Only because we live in a rural setting, and have all sorts of different situations than urban dwellers do I say “FWIW” here’s how I handle rats in trees: Rats in trees mean to me big nests consisting of trash, and twigs. I blast them out of the trees with a 3000 psi high pressure washer. Takes about 8-10 minutes of blasting from different angles to get the nests out of the trees, AND DOES NO HARM WHATSOVER TO THE BARK OF THE TREES AS SUGGESTED BY SOMEONE ON THE GARDENING THREAD A COUPLE OF YEARS AGO.
No nests, no rats. Last time I blasted the little b-stards nests was several years ago, and they never returned. I had three nests in three different trees around the property.
So if applicable to your situation OK, if not.....well good luck. I would believe the air guns would be only worth it if the rats make themselves available as targets often enough that you wouldn’t be wasting your time away from FREE REPUBLIC for hours waiting for them to appear so you could blast away at their worthless little carcasses.
Tree Rats = Squirrels, not actual rats...