Because chicks of the same clutch hatch over several days, the older, larger chicks are better able to compete for food. Scientists believe asynchronous hatching is an adaptation to unpredictable food supplies. If food is plentiful, all the chicks survive. If food is scarce, the brood is rapidly reduced. As with incubation, large species have proportionately longer nestling periods than small species.
Small species grow more rapidly than large species.
Small species may reach adult weight by fledging, while large species may not reach adult weight until sometime after leaving the nest. As contour feathers replace downy ones, chicks begin to preen after meals. Once wing feathers have developed, chicks spend a considerable amount of time flapping them at the nest. After fledging, young birds begin to make short flights to nearby trees or perching sites. They usually return to the nest when a parent returns with food.