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.
Pollution is also responsible for many bird deaths.
Interestingly it is not uncommon to find dead birds around the city of Philadelphia during the spring and fall migrations. City lights and buildings disorient many migrating birds. Many of these confused birds will die and can be found on city streets. Only yesterday for example, I picked up a dead Brown Creeper that I found while walking to work on a street near the Academy. Birds such as that creeper are actually a very important source of specimens for our collections.