/fiːt/ (n.) a difficult act or achievement Modern-day researchers have attempted to understand this _______ .

/ˈspiːʃiːz/ (n.) type; a basic group in biological classification Migration is an excellent example of how nature has responded to the biological imperative for _______ to evolve and spread out

/ˈfjuːəl/ (v.) to provide energy About 55 percent of bird body weight is made up of the fat necessary to _______ this amazing journey.

/ˈheməsfɪə $ -fɪr/ (n.) one half of the Earth; also, one half of a sphere … migration of this type takes place primarily into the higher latitudes of the Northern _______ .

/ˈæspekt/ (n.) a part or feature How birds manage to unerringly travel between distant locations is one _______ that has fascinated observers for centuries.

/ˌnævəˈɡeɪʃən/ (n.) finding the way from one place to another Birds use the sun, stars, and landforms for _______ .

/ɪˈvɒlv $ ɪˈvɑːlv/ (v.) to develop gradually Scientists believe that birds _______ d from dinosaurs.

/ˈwɪndswept/ (Adj.) unprotected from the wind Many rock ptarmigan never leave the high arctic tundra, spending their breeding season atop _______ arctic peak

/ˈfæsəneɪt/ (v.) to interest greatly The study of the lives of birds _______ s many people.

/əbˈskjʊə $ -ˈskjʊr/ (v.) to make difficult to see When the stars are _______ d by clouds, nocturnal migrants may become confused and return to land or stray off course.