Why the birdbath turns red

It happens every summer, at least at our house.  The water in the bird bath starts to turn red, obviously contaminated by some microorganism carried in by the birds.  It takes a good soaking in bleach to get it clean, and has to be repeated several times.  Applying my C+onthefinalexam knowledge of microbiology, I assumed the culprit was Pseudomonas aeruginosa, but this week I stumbled across an article about Hematococcus:

This member of the Volvocales is usually encountered by people who have the good sense to install birdbaths in their gardens. Very often the first indication that something strange is happening is that the water begins to change to an orange-red colour. If you're lucky enough to have a microscope at hand a strange sight awaits you. The culprit for this unusual coloration is a micro-organism called Haematococcus pluvialis. The red colour is due to the pigment called astaxanthin, which possibly protects the organism from the harsh sunlight, especially the ultraviolet rays from the Sun...

It has been my experience that whenever these micro-organisms are found they nearly always appear to be in a state of encystment at the bottom of the bird bath. Only when the cell is placed under the cover slip does it appear to come to life. It is remarkable that this organism can withstand extremes of climate, many bird baths completely dry up for long periods of time... Eric Hollowday wrote a small piece stating that there are recorded instances of these animals being kept dry for up to 7 years in the laboratory