* Number of ships waiting to move in or out down 30 to 57
* Coast Guard stretches shipping cutoff to midnight from 6 p.m.; to resume at daylight (Adds details on shipping and cleanup of oil)
HOUSTON, March 26 (Reuters) - The backlog of ships waiting to sail in or out of the Houston Ship Channel fell on Wednesday as crews cleaned a fuel oil spill in Galveston Bay, the head of the Houston Pilots said.
The U.S. Coast Guard also reduced the so-called daylight restriction, allowing ships to sail until midnight CDT (0500 GMT) before movement stops until Wednesday morning, which is expected to further reduce the backlog, Houston Pilots Capt. Clint Winegar said.
He said the number of ships waiting to move to or from the port of Houston through the channel, the waterway through which more than a tenth of U.S. refining capacity receives crude oil, slid by 30 to 57 by Wednesday afternoon.
Affected tankers also included refined product ships waiting to move out, largely to export markets. The thinning backlog gave U.S. Gulf Coast cash gasoline differentials a double-digit boost on Wednesday, reversing a similar decline on the backup the day before.
On Tuesday, when both inbound and outbound traffic resumed, the Coast Guard stopped movement at about 6 p.m. CDT (2300 GMT) to limit the spread of fuel oil floating in Galveston Bay. On Wednesday, the Coast Guard stretched that window to midnight, Winegar said, when movement would stop until about 7:30 a.m. CDT (1230 GMT) on Thursday.
The channel and offshoots to Texas City and the port of Galveston, were shut on Saturday after a Kirby Inland Marine barge collided with a cargo ship and spilled heavy oil that is primarily used as fuel for ships.
Vessels in the channel must move very slowly and stop at inspection and decontamination stations.
The Coast Guard reopened the channel and the Intracoastal Waterway on Tuesday.
Winegar said that late afternoon on Wednesday, 35 ships were waiting to sail to the port of Houston and 22 were waiting to sail outbound.
Of those, he said he expected 15 ships to sail inbound and another seven to move outbound before the midnight shutdown.
The closure of the shipping channel forced the second-largest refinery in the United States, Exxon Mobil Corp's 560,500-barrel-per-day plant in Baytown, Texas on the east side of Houston, to cut production earlier this week.
Not all ships waiting to travel are tankers. There are also container ships, bulk carriers and a few cruise vessels.
The spill of 4,000 barrels, or 168,000 gallons (636,000 liters), is far smaller than the 260,000 barrels, or 11 million gallons, of crude oil released when the Exxon Valdez struck a reef in Prince William Sound in 1989.
Cleanup efforts continued as commerce slowly resumed.
70 ships were involved in cleaning the fuel oil that spread across the southern portion of Galveston Bay and into the Gulf of Mexico. The Coast Guard and responders on Wednesday continued setting up equipment nearly 100 miles from Galveston in the Matagorda Bay area to prevent the spread of oil further south on the Texas Coast.
While known for their energy infrastructure, the bays along the Texas Gulf Coast are nesting areas for migratory birds that move south during the cold winter months.
As of Wednesday, 30 dead birds had been recovered while 12 other oiled birds were being treated, according to U.S. and Texas wildlife agencies. (Reporting by Erwin Seba and Kristen Hays; Editing by Lisa Von Ahn and Grant McCool)