Steve,
Flickers, are the only species of woodpeckers that feed on the ground near tree trunks. They tend to catch and eat any ground bugs or insects that take flight in front of their food search.
Trees / Shrubs attract a lot of bugs which are either using the tree for shelter or feeding upon it. The birds will go after grubs within the lower trunk or roots.
Observe how much attention they are paying to the lower bole (trunk) if they are attacking the bark. This would be cause for concern of bark / boring beetles.
Now for the obvious question? Are you sure they are 'flickers' and not sap suckers?