My father has a decent size Silver birch in his back yard, and he's remodeling, so i want to grab part of the birch to turn into a bonsai... that and my wife says im getting to many bonsai's, so i figure my get out clause is if i get one for free!

I want to air layer it, but im worried its too late in the year to get that started? Im in Melbourne, so we have a couple of months of reasonable weather yet before tree's will start dropping their leaves and winter really kicks in.. i think sometimes they hang on to closer to may? But im just thinking, the growth pattern of the tree's is it to late to try? How many months will it need to sit for before it gets some roots?
Also as an unrelated question, can someone post me an image of a haunted forest bonsai formation? Was reading about it the other day, but googling it, it doesnt seem to be a common term.
Cheers!!