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When your sugar mixture turns cloudy, it needs to be replaced. Yeasts eat sugar, causing fermentation that could potentially harm a hummingbird.  A warm, sugary mixture is also a great place for mold and bacteria to grow. If possible, check your feeder every other day. Keeping an eye on your feeder will keep any harm from coming to the hummingbirds. If you find mold, mix ¼ cup of bleach in a gallon of water. Soak the feeder for an hour in this bleach mixture. Scrub any mold off and then rinse the feeder thoroughly before refilling it. Flush the feeder with hot tap water. Do not use soap--hummingbirds do not like the taste that soap leaves behind and will avoid your feeder if it has soap residue. Be aware that the amount of time you can leave hummingbird food outside depends on the temperature in which the feeder is hung in.  If temperatures are 70 to 80 degrees Fahrenheit (21 to 26 C), change the food every 5 to 6 days. When temperatures are 81 to 90 degrees Fahrenheit (27 to 30 C), change the food every 2 to 4 days. If the thermometer tops 91 degrees Fahrenheit (32 C), change the food daily.
Know that your food can cause harm if left to ferment or mold. Check your feeder for black mold as often as you can. Clean your feeder before your refill it. Change the food in the feeders regularly.