Birders

You might try trimming the bottom of the bush so you can see in there. Then keep an eye on the bush for the cat hanging around.

Get a spray bottle and mix some cayenne pepper mixed in. Keep an eye out for the cat and spray its face with the spray bottle. Adjust the spray to a stream that way you get better distance and aim.

After you get the cat a few times with the spray it might get the hint and find someplace else to bird hunt. Be vigilant in case the cat is persistent as cats can be.

The cat knows the birds are nesting and hopes to catch the birds off guard. When the eggs hatch keep an extra eye out for them because little birds lack the defense skills of an adult bird so they are pretty much sitting ducks for the cat to prey on.

You might have to cover the handrail with a tarp and tie it on there to keep access to the bush to a minimum for the cat.

Keep us posted.

A bell collar on the cat might help alert the birds the cat is lurking around. But the owner might remove it. If it was your cat it might be worth a try.

How about little bells in the bush around the proximity of the nest ?

Out doors cats are the biggest danger and killers to nesting song birds.