Last week, my wife and I were walking through the eastern Iowa woods and
noticed a few small trees with lustrous red berries.
What function do the poisonous berries serve?

What makes you think they are poisonous? Maybe they are Viburnum, which the
birds will eat later in the winter. I am still trying to figure out the purpose
of Callicarpa berries, which were apparently one of those random mutations that
people love to puzzle over.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"The trouble with people is not that they don't know but that they know so much
that ain't so."
Josh Billings (Henry Wheeler Shaw), 1818-1885