Birds get stressed too!
Stress can be defined as the physiological response of a bird to a situation that causes it strain or tension, such as overcrowding, too high or too low temperature and/or humidity, and the inability to hide from real or imagined threats.
There are two types of stress - physical and psychological. Physical stress can be principally divided again into two types: emergency stress, such as a situation that poses an immediate threat, and continuing stress such as overcrowding in a cage. Stress can be beneficial as it keeps the bird alert and allows it to react more competently to an emergency.
A bird reacts to emergency stress, such as you trying to catch it, by releasing a large amount of adrenaline into the blood stream. This has the effect of raising the blood pressure, emptying sugar supplies into the blood stream and dilating the blood vessels in the muscles to give them immediate use of this energy - such as flying away.
If stress is overwhelming, such as in the case of an uncontrollable infectious disease, exhaustion of the adrenal gland sets in, usually with fatal results.
A review of the most common cause of death in birds in one zoological collection showed that most of the disease factors were of relatively low virulence indicating the pre-disposing factors such as stress added to the circumstances that allowed these opportunists to cause death.