Which bird is considered a keystone species in many ecosystems?

The story behind the answer

Woodpeckers are keystone species because they’re ecosystem engineers: their powerful bills excavate tree cavities that become scarce, essential nesting sites for dozens of “secondary cavity nesters” that can’t dig their own—think bluebirds, chickadees, owls, ducks, squirrels, and bats. By hammering out dead or decaying wood, they also help control bark-boring insects and create sap wells that feed other animals, including hummingbirds and insects, boosting forest biodiversity.

Remember it this way: the bird that makes holes holds the ecosystem together. Unlike bald eagles or herons (top predators) or hummingbirds (pollinators), woodpeckers literally build the real estate many species depend on. Remove woodpeckers, and the housing market for wildlife collapses—classic keystone effect.