A large enough and hungry enough praying mantis can and will catch and eat a hummingbird. The non-native Chinese Mantis that is commonly sold in garden centers is the most likely culprit, but the smaller native ones such as the Arizona Bordered Mantis can also catch a hummingbird. They commonly hang out on hummingbird feeders because some of their favorite meals such as bees and wasps are attracted to sugary solutions. If you want to protect your hummingbirds, let the mantid walk onto a stick (or your hand if you're not afraid of them) and relocate them to an area far from your feeders.
Cats are an invasive species and they are superpredators. It is estimated that both outdoor domestic and feral cats kill billions (yes billions) of birds yearly in the United States alone.
Hummingbirds feed repeatedly at the same food source, so a cat can easily predict where the bird will be. One swipe of the cat's paw can kill a hummingbird where a larger bird might have escaped. Even if it escapes, a minor scratch from a cat can transfer a lethal bacteria that will end up killing the bird by infection in a matter of days.
Climate change is affecting the routes of species that migrate, so hummingbirds are being found in locations far from their normal range and their usual food source. Hummingbirds remember past food sources, so finding new ones in unfamiliar territories might be difficult. They may also be unable to find mates.
Photo by Oscar Palmquist
Many people love the thought of hummingbirds flitting around their house so they buy a hummingbird feeder. They fill it with nectar (often red and bought with the feeder), hang it and forget about it.
Hanging a hummingbird feeder is a responsibility. Spoiled nectar filled with bacteria is dangerous to them. They won't continue to drink it if it is spoiled, but an uninitiated hummingbird will try it assuming it is good. Black mold grows easily inside the feeders and can give the bird a fungal infection that swells their tongue making them unable to eat. A mother will also pass this infection onto her chicks.
Hummingbird territories are rather small, so even minor development can have a large impact on the species. Urbanization, alteration of the landscape and habitat loss due to land clearing for agriculture (some of it illegal) and illegal logging continues throughout the Americas. In 2013 it is estimated that 80% of the timber exported from Peru was illegally obtained. This is going on all over Central and South America, where most hummingbird species reside.
Humans walk into glass doors all the time. If we are seen, we usually are embarrassed and laugh it off. When a bird collides with glass at high speed, they will either be killed outright or die later of their injuries.
The urban environment is filled with buildings and other structures made with reflective or nearly invisible surfaces. They are pleasing to our eye, but these structures kill nearly a billion birds per year.