Birds are unique in the way they respond to and exhibit their diseases. You, as a bird owner, must appreciate this fact if you are to recognize illness in your birds and respond promptly to their needs.
All animals have limited ability to physiologically compensate for one or more medical problems, involving one or more organ systems. When the animal can no longer compensate, obvious signs of illness are expressed. The term for the period during which an animal is sick but does not show obvious signs of disease is sub-clinical illness. Caged birds seem to have an even greater compensatory ability, and sub-clinical illness is common in birds. Birds, in fact, appear to "hide" signs of illness. This is actually a preservation response, rather than a deliberate or conscious act.
This preservation response is, no doubt, an evolutionary adaptation. Many birds are flock dwellers, with social hierarchies ("pecking orders"). Any individual struggling to maintain its position in this hierarchy tends to be eliminated from the group. This tendency for members of the same species to harass weaker members of the group eliminates the weaker members' genes from the gene pool, thereby strengthening the species' chances for survival. In spite of the benefits for the group, it behooves the individual to look healthy, for as long as possible, to avoid this harassment.
Animal behaviorists believe that the evolutionary establishment of communal groups (flocks, herds, etc) resulted from the increased survival of individuals (more eyes and ears to detect predators and other life-threatening situations). Predation often eliminates the weakest members (very young, old or sick) of the group. Again, it behooves the individual to stay as healthy as possible, for as long as possible, to avoid attracting predators to the group and predation on itself.
Sub-clinical illness in caged birds makes diagnosis of disease difficult. Consequently, the avian veterinarian must rely heavily on diagnostic laboratories to provide tests that can aid in the diagnosis and treatment of disease, as well as help monitor the clinical course of the avian patient. Bacterial and fungal diseases can be diagnosed in the microbiology laboratory, and the proper and most effective antimicrobial drugs (antibiotics, etc) to use in treating these diseases can be determined.
Samples for certain special tests must be submitted to other laboratories for analyses. Diagnosis of lead poisoning (a common intoxication of caged birds) requires a specific analysis of the patient's blood. Diagnosis of chlamydiosis ("parrot fever") requires submission of feces (or cloacal swabs) or tissue samples to the State laboratory for very specific testing procedures. Blood samples can be submitted to other laboratories for serologic analyses (detection of antibodies to certain disease agents, such as chlamydiosis). Viral diseases are difficult to diagnose in caged birds. Virus isolation and identification require submission of specific tissues or body fluids, secretions or excretions, and must be performed by State laboratories, or laboratories located within veterinary colleges. These tests are time-consuming and costly.