Some of us have birdbaths in our garden with a large plastic receptacle fashioned from a plant-pot saucer to hold the water.
Many birds use it each day, but it is common to have algae build up very quickly on the surface covered by the water.
This can happen even if the plastic receptacle was bought new from the shop and the water put in it is good drinking water. The birds bathe in it and drink the water without any obvious ill effects.
Where do the algae come from, and how do they survive and grow?
Freshwater algae are well adapted to distribute and establish themselves in new, often temporary, ponds and puddles.
Most green alga species can make tough-walled resting spores when conditions get difficult, usually when their habitat dries up. These dry resting spores can survive a long time and are small enough to be picked up and carried by the wind or in mud that has dried onto the feet of birds.
Any newly filled birdbath will very rapidly be colonized, with the spores revitalizing as soon as they enter water.
But the algae also need nutrients to grow, and wouldn’t survive well in distilled water.
Ordinary drinkable tap water, processed to be free of toxins and microorganisms, will still contain quite high concentrations of plant nutrients such as nitrates and phosphates. Other sources include dust carried in the air and, of course, general muck on the bodies of the birds bathing in the bath.
For these reasons, repeatedly topping up a garden pond with tap water usually leads to the unsightly green algal overgrowth your correspondent has reported.
So it is absolutely normal for algae to grow in birdbaths, which are just another pond habitat for them. The good news is that the algae are harmless and won’t worry the birds.
Given light and water containing dissolved nutrients, a microscopic amount of algae or their spores will grow to a visible film before too long.
But where does the microscopic amount of algae come from to start with?
It might have been on the receptacle, which was not sterilized, or in the water, which also does not have to be sterile to be drinkable. Or it could have come from the surrounding environment, either airborne or carried in by birds.
Those who work in offices that have watercoolers equipped with large, clear plastic bottles will know that drinking water in a plastic container closed to the environment can grow algae if it is left in sunlight for too long.
In fact, many of us have probably drunk a glass of such water before we noticed. Although it tastes slightly different, you will likely suffer no obvious ill effects.
There are many commercial products available to control pond and pool algae, and most are effective. Just do your homework before running out and buying something blindly.