Birds that fly to the tundra to nest during the summer are said to be

Migratory

Bird migration is the conventional periodically movement, often north and south along a flyway, within breeding and wintering terrains. Many varieties of bird migrate. Migration carries high costs in extinction and mortality, from poaching by people, and is motivated essentially by the availability of food.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.