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This search makes me so sad, since chances are slim that these deaths are from natural causes. The leading suspects would be window collisions and pesticides, but a disease such as West Nile virus could also be involved.
If you find dead hummingbirds near windows or glass doors, they probably died from colliding with the glass while trying to escape from a predator or an aggressor. To prevent future deaths, you’ll need to reduce the mirror-like quality of the glass. This page from the Fatal Light Awareness Program busts many myths while providing a variety of options for reducing or eliminating collisions:
The American Bird Conservancy has just released a new product to apply to windows to save birds’ lives: BirdTape.
Pesticides are major killers of birds, and not just on farms. If you use chemical pesticides in your yard and garden, stop. Switch to low-toxicity alternatives such as insecticidal soaps, diatomaceous earth, pheromone traps, and biological controls. Don’t put leftover pesticides in your household trash; call your city or county for instructions on proper disposal. Lawn-care services are major offenders, applying harsh pesticides even when there isn’t a problem. If your current lawn-care service uses chemical pesticides, switch to a “greener” one that uses low-toxicity alternatives. If you suspect poisons might be drifting in from a neighbor’s yard, explain to them that anything that can kill a hummingbird can hurt people and pets, too. If your city or county sprays for mosquitoes, call the agency and ask what pesticide they’re using and whether it’s known to be toxic to birds and other wildlife.
To prevent diseases, clean and refill your hummingbird feeders every 2 to 4 days (more often in warmer weather), and disinfect water features weekly with a 10% solution of chlorine bleach. Search your yard for standing water that might be breeding mosquitoes. If you can’t eliminate the source, add a product containing Bacillis thuringensis israelensis or Bti, a naturally-occurring pathogen that kills mosquito larvae.
If all else fails, report the problem to a health and/or environmental quality agency at the city, county, or state level; the agency may want to test the carcasses for various contaminants and diseases. If the agency seems disinterested in the problem, remind them that birds can alert us to problems before they affect humans, and that they are protected under federal law (the Migratory Bird Treaty Act).