Every fall and winter at sunset, the sky above Staten Island fills with majestic sandhill cranes alighting in the fields. The sight is more spectacular than usual this year, as the number of cranes wintering on the island in the Sacramento-San Joaquin Delta has doubled over the same time in 2013.

Scientists say they’re not sure what’s causing the population boom but suspect the drought and a shift in what farmers are growing may be at its root.

The latest count by The Nature Conservancy, taken last week, found nearly 3,500 sandhill cranes at Staten Island. The geese population – mostly Aleutian cackling geese and greater white fronted geese – was also much larger than expected, with 14,000 counted, said Dawit Zeleke, conservation scientist with The Nature Conservancy.

Staten Island is an important destination for sandhill crane populations. Fifteen percent of all sandhill cranes that fly into the Central Valley each winter along the Pacific Flyway alight there. The flyway is a crucial path for more than 5 million birds, including cranes, swans, geese and ducks.

“We can’t say with any certainty why the numbers are higher, but we believe the birds are having to congregate in one area because of shrinking wetlands due to drought,” Zeleke said.

The Nature Conservancy bought the island in 2001 to safeguard wetlands for migratory birds. It spent $30 million in public grant funds approved by voters in Propositions 204 and 13. The organization also owns the nearby McCormick-Williamson Tract.

The crowding of cranes and geese at Staten Island this year is expected to continue, said Laura Shaskey, program manager for Conservation Farms and Ranches, the organization that manages the land at Staten Island for the Nature Conservancy.

Staten Island is managed to be attractive to cranes. Most of its 9,200 acres are used for growing corn. When that corn is harvested in September and October, the leftover plant material is ground up as mulch – which cranes like to feed on. The field is then flooded to provide nightly protection from predators and a feast of invertebrates for cranes.

This year, cranes and other water-loving birds have fewer alternatives when it comes to flooded fields. The expanse of rice fields – which function as surrogate wetlands – shrunk this year in the Sacramento Valley because of reduced water allocations that prompted some farmers to fallow their land. Only 428,000 acres of rice will be harvested in 2014, a 24 percent drop from 2013, according to the U.S. Department of Agriculture.

Shaskey said he thinks an ongoing shift from growing corn and alfalfa to grapes or almonds has also served to lessen habitat for migratory waterfowl, which typically don’t use almond groves or vineyards for either food or protection.

The acreage devoted to almonds in California more than doubled from 1995 to 2013 – from 418,000 acres to 840,000 acres, according to the California Department of Food and Agriculture.

“We’re starting to see more of that kind of cultivation here, in areas between Twin Cities Road and Thornton Roads,” Zeleke said.

He sees the switch from pasture land or corn growing to almonds as part of a long-term trend given the state’s powerhouse status as an almond producer. California grows about 80 percent of the global supply of almonds and nearly 100 percent of the domestic supply.

Driven in part by the lucrative almond industry, the average value of cropland in California rose from $3,800 in 2004 to $7,300 an acre last year, despite uncertainty over water supplies, according to the USDA.

Zeleke said the conservancy hopes to buy more existing pasture or farmland before it is bought for use as almond farms or vineyards. But the rising price of land and commodity prices may make accomplishing that goal more difficult.

“It’s becoming harder to compete with that,” Zeleke said. “It’s a challenge that we’re going to have to deal with.”

Call The Bee’s Edward Ortiz, (916) 321-1071. Follow him on Twitter @edwardortiz.

Lodi holds its annual sandhill Crane Festival Saturday from 10 a.m. to 6 p.m. and Sunday from 10 a.m. to 5 p.m.

Where: 125 S. Hutchins St., Lodi

What: Tours to see cranes, an art show, presentations and workshops

For more information: (800) 581-6150 or