When I first put together the water barrel fountain, I called the Santa Clara Vector Control District for help. At that time, they still had staff that came to your house and did a backyard inspection, and they gave me some good advice and left some mosquito fish. Now, I'm sorry to say, they no longer come though they have a lot of information on their website.
I then bought a 50 cent gold fish and added it to the water barrel, and while the mosquito fish seem to come and go (maybe they go to the bottom in winter), I can often see the gold fish cruising somewhere when I walk past the barrel. Still, I was surprised to find it really close to the surface a few days ago, Looked almost as if it wanted a breath of fresh air.
Very odd, I thought. And I didn't completely understand what was going on until I came to the sandy bird bath and saw small wriggly black creatures in there. I had assumed that because the birds stir up the water frequently, mosquito larvae would not be a problem. But I realized that many of the birds had flown back home, and others had found other pleasant places to bathe. And I suspected the fish might also have been eating the larvae that accumulated at the top of the barrel (or maybe not, but it's a cool photo, isn't it?).
So I went to the shed and got my shaker of Mosquito Bits. I actually don't even remember where I bought them, but do remember that the vector control guys told me not to use the donut-shaped mosquito dunks, which were not very effective. They endorsed the bits as effective and non-toxic to birds and other creatures.
It took just one shake, and about an hour, for the problem to be solved. I was greatly relieved. On the one hand, I like to offer bugs to the birds who visit my garden. But with West Nile disease and some other mosquito-based illnesses prevalent around here, this seemed the prudent way to go. And, also kind: Birds get West Nile before humans do.
So, dear friends, take stroll around your garden and check for mosquito breeding areas. It's good for you, and for the birds.