But occasionally there can be a connection. One possibility is that the birds can be hosts for a microorganism that causes a disease in humans. Another possibility is that when humans make changes to an environment, they can create favorable conditions for parasites to enter new areas. Once they are introduced to the area they can invade a previously 'healthy' bird population significantly increase the death rate. This is especially true on islands like Hawaii where there has been a decline of many of the native Hawaiian birds.
Birds can be an intermediate vector.
This means that while the bird might carry the disease-causing microorganism, in order to infect a human there must be another carrier first. If a mosquito bites an infected bird and the parasite is transmitted through the blood to the mosquito, it will then become a host organism itself. If the same mosquito then bites a human, it can pass the disease-causing microorganism to the human. Falling victim to predators is a common cause of death, especially for old and young birds that make easy prey. Extreme weather conditions, such as drought and storms, can also cause many deaths.