Answer: Birds tend to peck holes while looking for insects. They seem to know when a tree is distressed and will explore the bark while looking for insects. Woodpeckers can do damage, as can sap suckers. Both make patterns of holes in the bark. Generally they won't damage a tree enough to kill it, though. It's usually the other way around - they will frequent a tree that is already in trouble.
Willows can develop Bacterial Twig Blight which causes blighted twigs and branches to die back, and leaves to turn brown and wilt. This bacterium overwinters in twig and branches cankers and you can find brown streaks in sections of affected wood. Trees can be completely defoliated by this disease. If practical, you can cut out diseased twigs and branches. A bordeaux (lime-sulfur) spray applied in the fall may help to control this problem.
Hope this information is helpful!
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