US STOCKS-Wall Street ends down; Twitter rallies after-hours
* UPS shares fall after outlook, weighing on transports
* Windstream rallies; set to spin off assets into REIT
* Merck up as results beat expectations, but Pfizer slips
* Indexes: Dow flat, S&P off 0.1 pct, Nasdaq up 0.2 pct (Updates to close; adds Twitter and American Express results)
By Ryan Vlastelica
NEW YORK, July 29 (Reuters) - U.S. stocks fell on Tuesday, dropping in a broad selloff as a weak outlook from courier company UPS weighed on sentiment and pressured transportation stocks.
Although a rally in telecom stocks helped to limit the broader market's decline, the S&P 500 ended below its 14-day moving average for a second straight day. Nine of the 10 primary S&P 500 sectors fell on the day.
United Parcel Service shares fell 3.7 percent to $98.86 after the world's biggest courier company slashed its earnings forecast for the year due to spending to boost capacity. It also reported second-quarter earnings that were below expectations.
Rival shipping company FedEx Corp was down 1.6 percent at $147.14. The Dow Jones Transportation index, which is often viewed as a proxy for business activity, fell 1.4 percent. It is about 3 percent away from the record closing high hit last week. Continuación...