i have seen some postings ob the duck threads of ducks eating metals and getting botulism type symptomsBased on a quick internet search, I believe you have been misinformed by a person who should know better.
Botulism: Causes, Symptoms... states
"Botulism (or botulism poisoning) is a rare but very serious illness that transmits through food, contact with contaminated soil, or through an open wound. Without early treatment, botulism can lead to paralysis, breathing difficulties, and death."
The vast majority of botulism information available is related to humans and likely very limited knowledge about it as related to poultry. If botulism can be contracted through contaminate soil, I see no reason that dirt scratching/eating poultry would not be able to contract botulism without eating grubs.
Unfortunately this particular site does not give any information on symptoms that would be of value when trying to diagnose botulism in poultry.
Overview of Botulism in Poultry referred to as limberneck or Western Duck Sickness and is definitely not confined to contract by eating maggots.