U.S. government bonds strengthened on Monday as lower oil prices and concerns over the U.S. economic outlook stoked demand for haven assets.Trading was thinner than usual following the Easter weekend, as many markets in Europe were closed Monday. Traders say this may have magnified part of the price moves in bonds.In late afternoon trading, the yield on the benchmark 10-year Treasury note was 1.870%, compared with 1.902% on Thursday. The bond market was closed Friday. Investors are awaiting comments from Federal Reserve Chairwoman Janet Yellen on Tuesday for clues about the timing of an interest-rate increase. Also, the nonfarm jobs report for March, a key indicator of the labor market, is due Friday.