The distinct call of a woodpecker carries through the forest. It's not drilling or pecking, it's calling to its mate. These woodpeckers have a nestful of babies in a cavity in a nearby tree, and the hungry babies are keeping both parents busy. These are sapsuckers, woodpeckers that don't just drill into trees for bugs. Like people tapping trees for sap to make birch or maple syrup, sapsuckers create elaborate networks of sap wells and maintain them to ensure sap production. The sap wells are rows of pencil-eraser-size holes pecked through the bark of trees, and the birds lick the sap that oozes out. They also eat insects that get caught in the sap.
Sapsuckers defend their wells from other sapsuckers, as well as from other birds. Rufous Hummingbirds are known to nest near sap wells, follow sapsuckers in their daily movements, and may even time their migration to coincide with sapsucker migrations so they can feed off the sap wells.
Like other woodpeckers, sapsuckers excavatle a new nest cavity each year, and their "leftover" nest cavities are important to other cavity-nesting birds that aren't equipped to build them. Many species of flycatchers, swallows, falcons, and owls nest in cavities, as do ducks like mergansers, goldeneyes and wood ducks - which is why they like nest boxes. Even some mammals like northern flying squirrels nest in woodpecker-made cavities.