I think your problem is possums not birds. Hardware stores like Bunnings and Masters and some nurseries sell resin owls and owls with rotating heads and flashing eyes that will deter possums at night. However, if you are certain that birds are the problem then try the $2 shops or The Reject Shop to buy large novelty birds on a stick that will rotate e.g Hawk or Sulphur Crested Cockatoo. Alternatively, try a toy shop and buy plastic hand held windmills that you can attach to a long stick and place around your roses. Whatever you manage to buy it must be able to rotate in the wind day and night. Remember roses are possums favourite food. I hope I have been of some help. Good luck.