/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

/daɪˈɜːnəl $ -ˈɜːr-/ (Adj.) active during the day _______ migrants have also been shown to use geographic features.

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

/ˈɒptəməl $ ˈɑːp-/ (Adj.) best, most favorable This form of migration allows birds to breed in areas that provide _______ conditions for nesting and feeding their young.

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

/ɪnˈdjʊə $ ɪnˈdʊr/ (v.) to live under difficult conditions … winter season in nearby valleys, enduring some of the coldest conditions on Earth.

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

/nɒkˈtɜːnl $ nɑːkˈtɜːr-/ (Adj.) active at night _______ migrants, those species that travel at night, seem to take their navigational cues from the stars.

/maɪˈɡreɪʃən/ (n.) movement from one place to another Bird _______ generally takes place twice a year. In the spring and autumn.