Why do parrots squawk so loudly in a cage?

From Konrad Lorenz's King Solomon's Ring (pgs. 72-3):

But of all animals that suffer under the inefficient methods of many zoological gardens, by far the most unfortunate are those mentally alert creatures of whom we have spoken above. These, however, rarely awaken the pity of the zoo visitor, least of all when such an originally highly intelligent animal has deteriorated, under the influence of close confinement, into a crazy idiot, a very caricature of its former self. I have never heard an exclamation of sympathy from the onlookers in the parrot house. Sentimental old ladies, the fanatical sponsors of the Societies for Prevention of Cruelty to Animals, have no compunction in keeping a grey parrot or cockatoo in a relatively small cage or even chained to a perch. Now these larger species of the parrot tribe are not only clever but mentally and bodily uncommonly vivacious; and, together with the large corvines, they are probably the only birds which can suffer from that state of mind, common to human prisoners, namely boredom. But nobody pities these pathetic creatures in their bell-shaped cages of martyrdom. Uncomprehendingly, the fond owner imagines that the bird is bowing, when it constantly repeats the bobbing head movements which, in reality, are the stereotyped remnants of its desperate attempts to escape from its cage. Free such an unhappy prisoner, and it will take weeks, even months, before it really dares to fly.