Contrary to people's previous knowledge, it's food shortage not dietary preference that motivates birds to migrate thousands of miles back and forth between breeding and nonbreeding areas each year, a new research shows. "It's not whether you eat insects,fruits or candy bars or where you eat them that counts,but how reliable that daytoday food source is," said the study leader W.Alice Boyle of the University of Arizona. To figure out the underlying pressure that drives some birds to leave home for the season,Boyle examined 379 related species of New World flycatchers  and compared their sizes,food types, habitats, migratory behaviors and whether or not they fed in flocks. To compare the birds,the researchers constructed a "supertree"  showing the exact evolutionary relationships among different species. A computer analysis then determined whether a particular species was migratory because it ran in the "family" or whether something in the bird's environment was forcing it to leave each season.Boyle and her colleagues found that food shortage was the number one issue that predicted a species' migratory behavior. "Food availability is the underlying pressure, not diet and habitat," Boyle said. An alternative strategy that the birds use to handle food shortage is to hunt in flocks, since a group is more likely to find a new source of food than an individual. "If you suffer from food shortage, you have two options," Boyle said."You can either hunt with other birds or you can migrate." The passage implies that   _  .
the birds' food sources are getting rare in the world the living surroundings are severe for birds birds search for food separately birds can survive only by hunting in flocks
Question: Contrary to people's previous knowledge, it's food shortage not dietary preference that motivates birds to migrate thousands of miles back and forth between breeding and nonbreeding areas each year, a new research shows. "It's not whether you eat insects,fruits or candy bars or where you eat them that counts,but how reliable that daytoday food source is," said the study leader W.Alice Boyle of the University of Arizona. To figure out the underlying pressure that drives some birds to leave home for the season,Boyle examined 379 related species of New World flycatchers  and compared their sizes,food types, habitats, migratory behaviors and whether or not they fed in flocks. To compare the birds,the researchers constructed a "supertree"  showing the exact evolutionary relationships among different species. A computer analysis then determined whether a particular species was migratory because it ran in the "family" or whether something in the bird's environment was forcing it to leave each season.Boyle and her colleagues found that food shortage was the number one issue that predicted a species' migratory behavior. "Food availability is the underlying pressure, not diet and habitat," Boyle said. An alternative strategy that the birds use to handle food shortage is to hunt in flocks, since a group is more likely to find a new source of food than an individual. "If you suffer from food shortage, you have two options," Boyle said."You can either hunt with other birds or you can migrate." The passage implies that   _  .
A. the birds' food sources are getting rare in the world
B. the living surroundings are severe for birds
C. birds search for food separately
D. birds can survive only by hunting in flocks
Answer: