题目内容：

    When fisheries biologist James Drymon noticed feathers in the vomit of a tiger shark, he first assumed they belonged to some unfortunate seabird: a gull, perhaps, or a pelican. But when he and his team genetically sequenced the feathers, the results surprised them: the feathers came from a land-based songbird called a brown thrasher. So what was it doing in a tiger shark’s stomach in the Gulf of Mexico?

Drymon, a researcher at Mississippi State University’s Coastal Research and Extension Center, and his colleagues sorted songbird from a tiger shark’s stomach through the stomach contents of 105 juvenile tiger sharks between 2010 and 2018. Nearly 40 percent had recently feasted on birds that come from dry land. In all, the scientists counted 11 terrestrial bird species showing up on the sharks’ menu. The results were published online in May in Ecology. Researchers have known since the 1960 s that sharks sometimes eat songbirds. “But what was interesting to us was the prevalence of the behavior,” Drymon says, “This is something that happens every year in a high number of sharks.”

Every fall and spring, songbirds undertake dramatic migrations across the Gulf of Mexico. If bad weather comes along, they can be forced to land on the water—which is effectively a death sentence. “The estimate for the number of migrants that die because of storm-related events is in the billions,” Drymon says. He suspects that sharks have long taken advantage of this twice-yearly nutritional bounty raining down from the skies, but scientists have only recently had the genetic tools to confirm this by identifying partially digested feathers.

“The results stressed how interconnected marine and terrestrial ecosystems can be,” says University of Miami marine ecologist Neil Hammerschlag, who was not involved in the study, “It shows how opportunistic these sharks are.”

1.How did James Drymon and his colleagues identify the feathers?

A.By making comparison. B.By checking their DNA.

C.By observing them closely. D.By analyzing previous data.

2.What do we know about tiger sharks?

A.They dislike terrible weather. B.Most of them feed on land birds.

C.They actively catch songbirds for food. D.They are good at making use of opportunities.

3.According to the passage, what do songbirds fear most when they migrate?

A.Sharks in the sea. B.Other larger birds.

C.The stormy weather. D.The long distance.

4.What is the text mainly about?

A.Tiger sharks’ eating habit. B.Researchers’ identification of feathers.

C.Land birds’ suffering a lot when migrating. D.Land birds’ being on the menu of tiger sharks.