All wildlife faces hazards to survival, including weather, humans, pesticides, and so on. For tiny hummingbirds, however, predators are a significant source of risk.
Cats, both domestic and feral, are probably the most common predators of non-nested hummingbirds. Reports to the Society come in regularly, for example, of people whose cats carry a hummingbird in their mouth--sometimes dead, sometimes not. Reports of tailless hummingbirds are less likely to indicate a hummingbird in molt than one that narrowly escaped being caught by a cat. In the case of one species, the Critically Endangered Juan Fernandez Firecrown (Chile), cats are actually an important reason for the bird's being close to extinction.
To reduce the risks from cats, feeders should be hung high: at least five feet (1.2 m.) above ground, and preferably positioned so that a stalking cat will be quite visible. If you have a cat and love hummingbirds--or any other birds--keep the cat inside!
Other birds, such as hawks, have been documented catching hummingbirds. Considering the differences in size, one wonders just how much nourishment a tiny hummingbird (typically only 3-4 grams, or 0.1 ounce) can provide to a large bird, but it happens nonetheless. In contrast to this, many people have observed hummingbirds pursuing or confronting a hawk, most probably in defense of a nest. This fearless behavior against overwhelming odds is but one of the reasons that hummingbirds evoke our admiration.
In the nest, eggs and chicks represent an easy target for a variety of predators: other birds, such as blue jays and crows in the U.S., and snakes, particularly in the tropics.
Hummingbirds share some of the same predators as birds of greater size. Their very small size, however, causes them to have more--and very unusual--predators.
Praying Mantis: Most people are surprised to learn that a praying mantis will successfully capture, kill, and eat a hummingbird. Typically the insect will position itself on a plant or a hummingbird feeder to which it observes a hummingbird coming repeatedly. Its lightning-fast strike often assures it of success. Because of the relative size difference, it make take over a day for the bird to be consumed. While praying mantises are very beneficial insects in a garden, they should not be allowed on hummingbird feeders.
Spiders: Hummingbirds use spider webs as a source of spider's silk in nest construction, being necessary to bind the nest to the tree branch or other substrate and to hold the nest together. Even so, the hummingbird must be careful when removing the pieces of webbing, for it may become entangled and be trapped there. Spider's silk has a tensile strength comparable to steel on a weight basis. In one report, a Ruby-throated Hummingbird was caught in an active web, and quickly wrapped and encased by the spider, much as an insect might be.
Other insects: Bees and wasps can sometimes out-maneuver and attack a hummingbird. A single sting may be fatal to a hummingbird, because there is so little body mass to absorb the venom, but there are few data to know the extent of this. However, bees are very important pollinators, so it is important to support them. Just keep your feeders clean and use bee guards if necessary to keep your feeders bee-free.
Frogs can capture a hummingbird, although the result is probably death to both. One image in the files of the Hummingbird Society shows a frog with a half-swallowed Anna's Hummingbird, which almost certainly had tragic results. There is at least one report in the literature of a fish (bass) capturing a hummingbird.