we have a very clever robin in our garden

[hall7274]

It stands watch for our cat Gary, whenever gary is in the garden the bird will make a certain call and actually follow it around keeping the cat insight, if gary goes next door the bird will also follow, i'm not sure what other birds it's warning, as gary has attacked various birds over the years, i have visions that the birds have now ganged up and watching each other's backs, the bird is so timely, that i know when gary is on his way back from the garden travels, our garden is very quite when gary isn't around!! How amazing are these birds.

[Wrenna]

Robins are extremely territorial, and the behaviour you mentioned is typical of a robin trying to defend it's territory from another bird!!
How comical for it to be trying it's luck with a cat though!