By eating beetles and their viruses and by defecating these viruses along tree trunks, birds inadvertently spread it to bark beetles in the same tree and throughout the forest. The breeding season for birds occurs when the insect populations are their highest. During insect outbreaks, some birds will increase the number of offspring that they raise to take advantage of the abundant food supply. Birds are highly mobile and many species of birds will take advantage of a local insect outbreak by moving into the infected area. Some of these invasions can increase the normal numbers of birds in an area by 80 times.
Birds like to feed large, juicy insects to their young.
Relatively few insects survive past the egg and small, young larval stages. By feeding on large, late stages of caterpillars, and on pupae and adults, birds become a key force in depleting insect populations. Birds can alter their diets to feed almost exclusively on an insect pest during an outbreak, if it becomes profitable for them to do so. They can develop a search image for this new prey and can learn how to hunt for it more efficiently. Along with developing a search image, birds can change their foraging locations and foraging behavior in response to an insect outbreak.