Here is the basic mask setup for your tree. One thing I noticed when I add a Mask to the Multi-Pass, it also adds an Alpha too. The Alpha, whether this one or if I add one manually, always includes all the objects as there are no settings. If you look at my images, notice the Alpha mask for the tree looks a lot smoother and more detailed. I have no idea why that is.
Also, the small hill the tree sits on may also need a mask if the birds start that low. If it's a separate object, also drag it to the Mask list. Worst case you need to cut it off from the background mountain.