Late winter and early spring is usually the time I start to see blue snow. That’s right, blue snow – or more accurately, blue spots in the snow. If you have walked the woods at this time of year, you may have seen small blue spots in the melting piles of snow.

So, what’s up with the blue snow? A quick Google search finds ridiculous comments about what causes the blue spots in the snow. I was shocked to see some people suggested that the blue spots were the results of spitting mouthwash on the snow. Others strongly suggested it was bigfoot urine and pointed to the blue snow of evidence of their existence. Others suggested Smurfs. Obviously, none of these are causing the blue snow.

Upon closer inspection of the blue stained snow, you will also find the round woody pellets indicative of the droppings (feces) of the Eastern cottontail rabbit. The cottontail rabbit is the most common and abundant rabbit in the eastern half of the country. As it turns out, the blue stain in the snow is the urine of the rabbit.

But why is it blue? The diet of the cottontail changes as the seasons change. During spring and summer, their diet consists mainly of green plants such as grasses, sedges and the leaves of hundreds of plants (including many in your garden). But as winter rolls around and the green plants are no longer available, the rabbits switch their diet to woody vegetation such as twigs of shrubs and small trees.

In late winter, the rabbits have often eaten all the twigs within its reach and now turn to a less desirable food source, European buckthorn. The European buckthorn is a non-native and invasive shrub or small tree that is now common in many parts of the country. It is well known for holding its green leaves well into winter. If you have any woodlands or natural areas, chances are you have the buckthorn growing there.

The buckthorn is well known for producing hundreds of dark blue berries which are toxic and causes upset stomach and vomiting in people, which explains its scientific species name is cathartica, meaning “to cause purging or cleansing.” In other words, it causes a lot of vomiting. Having said that, birds seem to eat the berries with impunity and rely on the berries in winter for survival.

However, the cottontail rabbits don’t eat the berries. Usually the berries are either too high on the tree for the rabbit to reach or all the berries are eaten by the birds. No, the rabbits are eating the bark and small twigs and branches of the buckthorn. The buckthorn produces phytochemicals through primary and secondary metabolism. Usually the phytochemicals have biological activity in the host plant and help in the plant’s growth or defense when fighting competing trees, pathogens such as plant killing fungus or predators such as insects.

In this case, it is believed that the phytochemicals are consumed by the rabbit when it eats the small twigs and branches. The compounds pass through the rabbit’s system and come out in the urine. Normal rabbit urine color is yellow. When they are eating buckthorn, the rabbit’s urine comes out yellow, but it is widely reported (not tested) that once the urine, tainted with the phytochemicals, is exposed to sunlight, it turns blue in about ten minutes. And therefore, you get blue spots in the snow. Of course, if the rabbit urinates on the bare ground, you won’t see it because the urine soaks into the soil.

Well, going back to college chemistry, I take an exception at the sunlight theory. You may remember from your own chemistry classes that many things when exposed to oxygen turn blue. This is called oxidation. There are many chemistry experiments demonstrating liquids turning blue when exposed to oxygen. It makes more sense to me that the urine turns blue when exposed to the air, not sunlight.

Either way, if the rabbit’s urine turns blue with sunlight or oxygen, the blue snow mystery is solved. Until next time…

Stan Tekiela is an author / naturalist and wildlife photographer who travels the U.S. to study and capture images of wildlife. He can be followed on or He can be contacted via his web page at

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