(FORTUNE Magazine) – Lee Iacocca had barely blown out the candles on his 61st birthday cake when 70,000 Chrysler workers in the U.S. and 10,000 in Canada went on strike as their contracts expired. Picketers sounded almost apologetic -- ''We had to do it,'' one of them explained -- and both Chrysler management and United Automobile Workers representatives predicted a quick settlement. The UAW wants an increase in pension fund contributions, a profit-sharing plan, limits to the amount of work that the company can farm out to suppliers or overseas, and pay parity with General Motors and Ford. Chrysler assembly-line workers average $13.23 an hour, six cents an hour less than their counterparts at Ford and GM. Labor disputes, including a 27-day hotel strike in New York City, a one-day baseball strike, and several airline strikes, have made the front pages this year, but the number of strikes continues to decline. In 1983 the Bureau of Labor Statistics counted 81 strikes involving 1,000 or more workers. The tally dropped to 68 last year and only 37 through September of this year. ''Workers don't feel strikes are a very effective weapon, and they've watched too many workers take a severe beating on picket lines,'' said Ray Rogers, director of Corporate Campaign, a consulting firm to unions.