Outdoor cats will often pursue birds in trees, especially vulnerable baby birds. Fortunately, there are devices and barriers that can be placed around trees to prevent cats from climbing up after birds. You may have to experiment with a number of different devices or barriers before you find the one that is most effective as a deterrent for your cat.
Position a barrier such as aluminium sheeting or chicken wire all the way around the trunk of a tree to prevent a cat from being able to climb up. There is also a product known as Cat Deterrent that is a metal-spiked belt. It comes in two different sizes and fits around the diameter of a tree.
A product known as Catwatch that is sold online claims to keep cats out of certain outdoor areas. It is an electronic device that is activated by a cat's movement and body heat. A cat going near the device will set off an ultrasound alarm that will repel the cat from that area. The alarm stops as soon as the cat leaves the area, so put it by the tree that has the baby birds.
Spray the Tree
There is a spray product that mimics a cat's facial/cheek pheromones. If you spray the tree trunk with this product, it is supposed to calm the cat and deter any aggressive behaviour such as pursuing baby birds. The product is also available in the form of an electric diffuser.
Restrict Access to the Tree
Restrict access to the tree that has the baby birds by keeping your cat indoors at least until the baby birds have left the nest. Another option is to allow your cat to go outdoors but keep them on a leash attached to a fence or clothesline that will not permit them to get close to the tree. If all else fails, you could have your cat declawed, however, that would leave it defenceless against neighbourhood dogs.