In the previous post, I included an image of a really interesting Greyhound with white dots on his coat. In horses, small white dots on a colored background are often called Birdcatcher Spots. They are not usually as large or as abundant as the spots on that particular dog, which is probably why they go unmentioned in most registry applications. The horse above is a good example of this kind of spotting. You might want to click on the image to get the larger version, and even then the small spots on his hindquarters, barrel and neck are easy to miss if you are not looking for them.
Here is a close-up of some similar spots. Both the horse above and this one are Saddlebreds, where the trait is not uncommon.
To my knowledge, there have not been formal studies on Birdcatcher Spots. My own experience has been that they are more often seen on chestnuts than any of the black-based colors. (A reader did share an image of a dark bay with very prominent white spots on our Facebook page back in late December.) The spots also seem to occur more often in the “thin-skinned” breeds with finer coats – breeds like Thoroughbreds, Arabians and Saddlebreds. Some have noted that horses with these flecks are more prone to getting white hair growing back over skin abrasions. The horse above did have what appeared to be a few minor abrasions that were growing white hair. A similar tendency might also explain why some roans become covered in dark lines and specks, since injuries on roans tend to grow back with dark hair rather than white.
That does not seem to be the explanation for all cases, however. Here is another Saddlebred from the same show, with unusual white spots on her face. One is just visible on her right ear in this image, and another just behind the browband. A third can be seen in front of the cavesson, and there was another on the left tear bone (not visible here). When asked, her owner said she had always had odd spots like this. Other than her broken blaze, she was a seal brown horse with no other white markings. (She was a very striking horse with a slightly baroque head shape that reminded me of some of the older images of carriage horses I saw while writing my book, and I would have happily taken her home with me!)
Here is another instance of random white spots, this time on a tobiano.
Although this fellow had roan hairs around the edges of his pattern, a bit like the horse at the top of this post, and he did have the kind of blaze that might suggest that one of the sabino patterns was present, these were the only round spots like this on his coat so they seemed almost out of place there.
It seems likely that there are multiple causes, either environmental or genetic or some combination, that cause white spots. Reports of how the spots appear, and whether or not they are permanent, vary. Some owners report that their horse was solid colored and then became spotted with a single shedding. Others report that the spots appeared over time. Some say the placement of the spots shift with each shedding. Still others seem to have spots that get progressively larger and more roaned over time. Unfortunately with subtle color variations like these, it is often hard to assemble enough information to draw firm conclusions.