Last week, my wife and I were walking through the eastern Iowa woods
and noticed a few small trees with lustrous red berries. They were
beginning to wither, and most of the other deciduous trees had dropped
their leaves. This got me to thinking:

What function do the poisonous berries serve? I would assume there's
some animal that can digest them and disseminate the seed, but then
why were the berries withering on the vine? Is this a sign of a
missing bird species (e.g., a bird species fallen prey to the West
Nile virus)? Against that idea, I seem to recall often seeing such
bushes full of untouched fruit. What gives?

Thanks in advance,
A nonbotanist dilettante