In the summer the local rabbits are a menace, a relentless gnawing horde determined to wreak havoc on our gardens. More than once they have nearly moved me to violence. Extreme, you killed my third planting of snap peas type violence.
But in the winter, we tolerate them when they come in to scrounge for food under our multiple bird feeders. Early in the winter there was one or two. Now, in mid-February, the rabbit roll call is up to ten. They leap and jump at each other, try to emulate squirrels by attepmting to climb up onto the lower feeders. They are far more amusing this time of year than in summer, and we are too soft-hearted (and soft-headed) to chase them off. We know we will regret it.
So we get bunny berries, a treat our old dog finds hard to resist. And out there in the snow, mixed in with the yellow holes created by the 'bunnybirds' and dog, we sometimes find these other, somewhat alien-looking pee-holes. It is not some wayward extraterrestrial using our side yard as a latrine making them, it is the rabbits.
The most common, and probably true explanation is this: when they chew the bark of the buckthorn, an ornamental/invasive shrub common to our area, their pee comes out a darker yellow that turns blue on exposure to sunlight. I don't know the particular chemical responsible for this, but since the buckthorn has a dark blue to dark purple berry this side effect makes a certain amount of sense.
A long time ago the late and still-lamented Frank Zappa warned us not to eat yellow snow—though those of us raised in northern New York state already knew that. Now we can add blue snow to that list; in spite of its origins it is unlikely to be berry flavored.