig up and eat eggs of sea turtles and Hawaiian petrels, rare birds found in the island highlands. "If nothing is done," Steve said, "these petrels will soon be extinct."

Humans, through their innocence, have also caused problems. The land iguanas normally feed on cactus fruits and yellow cactus flowers. Well-meaning visitors' found that the tame iguanas would eat yellow flowers from their hands, and the iguanas found that it was simpler to wait for the visitors than to forage for their own food. The park service now bans any feeding of Galapagos animals.

The vegetation in the Galapagos is varied, ranging from mangrove in the wet areas to cactus in the drier areas. But here, too, humans have caused problems. To develop locally produced quinine, settlers many years ago imported cinchona trees. Now, in some areas, chinchona trees are overpowering the native vegetation.

For the group of visitors, five days on the Galapagos was too short. They never got to see the flightless cormorants and the Galapagos penguins. These penguins are able to thrive on the Equator because the cold Humboldt current sweeps northward to the Galapagos from the Antarctic and provides the penguins with a cold-water habitat.

And they never got to see the giant tortoises of the Galapagos in the wild.

"It's too difficult to bring visitors to the areas where the tortoises live," Pepe said. "so we bring the tortoises to the visitors."

The tortoise pen at the Darwin Research Station was the only area of the island that resembled a zoo. The giant tortoises were brought there so that visitors could walk among them. It was the only area where the guides permited visitors to touch the animals. The visitors quickly discovered that touching the shell caused no problems for the tortoises, but that tapping the shell (which one visitor did) produced an instinctive tortoise reaction -- head, neck, legs and tail promptly disappeared into the shell.

Over the years, because of the actions of both humans and wild animals, the population of the famed giant tortoises has dwindled to 10,000. Once there were 14 subspecies of tortoises. Now there are 11.

The Darwin Research Station is making special efforts to breed more tortoises. They are raising baby tortoises of all the available subspecies at the station and returning them to the various islands after they are large enough to fend for themselves.

"We recently collected 12 adult female tortoises and one adult male from Hood Island," Pepe said. "The male was excited at the beginning, but then he got tired. No competition."

To protect the fragile ecology from humans, the park service has set up four zones among the islands -- areas where up to 90 persons may visit in one day, areas where only 12 persons a day are permitted, areas where only scientists and guides may go and primitive areas where no one is permitted.

The goal of the Ecuadorian park service is to somehow save 60 percent of what the Galapagos once were.

The perspective of the guides and the park service was expressed by the Dutch-born guide, Dolph, at a meeting with the visitors at the end of the trip:

"The future of the Galapagos is not the most important thing in the world. But if you come away with the fact that it is important to protect nature all over the world, we will have succeeded."