...they feed the insect eating birds. The birds are beneficial because they eat the icky bugs that bite people and animals.
I read on my state's wildlife website that it's so hard for birds to find bugs to eat during the cold winter season that they migrate to warmer, bug infested areas. I just thought it was because they were cold and maybe looking for berries and other foods not available in winter. It didn't occur to me that the insect eaters were subject to food shortages.
Our state bird is an insect eater but it winters here and forages for food. It's a hardy little thing, like the state it's named for.