Michael Crockett Jr. combined his birthday and elementary school graduation money with a gift from his grandmother and bought his dream bike: a $230 Mongoose dirt racer he had long wanted.

A few days later, the 12-year-old parked his shiny new prize outside a store near his Locust Road NW home and went in to buy a popsicle. Michael returned to find an old, rusty 20-inch girl's English racer in its place. He had become a victim in the annual warm weather surge of bicycle, motorbike and outdoor equipment thefts.

Each year, from April through September, such thefts increase markedly, despite efforts by police to curb them and warnings to owners to take extra precautions.

During those months last year, the "seasonal trend" resulted in 1,318 bicycles reported stolen, an 18 percent increase over the rest of the year, according to the police department. From January through March of last year 112 bicycles were stolen. There were 133 reported thefts of mopeds and motorscooters during the warmer months of 1981.

During the summer, District police increase their efforts to retrieve stolen bikes. The youth services units in some police districts assign officers to patrol on bicycles and stop youths riding new, expensive models, especially if they are not obeying traffic laws.

"The reason most people steal bikes is to sell them or the parts," said Sgt. Charles Cephas of the community relations office. "Some stolen bikes, especially racing bikes, can be sold for $400 to $1,000."

The police urge owners to register their bikes, as required by law, to improve the chances of recovery in the event of theft. District law requires that all bicycles be registered within 14 days after purchase. Failure to do so could result in a $5 dollar fine.

Registration forms are available at stores that sell bicycles. The forms can be submitted between 8 a.m. and 8 p.m. at any police station, for a $1 registration fee. The registration is good for five years.

Although registration will not prevent theft, it can increase the chances of recovering a stolen bicycle, said Clifton Porter, clerk of the police property division.

"All stolen equipment that is not recovered is sold at a public auction," said Porter. "But . . . if the stolen bike is registered, we first notify the owner by certified mail. But even then, some owners never claim them." He said the monthly police auctions of unclaimed stolen property net the D.C. Treasury an average $3,000 to $4,000 in revenue.

Police also recommend securing your bike with a cable to a bike rack or any fixed object except a tree. It is illegal to secure bikes to trees in the District.

"A heavy-duty cable offers the most resistance to wire cutters," said Cephas. He also suggested taking the front wheel off the bike and carrying it along, as another theft deterrent.

Michael Crockett Sr. says he has observed several bike thefts in recent months. "They thieves ride old bikes until they see a newer one. Then they wait outside stores or arcades until a bike is left alone. They take the newer one and leave the old one behind," said Crockett, a technical representative for Xerox Corp.

Crockett's son was one of the more fortunate victims. As a result of the $50 reward that the Crocketts offered, three neighborhood boys tracked down a youngster that one of them had seen riding the Mongoose. The three boys recovered Michael's bike and claimed the reward.