The Mystery of the Blue Pee

Friday, January 30, 2015
About a year ago when I was out in the backyard filling up my bird feeders I noticed a patch of blue snow.  And then another.  And another.  And yet another.  In fact there was blue snow patches all over the yard.  Then I noticed beside some of the blue patches were patches of rabbit scat- an Eastern Cottontail (Sylvilagus floridanus) to be precise.  Hmmm. 

Blue Eastern Cottontail Pee

Blue Eastern Cottontail Pee

Since I assumed that Eastern Cottontail pee is not always blue I did some research.  The reason for the blue pee is not a shocking one.  The blue urine was the result of diet of European Buckthorn (Rhamnus cathartica).  While this may not be surprising to those familiar with the plant (European Buckthorn has dark purple berries), what is strange is that this colour seems to be the result of eating the twigs and bark and not the berries.  Even more bizarre, the urine of domestic buckthorn eaters is initially yellow or brown, but within ten minutes of exposure to sunlight turns bright blue in the snow.  Eastern Cottontails do not regularly eat European Buckthorn, so these blue spots in the snow can indicate that the rabbits are stressed for food.  

European Buckthorn Berries

European Buckthorn

Mystery of the blue pee- solved!  You never know what you will find in nature but there are always interesting surprises. 

~Kristen Martyn

9 comments:

  1. I just landed here on this page from googling what are the blue spots near backyard wildlife- rabbits and squirrels are. This makes sense!

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    1. So glad you found this useful! I wrote the post back in 2015, but am seeing lots of blue pee this year too! :) ~Kristen

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  2. Just discovered this in 2019...I figured it was something like this. Thanks!

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    1. Indeed! I've been finding it a lot this winter in my yard as well. :)

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  3. A friend of mine, who is a naturalist, shared this the other day. Today, mt boys found blue snow while we were out on a nature walk. There was plenty of buckthorn around. They were so amazed by this! Thanking for sharing this information. It was fun both to find the blue snow and to be able to answer their questions about it.

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    1. Hi Jessica! Glad we could help solve a mystery ;) Kristen

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  4. Haven't had rabbits in many years due to large owl population and this year's 2020 owls are down with the rabbits all over the place. While picking up after dogs in back woods blue spots threw me. Lots of snow and buckthorn back there. Thanks!

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    1. How lucky you are to have so many owls! Glad you liked the article :) Kristen

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