Happens somewhere this time of the year in NZ.
Our local Acclimatization Society Club had a F&G field officer answer feedback on this topic.
Apparently when the bird dies and the carcass gets fly blown,it is the maggots that become highly toxic to other birds.
Some ridiculous figure like 1 maggot has enough toxicity to kill 5 mallards.
So if you see a dying or dead gamebird on your pond/land/park the bird has to be gathered up and burned/buried asap.
As a huge number of maggots (100s) can be laid on a dead gamebird.
And the birds find the maggots attractive to eat.
Of course a infected bird can fly away from the initial area of contamination.
So one needs to be alert to the early symptoms in birds and deal to the issue then.
Only about 8 weeks to opening W/E already.
Hope this helps educate  those concerned to protect our hunting resources.
Cheerz