We've had discussions here locally with two representatives of the US Humane Society (at a national level, not local.) One was their national Urban Wildlife Director. With the USHS we are exploring long term, humane methods of curtailing the growth of herds so it does not exceed the shrinking amount of grazing land available to them.
The details are complex and the costs high, but there is some possibility of getting something done to try to dramatically slow the growth of the herds. It is a long term project of major proportions.
Meanwhile, the USHS has recommended that after donkeys are lured to the corral by gentle means, the males be gelded before being released. (Previously, when donkeys were removed from the Hualalai resort area and moved to Waikoloa, they were not gelded.)
All of this has also been discussed with a large-animal vet here on the Island, so someone who is knowledgeable about equine matters is providing lots of expertise.
I'll revisit this subject in a future post.