I understand from various sources on the internet that, if left out too long, sugar water will grow mold and bacteria that are harmful to the birds. What I discovered recently is that this can happen really quickly in hot weather like we are having now. I'm embarrassed to say that my hummingbird feeder, which had probably been rinsed out and refilled about a week ago, had visible signs of mold. I found a brush and made a solution of bleach water to clean it. I used a toothpick to clean the little tubes inside the "bloom" where the hummingbirds feed. Then I rinsed it multiple times to rid it of all traces of the bleach. The entire process took quite a long time.
Most sources that I read recommended changing the water daily if the temperature is above 90 degrees. This article suggests that small feeders are better since the sugar water should be changed often anyway. It also says several small feeders will attract more birds than will a single, larger one with multiple perches. I wish I had read this before I bought that big new feeder this spring!
My theory about why hummingbirds are so late coming to our garden is that they are awaiting the bloom of the cardinal flower, which should be any day now. Although they do visit the black and blue salvia outside my kitchen window, they seem to have a strong preference for cardinal flowers. It seems to make sense to me that the actual nectar of either flower would be better for them than sugar water.
I have to admit that I have been neglectful of my hummingbird feeder. I had not realized how often it needed to be cleaned or how much labor is involved in cleaning it. So I'll keep sugar water in the feeder until our cardinal flowers make their appearance, then I'll take the feeder down. I'm afraid I'm not vigilant enough to keep the sugar water fresh, and I certainly don't want to harm the little birds I'm trying to help.