a reliable source of forage. The urban heat island effect may also enable them to live further north than they otherwise could. White-winged doves typically migrate into Arizona beginning in March. In California, birds arrive in April and depart by August. Texas migrations run from April through June, peaking in May, and departures run September to October. Migratory groups may include as many as 4,000 individuals, though typically less than 50. A combination of weather, food availability, and hunting pressure can affect the timing of migration. As populations expand in Texas they are becoming less migratory; about 1/3 of birds now overwinter in Texas. Migrations are tracked via traditional banding methods, but the isotope composition of hydrogen and carbon in the feathers can also be used. A 2015 study showed that by tracking the amount of various isotopes, researchers could accurately identify a white-winged doves migration origin. They could also identify if it had been eating from saguaros because of the unique carbon signature that cactus photosynthesis produces. Behavior and ecology Normally, up to 4000 birds are seen migrating, but in Texas, a flock of up to a million birds was recorded. Z. asiatica may fly 25 or more miles to find water, though they can be sustained solely by the water in saguaro cactus fruit. Breeding and nesting To impress females, males circle them with tail spread and wings raised, and also fan or flap their tails, or engage in cooing and preening. Males may aggressively defend territory from other males, sparring with wing slaps. Males and females work together in raising the young. The white-winged dove builds a flimsy stick nest in a tree of any kind, and lays two cream-colored to white, unmarked eggs. One chick often hatches earlier and stronger, so will demand the most food from the parents. A dove may nest as soon as 2–3 months after leaving the nest, making use of summer heat. The dove nests as long as food and enough warmth are available to keep fledglings warm. In Texas, they nest well into late August. Families and nestmates often stay together for life, perching and foraging together. They return to the same nesting site year after year, sometimes rebuilding a nest in the exact location it was the year before. They have better nesting success when in mesquite or tamarix trees. Breeding occurs in two peaks in the summer, although