The active-duty strength of the nation's military services declined in February from both the previous month and the year-earlier period, the Pentagon said Thursday. As of Feb. 29, the Army, Navy, Marine Corps and Air Force had 2,163,193 men and women on active duty, down 2,625 from Jan. 31 and a decrease of 8,914 from the Feb. 28, 1987, tally. It is not unusual for military strength to swing up and down on a monthly basis during the year, primarily because of differences in the flow of new recruits. The strength totals are expected to vary even more widely than normal this year, though, because the Pentagon is reducing its active-duty strength in a budget-cutting move. According to the latest ``Military Strength'' assessment, three of the four armed services reported personnel reductions in February. The Marine Corps reported 198,231 men and women on active duty, down just 12 from January. The Air Force had 603,599 on active duty, down 975; and the Army listed 776,415, down 2,308. The Navy, meantime, reported an increase of 670 personnel in February, with 584,948 on active duty at the end of the month. The Marine Corps and Air Force reported declines in strength compared with Feb. 28, 1987, while the Navy and Army reported slightly increases.