Final answer:
An exaptation is a trait originally selected for one purpose but later co-opted for another use. Feathers, which initially evolved for thermoregulation, later became used for flight, making them an example of exaptation. Birds also exhibit adaptations in beaks and feet correlating with their diets and habitats.
Step-by-step explanation:
An exaptation is a trait that has been co-opted for a use other than the one for which natural selection has built it. This can occur when a trait that originally evolved for one function later provides a selective advantage for an alternate function. Feathers are a classic example of exaptation, having evolved for purposes of temperature regulation, and later becoming instrumental in the capability for flight. Though birds use feathers for flying today, the original function of feathers was to help with thermoregulation or possibly in display and camouflage.
Two examples of flightless birds are the ostrich and the kiwi. An ostrich's beak and feet are adapted for their life on the savanna, eating a variety of plant and animal matter. Meanwhile, the kiwi's beak is adapted for foraging insects and other invertebrates from the soil of forests. The beak variation in bird species correlates strongly with their preferred food source, such as the thick and strong beaks of fruit and seed-eating birds for breaking nuts, or the long beaks of nectar-eating birds for reaching into flowers. Bird feet too show remarkable adaptations, like webbed feet for swimming in ducks or strong talons in predatory birds for grasping prey.