When I did my course to become a parrot behavior consultant, I had to trace the journey of bird keeping up until the present time. It was fascinating to learn that landed gentry kept large parrots in cages at the door way to their homes to alert the occupants of the house that there were guests arriving.
My birds Sam and Sprog do much the same thing as soon as my gate starts opening I am aware that my husband or a guest is coming in. My husband gets the biggest welcome or did until my daughter arrived. Sam and Sprog have got it so taped that as soon as a car arrives chaos breaks out on my veranda. It is very handy when I am in anther part of the house, ooh but if I am on the veranda it is terribly terribly loud. My bird which is Sprog, screeches so loud that in the beggining we thought screech would be a good name, but I am glad we did not name him that. He is the baby of the flock so Sprog is more appropriate, or thats how I see it.
Noise is one of the things that you must take into consideration when buying a parrot. If I was living in a semi detached house or flat the noise would be too loud for my neighbours. So think carefully when purchasing a bird. Do some research on the web about the noisier parrots.
Are your birds your early warning systems ?