One of the few pieces of continuity, of normality, in the upheaval and tragedy that is Nicaragua today is the birds.

Dozens of species flourish in the lush tropical vegetation, from the ominous hawks and buzzards that glide silently over the battle zones, to small, brilliantly colored songbirds.

It is the songbirds that are remembered. In nearly every city and village, ears are assaulted by the drone of planes, the heavy thud of grenades, shells that still make even the most war-seasoned flinch involuntarily, the sudden rat-a-tat-tat burst of automatic weapons and the nearby rifle crack that means head for the nearest cover.

But after every spit of gunfire, there is an inevitable seconds-long lull. After a moment of silence, the birds begin chattering and singing again as if nothing had happened.

There are no reliable news reports in Nicaragua now - no newspapers save the government propaganda sheet, no radio broadcasts except military voices reading the latest communique, or the excited revolutionary chant of the guerrilla shortwave Radio Sandino. Claims from both sides are equally likely to be false.

Reporters attempting to cover Nicaragua's civil war find themselves reduced to listening to their own stories parroted back on the Voice of America, the BBC and other foreign stations.

Because there is no unbiased local information agency, no even remotely reliable source of accurate reports of troop movements, there is no indication until a battle is long over of who is winning.

War in Nicaragua is rather a series of isolated sights and sounds. But from the little available each day to one person's eyes and ears, a picture emerges.

Nicaragua is physically a very gentle country, where soft, jungle-covered hills slope into wide fields. The road to Matagalpa, a small city 80 miles north of here, passes through some of Nicaragua's most beautifual scenary, and the city itself lies in a narrow green river valley.

Two miles outside Matagalpa on Sunday, the National Guard had set up a roadblock - a concrete telephone pole and several boulders laid across the potholed two-lane highway. Three smiling middle-aged soldiers walked down from their sandbagged post above.

"If you go up there," one pointed cheerfully up the road, "you'll die."

Ahead, less than a half mile from the edge of the city, heavy cross fire from the surrounding hills forced us to abandon the car and head for cover in the bush. Fifty yards ahead, another car in which an NBC television crew had been traveling, sat empty beside the road with a door hanging open.

Planes flew overhead, launching rockets into the city, but fleeing residents scrambling through the bushes said most of the government ground troops were outside the city, firing in from the hills. The streets were controlled by the Sandinista guerrillas.

As they huddled in a abandoned roadside house, the NBC crew was surprised by two armed guerrillas. One smiled, shook hands and asked if they would like to meet his commander. The other pushed them out with his gun and said it would give him great pleasure to kill some North Americans.

Taken to a house on the relatively quiet city streets, the crew was greeted by several men, all in their mid-thirties, dressed in fatigues. The guerrilla leaders cordially answered their questions, offered them food and coffee, and allowed to them to take pictures.

The crew as escorted back to the car, and we all took off back toward Managua, crouched low and not knowing if the loud gunfire was directed toward us or we were being irrationally afraid and were merely deceived by the echo of shots around the valley.

Both the guerrillas and the National Guard soldiers can be alterntively nice or nastily aggressive. Since there are few front lines, and usually what looks and sounds like random gunfire in all directions, one often doesn't know whose territory one is in.

Many on both sides are young, some barely teen-agers. Driving through a small town on the road back from Matagalpa, we stopped suddenly when a husky guardsman, dressed in camouflage fatigues, stepped into the road. As he approached the car and we scrambled for identification, we saw he was little more than a child.

The youth said he was 14 years old and had been in the National Guard for 1 1/2 years.

"How is it up there," he asked anxiously, looking toward Matagalpa. "It's bad, isn't it?"

"Senora," he said politely, "could you please give me a ride back to Managua? I'm very sick." We explained that, with the situation so uncertain, it is unwise for us to carry strangers - Guard, guerillas or the hundreds of civilian refugees trying to get away from the fighting.

"Of course. You are very wise," he answered. "Godd-bye then, and go with God."

Since the civil war began in earnest last September, those who have reported regularly on the Nicaraguan situation have looked on Managua as a refuge. After a long day driving from city to city outside the capital, crouching in doorways, listening to and seeing horror stories, the slightly seedy Intercontinental Hotel is another word of clean clothes and quiet.

But since the shooting began two days ago in Managua, the atmosphre has changed. There is a high level of tension, tempers are quick, and laughter sometimes borders on the hysterical.

Last night, reporters read aloud to each other from "Viva Vargas," a hilarious Wood Allen takeoff on a bumbling Latin American revolution.

The hotel is running out of food, and the menu has been cut back to a few pasta and beef items. Telephones are cut as often as they are working, and there are frequent verbal battles around the one telex machine.

The few hotel employes remaining as parts of the city become cut off are exhausted with the demands of more than 50 journalists and a growing assortment of wealthy city residents who have sought refuge here. There are four waiters now working in the restaurant, and they all sleep here. The others cannot get to work.

Most high government officials are now living either in the hotel or in the "bunker," President Anastasio Somoza's office inside the National Guard garrison across the street. Two nights ago, a Cabinet minister and Somoza's military aide sat in the hotel with a few reporters over drinks.

"You tell us," the minister said to the journalists. "What is the solution? What can we do?" He agreed that even if the Sandinistas are defeated this time, they will be back.

The military aide, whose black arm sling lost much of its fascination when he revealed he had fallen in the bathrub, said, "There can be no [political] solution here until there is peace," by way of a National Guard victory. "Somoza will not leave.""No," said the minister, "there can be no peace until there is a solution."

Yesterday, the day after temporarily victorious guerrillas were driven out of Masaya, 20 miles south of here, the city's empty streets were filled with debris. Remaining doors were tightly closed and a light drizzle fell.

It was as if some sort of poisonous gas had spread through the city, causing every living thing to disappear. As usual after battles here, there were few bodies. The National Guard moves its dead and wounded insided its outposts. The guerrillas ad civilians pull their bodies inside the houses and hide or bury them in interior courtyards.

The closed doorways were still decked with white flags, like as sad celebration had been stopped in mid-parade. On one door, residents had scribled a sign with chalk: "Two old people and two children live here."

The fact that there are people behind the doors and the rubble is clear only when sniper shots occasionally crack the silence, followed by the big volleys of National Guard patrols. A door quickly opens and hands reach out to two lone reporters walking timidly with their own white flag. "Quick, come in here. Come on."

"It was like Vietnam here," said one man inside a darkened doorway. Up the street, his grocery store lay in ruins, first rocketed by government aircraft and then, he said, looted by the National Guard. "Don't use my name. Don't say who I am."

At the end of the main street, around the corner from the local National Guard garrison, two bodies lay frozen in a grotesque sculpture. Someone had removed their clothes.

Suddenly the air filled with laughter and the incongrous strains of a radio playing a Bee Gees disco hit. Across from the garrison, the National Guard had set up a barracks in a church. The troops seemed calm and confident. A Sherman tank, several armored vehicles and a long line of military trucks were parked in front.

Next door to the garrison, in front of what used to be the Masay movie theater, a group of soldiers sat playing cards in the ashes.

The remains of a double bill advertising poster lay beside them. One movie starred Cantinflas, the American comedian. The title of the other was burned away, with only a line of promotion remaining. "Your heart," it promised in Spanish, "will be paralyzed with suspense." CAPTION: Picture, A group of Sandinista guerrillas chat and pose for pictures in Matagalpa, near the place where they surprised reporters taking cover from gunfire. NBC News via AP