Here's what they might do:
* Dive bombing: They will swoop down repeatedly, flying close to the perceived threat, making a loud, distinctive "chirp" or "chitter" sound.
* Distraction displays: They might feign injury to lure a predator away from the nest.
* Vocalization: They will sing loudly and persistently, alerting other birds and potentially scaring the predator away.
While they are likely to scare off a cat, it's important to note that:
* Cats are predators and may still attack a nest even if the robin is trying to deter them.
* Young robins are vulnerable and can be easy prey for cats.
If you see a cat near a robin's nest, the best thing to do is to try and scare it away, or if possible, keep the cat away from the area entirely.