bright-blue patches in the snow

So, here in Minnesota, Buckthorn is considered an invasive species, to be destroyed every possible chance.

Now, my rented house backs up to the Vento bike trail, and the undergrowth that borders each side of that path is owned by...the city? But yes, its got buckthorn growing there, big and wild, unchecked. I know its buckthorn because I saw the blue-stained pee spots of rabbits in the snow in my backyard last winter.

So I researched the matter and it turns out that in the winter, hungry rabbits eat the buckthorn bark, which has interesting repercussions:

 "..a result of rabbits consuming buckthorn (Rhamnus cathartica), a highly invasive tree in North America. One of the chemical constituents is excreted in the urine, and within a short time turns blue when exposed to sunlight."
[source]


 http://discovernature-teresa.blogspot.com/2013/02/strange-stains-in-snow.html has the best photo; 

So....that's interesting right?  Next winter, I will try to update this with actual photos of my own.