A bird that quacks and swims in the water may always be a duck, but a hand-rolled cigarette with a crimp on the end may not always be a marijuana joint.

So ruled a D.C. Superior Court judge this week, holding that a city narcotics detective exceeded his authority when he snatched a partially smoked hand-rolled cigarette from a car ashtray and charged the two occupants of the car with possession of marijuana.

"The mere appearance of a hand-rolled cigarette does not, without more, constitute probable cause to believe that it contains contraband," said Judge Henry H. Kennedy Jr., ruling that the detective violated the suspects' Fourth Amendment right against unreasonable search and seizure. He also ordered the cigarette and a pistol found in the car removed as evidence in the case.

Despite Kennedy's ruling, marijuana and gun charges still are pending against the two suspects.

In a 10-page memorandum, Kennedy delved into the virtues and ancient art of hand-rolled cigarettes and cited legal lore in cases as far away as Illinois and California on the uncanny resemblance of the traditional hand-rolled tobacco cigarette to the popular marijuana joint.

"Many people hand-roll tobacco cigarettes as a way of saving money and as a relection of individual taste," Kennedy observed, intimating that legitimate roll-your-own smokers risk being arrested by over-zealous police looking for marijuana.

In the case before Kennedy, D.C. police department narcotics detectives say they observed a parked car with a man and woman inside and a second man standing outside on March 20, 1980. When they approached the car, one of the occupants began emptying the dashboard ashtray, and the man standing outside fled when the officers showed their police badges, according to the Kennedy memorandum.

Next, Det. Albert Arrington said he spied a portion of a hand-rolled cigarette remaining in the ashtray. He described it as three-quarters to one inch long, blackened on one end and crimped or pinched on the other.

Suspecting it was a marijuana joint, Arrington reached inside the car and took it. Moments later, he also discovered a pistol in the car as well as additional suspected marijuana and a second pistol in the purse of the woman occupant of the car.

The man and the woman were charged with a variety of weapons and narcotics possession charges. The suspected drugs were later tested and found to be marijuana.

While police may seize an item "falling in plain view of an officer who has a right to be in the position to have that view," Judge Kennedy ruled that Arrington appeared to act only on "mere suspicion" and lacked more specific evidence giving him "probable cause" to make such a warrantless seizure.

Kennedy acknowledged that the dumping of the ashes, the flight of the man outside the car and Arrington's experience as a narcotics detective "may have engendered suspicion." But none of the factors, the judge asserted, "materially distinguishes this case from those in which it has been held that the mere appearance of a hand-rolled cigarette does not constitute probabable cause."

Government prosecutors say they are considering appealing Kennedy's ruling. One police official, who asked not be named, said Kennedy appeared to overlook two crucial factors distinguishing the cigarette seized in the car from an ordinary, hand-rolled tobacco cigarette: the cigarette was an inch or less long and "blackened" on one end, as though extinguished carefully for subsequent re-use -- all typical of marijuana usage.