Woodpeckers are a symptom, not a cause, of your tree problems. Think of the woodpecker as stage three in a process of decline, suggests forest pathologist John McLaughlin of the Ontario Forest Research Institute in Sault Ste. Marie. The birds are going after insects that have infested the tree, laying their eggs in galleries under the bark so their larvae can eat the tree's cambium layer. But the insects are only stage two. Stage one is an unhealthy tree, which attracts opportunistic insects in the first place. Odds are the tree has been deteriorating for some years.

Woodpeckers, such as pileateds, who can make impressions up to a metre long, also excavate cavities for nesting. When creating a new home, they usually exploit softer parts of trees that already have heart rot or other areas of decay.

Should you remove a woodpecked tree? Only if it's so damaged structurally that it's a hazard to buildings, power lines, or people, says ornithologist Mark Peck of the Royal Ontario Museum in Toronto. After the woodpeckers have fed and nested there, the tree can be habitat for other cavity dwellers, including small owls, tree swallows, flycatchers, and mammals such as flying squirrels. And even after it falls, says Steven Mann of Bartlett Tree Experts in Bracebridge, a tree is still a vital part of the ecosystem, providing habitat and soil nutrients at every stage of its decomposition.

If your trees really are a hazard, Mann and Peck suggest removing only the parts that could cause problems, such as large limbs or a high top. Leave them on the ground to decay, and leave the snag (the part that's left standing) for the birds and mammals.