Just like humans, hummingbirds can have illesses or injures.  Have you ever stopped to consider what types of things these might be?  Well, this post will give you answer. 

• Window strikes:  This can either stun or even kill the hummingbird.
• Cat caught hummingbirds:  The cat is the number one predator of the hummingbird, and therfore this is quite common and can either result in the hummingbird being injurd or killed.
• Sick hummingbirds:  Common hummingbird illnesses include fungal infections and the avian pox virus. 
• Trapped hummingbird:  You will want to tire the hummingbird by keeping it flying as much as possible, because this eventually make it easier for you to catch the bird and then release it.  Be sure to feed the bird prior to releasing it or the bird may not have enough energy to search for food.  You do not want to release a trapped hummingbird at night.   Unfortunately, I can not offer a reason for this, but read it from several sources.