Open terrarium and position birds inside. When you are happy with their placement, remove each one and attach a small amount of putty to the bottom. Return birds to their positions and press firmly in place.
Position a few trees of varying sizes around the birds. When satisfied with the composition, apply a small amount of putty to the bottom of each one and press firmly in place.
If desired, scatter a small amount of artificial snow around trees and birds for a wintery effect.