/briːd/ (v.) to reproduce Migration is the regular movement of animals between their _______ ing grounds and the areas that they inhabit during the rest of the year.

/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.

/əbˈzɜːvə $ -ɜːrvər/ (n.) a person who watches something If birds become aware of the presence of an _______ , they quickly fly away.

/streɪ/ (v.) to leave the correct route; become separated from the group When the stars are obscured by clouds, nocturnal migrants may become confused and return to land or _______ off course.

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