(2016·云南师大附中适应性训练)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(捕蝇鸟) and compared their sizes，food types, habitats, migratory behaviors and whether or not they fed in flocks.
