Flies are nasty little things. They don't take long to cause problems on a pasty vent in Spring, Summer, or Fall. The area gets irritated, the bird picks at it, a wound develops, a fly lays eggs, and maggots will burrow in there. I keep an eye on vents and trim feathers on some of the more bushy breeds. I check the birds' vents while they roost at night to make it easier. One time I had a hen with a infested wound, sprayed the wound with Prozap screw worm aerosol, and maggots poured out of that wound quick. I then cleaned the wound with betadine, and sprayed some permethrin around the the vent to prevent further problems.The bird healed fine and lived a healthy life. That was called learning the hard way. I also hang a few fly traps in strategic areas outside to cut down the population.