A pair of tropical boubous (pronounced boo-boos) were skulking, as is their wont, in the underbrush at the edge of the forest. Suddenly, they sang a duet.

The male piped three bell-like whistles and the female answered at once with a "kwee."

"Ooooh! I hear a tropical boubou," exclaimed Fleur Ngweno, bird-walk leader. In unified response, 29 pairs of binoculars scoured the bushes. Then, faster than a dusky flycatcher can gobble a bug, the bird-walk leader spun on her heels and proclaimed the presence of a black cuckoo shrike, a pin-tailed whydah and a marabou stork.

It was a typical Wednesday morning bird walk in Kenya, Africa's bird watching mecca.

In the near distance, under scudding white clouds, hovered the four peaks of the Ngong Hills, described by Isak Dinesen as "immovable waves against the sky." The highland air was cool, the breeze was gentle on the skin and the bird watchers, their eyes plugged into big, black binoculars, were joyous.

There are 1,066 species of birds in Kenya. Five hundred of them have been seen within 15 miles of the capital, Nairobi. Bird watching is a competitive event here. One local ornithologist, screaming around Kenya in an airplane and a road-rally car, spotted 308 species in a 24-hour day, an African record.

More than 5,000 persons a year fly from all over the world, spending an average of $3,000 each, to see Kenya's birds. Bird watchers have a name for someone who travels widely and spends money freely to add new species to his life list of birds seen. Such a person is a "twitcher."

The world's leading twitcher reportedly is Harvey Gilston, an Englishman who lives in Lausanne, Switzerland, and who has seen more than 6,000 species. Gilston, it goes without saying, often comes to Kenya.

"All the twitchers come to Africa at one stage or another because they can add so many birds to their life list so quickly," said Don Turner, an ornithologist who for 23 years has made a handsome living by squiring twitchers around Kenya.

"The average [bird watching] person on the East Coast of the United States can see maybe 500 to 600 species in a lifetime, if you count the migratory birds. But here, if you apply yourself, you can see 700 or 800 in a year," said Turner, managing director of East Africa Ornithological Safaris Ltd. Turner applied himself one day in November of 1984, traveled 1,000 miles in Kenya and saw 305 species.

The bird-sighting world record in 24 hours is not held here, but in Peru, where 331 species were seen. But Turner said he and his colleagues will beat that record in next November's Kenya Bird Watch.

Here in the lee of the Ngong Hills, along a forest road redolent with wild jasmine and African carissa, the Wednesday morning bird walk attracted no twitchers. They were 29 mostly local birders: expatriate wives, Kenyans who work for the government's Wildlife Department, a new employe in the bird department at the National Museum of Kenya, a visiting zoology student from Wyoming.

Fleur Ngweno, a white-haired, ruddy-cheeked woman with a lilting, high-pitched voice that is, well, birdlike, was in command, as she has been on Wednesday mornings here for 15 years. Local ornithologists described Ngweno, 47, who was born of French parents, raised in New England and has lived in Kenya for 23 years, as a first-rate bird identifier and an enthusiastic teacher. When she isn't birding, she edits Rainbow, an African children's magazine.

"Ooooh! What was that that zipped by? I think it was a Zambezi honeyguide. Yes, it was. Do you know that its song has not yet been recorded by experts? There are still lots of things to find out . . . . That's what is exciting about bird watching."

Ngweno said this about 10 minutes into the bird walk. During the next three hours, she rattled off a mesmerizing spiel that critiqued every bird she could get her eyes on. The black and brilliant yellow of a Reichenow's weaver reminded her, for some reason, that many birds become bellicose, fearing a rival, when they see their own reflection.

"I've seen a thrush fight with its reflection in a hubcap to the point where it wore itself out and started to lose its tail feathers," she explained. Sighting a fiscal shrike on a fence post, Ngweno observed, "That is very typical shrike. It sits on a fence post, flies down to eat, then perches back on the fence. You wonder what shrikes did before fence posts and power lines."

Her monologue touched on flora and fauna ranging from clawed frogs she spied beneath a bridge to Sykes monkeys that gamboled across a dirt road in front of her to a whistling thorn tree alive with big ants.

"It may be an advantage to the thorn tree to host those ants," she said. "Giraffes around here don't mind the thorns, they wrap their tongues right around them, but the ants bite the giraffes."

In more than 750 Wednesday morning bird walks in the Nairobi area, Ngweno and her bird watchers have sighted 300 or 400 species of birds. She said she does not keep count.

Her birders also have encountered Kenyan wildlife more substantial than birds.

"Our hairiest moment was when we had two groups of school children out and a python wrapped itself around one of the kid's legs," Ngweno says. "Happily, it soon unwrapped itself and went away."

Near the end of the walk, with the equatorial sun having burned the freshness out of the morning and the Ngong Hills seeming to smear in the heat, Ngweno insisted that all her sweaty followers take a moment and find the elusive white-starred bush robins. They were skulking beside a dirt road in thick forest underbrush.

In bird-watching usage, Ngweno explained, the word "skulking" refers to a bird's preference for hiding in thick foliage, not to its character. The robins, which flashed a golden yellow amid the brown-gray brush, were spotted by everyone only after 20 minutes of diligent, binoculared bush-gazing.

It had been "a medium day" for bird sightings, Ngweno said. The 35 species spotted totaled less than half the Wednesday morning record. Still, Ngweno was pleased with the appearance of the rarely seen robin.

"Everybody did see the white-starred bush robin?" Ngweno asked, her voice chirpy but concerned.

"Yes, we have," the birders answered. Turning her charges toward their cars, Ngweno said, "I'm so glad."