No bird soars too high if he soars with his own wings
No bird soars too high if he soars with his own wings. -William Blake
It is usual and customary these days to forewarn with disclaimers such as "the following material contains scenes of a disturbing and violent nature" and "discretion is advised." What typically follows are scenes of violence and its victims. Yet you will see no such cautions posted in "pet" stores or zoos, for one plain and simple reason: caged animals are socially acceptable and culturally normative. Screaming parrots, pacing tigers, swaying stereotypic elephants, and orcas with vacant eyes pressed to the glass aquarium wall  are not considered harmful to eyes and minds of children or others. Bars, glass, and other barriers behind which wildlife are interred are portrayed as only slight alterations of an animal's natural habitat and history. 
However, captivity is far from natural. The suite of maladies and premature deaths that haunt inmates of zoos and other captive institutions stand in stark contrast to the standards of health in free-living wildlife. Elephants who range across African savannahs and Asian jungles, and parrots who fly through the tree tops of verdant forests never exhibit the gaping, weeping wounds of self-mutilation that disfigure their captive counterparts. [3,4,5] Indeed, caged and confined birds—like POWs, concentration camp inmates, and vicitms of domestic violence—are all candidates for Complex Post-Traumatic Stress Disorder.
Psychiatrist and pioneer traumatologist Judith Herman created the category of Complex PTSD to bring attention to the profound effects that captivity imposes on the prisoner, because a PTSD diagnosis often fails to capture the severe psychological harm that occurs with prolonged, repeated trauma.  The US Department of Veteran Affairs describes victims of long term trauma as those who have been "held in a state of captivity, physically or emotionally. In these situations the victim is under the control of the perpetrator and unable to flee." They sustain "a prolonged period (months to years) of total control by another." 
Unremarkably, symptoms listed for humans suffering from chronic victimization commonly are found among caged parrots. Brain structures and processes governing emotion, cognition, stress regulation in birds and humans are comparable. Similar to humans "who [have] been abused repeatedly," parrot trauma survivors are misdiagnosed and blamed for the symptoms of their suffering and "mistaken as someone who has a ‘weak character'."  Parrot trauma is often dismissed as "bad" or "problem" behavior in need of "training" or punishment.
When examined through the lens of Complex PTSD, the symptoms of many caged parrots are almost indistinguishable from those of human POWs and concentration camp survivors. [7,8] These include alterations in emotional regulation, consciousness, and relationship:
Severely traumatized cockatoos who are rescued and receiving treatment at the innovative sanctuary, Midwest Avian Adoption & Rescue Services, Inc (MAARS)  commonly exhibit "rapid pacing in cage, distress calls, screams, self-mutilation, aggression in response to [human and other bird] physical contact, nightmares, insomnia, and self-mutilation."  Parrots are considered to be some of the most highly social species who bond for life and live in complex, closely knit flocks. However many rescued or abandoned parrots who come to sanctuary are so severely traumatized that they will not form relationships with humans or birds.
In a psychiatric study of male umbrella cockatoos (Cacatua alba) conducted at MAARS, the individual "B.B." was diagnosed with Complex PTSD:
He was captive-bred and was exposed to multiple caregivers who were themselves highly unstable (e.g., domestic violence, substance abuse). B.B. was passed to other family members and neighbors to care for and brought out at loud parties to perform. He appears to have some preference for humans, but generally speaking, is unable to successfully socialize with either humans or birds. For example, his relational overtures are conflictive and confusing. He will sing and "dance" as a way to get attention, but when a caregiver responds, his only response is sexual or highly aggressive (e.g., attacks, bites). He never shows affection to humans or other birds, and shows depression and lack of self-confidence and esteem (e.g., flat-crest, withdrawn, lack of affect). He will "fly into a rage" (e.g., scream incessantly, move erratically in his cage at the same time, and exhibit attacking behavior) if there is an unexpected noise or a stranger comes into the room. His moods and behavior are highly unpredictable.
Although this progressive treatment and care facility prescribed "a series of medications to attenuate his excessive reactivity (i.e., amitriptyline, clomipramine, Prozac), there were no significant results" and the prognosis was "poor."
Symptom 2. Alterations in consciousness. Includes having episodes in which one feels detached from one's mental processes or body.
Other cockatoos in recovery exhibit no response to social overtures, sit in the back of their cage day and night without moving, stare off into space in a trance and are difficult to "wake," and make loud distress calls and screams upon being left alone or removed from significant others. Another umbrella cockatoo in the psychiatric study, "T.C" (also diagnosed with Complex PTSD), was
depressed (e.g., did not groom, flat-crested, sat at bottom of cage, uninterested in enrichment projects, hypo-reactive). T.C. shows high levels of anxiety and depression, preferring to remain for the most part alone in his cage. Treatment has included prescribed psychotropic medications to aid in implementation of relational therapies; however, this has not yet proven successful so far. 
Symptom 3. Alterations in how the perpetrator is perceived and alterations in relations with others. Examples include isolation, distrust.
In captivity, a parrot's bond with humans is a double-edged sword. Humans are the source of food, water, and life itself, yet also an instrument of threat and death. When parrots are rescued and brought to sanctuary, they often have difficulty trusting their human caregivers. One case is Lola who was rescued and cared for by Marc Johnson and Karen Windsor of Foster Parrots, Ltd , another sophisticated sanctuary. Karen recounts a piece of his story:
Lola is an old, wild-caught, male Green winged macaw who was brought to Foster Parrots in 2002. He was missing an eye. He had broken bones in his feet and in his wings. He had no tail at all. A bald spot on the top of his head revealed a slightly concave skull fracture. These were all old injuries suffered at some point in his distant past, leaving him with a seizure disorder that would periodically grip, shake and paralyze him. It was speculated that he had been the victim of a dog attack. But the real tragedy lay in the fact that, subsequent to his injuries, he was delegated to a dog crate and kept in complete isolation in the basement of his guardian's home for several years. . .
After all of these years we've been together Lola still does not welcome petting. Only when a seizure has ripped through his body and left him partially paralyzed will he surrender to human love and comfort. I will lay him down on my chest and gently pet him and massage his stiff leg. He lays his head down and listens to my heartbeat. But when he's done with me, he's done, and then he rejects me completely. It's ok. For all the pain and all the loss this old bird has suffered at the hands of humans, I deserve to be rejected. At the same time I am also certain that he knows how much I care for him.
These are a few reasons why birds in cages do not sing. And if they do, we can only marvel at their love of life and perhaps hope for a future other than the ones we have condemned them to.
 The Orca Network. 2011. Retrieved October 16, 2011.
 Malamud, R. (1998). Reading zoos: Representation of animals and captivity. New York: NYU Press
 Bradshaw, G.A. 2007. Elephants in captivity: analysis of practice, policy, and the future. Society & Animals, 1-48.
 Bradshaw, G.A., Capaldo, T, Lindner, L & G. Grow. 2008. Building an inner sanctuary: trauma-induced symptoms in non-human great apes. Journal of Trauma and Dissociation. 9(1), 9-34.
 Clubb, R. et al. 2008. Compromised survivorship in zoo elephants. Science, 322 (5908), 1649.
 Herman, J.L. 1992. Complex PTSD: A syndrome in survivors of prolonged and repeated
trauma. Journal of Traumatic Stress, 5, 377-391.
 Whealin, J. & Slone, L. (2001). Complex PTSD. U.S. Department of Veteran Affairs. Retrieved October 16, 2011,
 Bradshaw, G.A., J. Yenkosky, & E. McCarthy. 2009. Avian affective dysregulation: Psychiatric models and treatment for parrots in captivity. Proceedings of the Association of Avian Veterinarians. 28th Annual Conference, Minnesota.
 MAARS, Midwest Avian Adoption & Rescue Service, Inc.
 Foster Parrots, Ltd. 2011.
 Windsor, K. 2011. Interview, The Kerulos Center. Trans-species Living Insights, Retrieved October 16, 2011.