Rabbits are what they eat

Rabbit debarking bittersweet (all photos in this post by Zas Ispa-Landa)

What: You are what you eat. And in this case rabbits are a whole lot of things. Zac and I were impressed at the abundance of rabbit sign (from the introduced eastern cottontail, Sylvilagus floridanus, as opposed to the native New England cottontail, Sylvilagus transitionalis) when we went tracking on Saturday down at Bread and Butter Farm in Shelburne. We even spooked a cottontail as we walked past the driveway. Rabbits seemed to prefer shrubby non-natives, or at least those were the most prolific species in the area and so were the most foraged upon. We found feeding sign (debarking and/or twigs clipped) on forsythia, common buckthorn, barberry, domestic apple, honeysuckle, staghorn sumac, and oriental bittersweet.

45o angle snips on twigs, charateristic feeding sign from cottontails
on woody species (forsythia in this case)

The story got even cooler at the end of the day when we found another high traffic area for cottontails under the powerlines. Buckthorn predominates in that area and the rabbits must have been eating fallen buckthorn berries as their urine was died blue (image on the right is blue tinted urine, on the right is more typical orange). Buckthorn berries are unpallatable, if not inedible to most things so the berries persist into late winter, when they become a last resort for many winter active species.

Ecological notes: Many plants contain phenolics (tannins, which cause the dry taste in wines, the bitterness in aspen bark) as a defense against herbivory. During spring and summer these are concentrated in leaves and flowers, but during winter they are translocated to the bark. Animals have a variety of ways of dealing with these phenolics (e.g. beavers will soak witchhazel before eating to leach tannins, birds and will eat clay, which absorbs phenolics). Buckthorn (Rhamnus cathartica) is high in phenolic compounds. Rabbits presumably can handle the phenolics by breaking them down in their kidneys, or maybe eating clay. The anthocyanins (pigments in the fruits) are also excreted in the urine. I'd assume that eating buckthorn fruits later in the season gives more time for the tannic acids to break down and makes the fruits more palatable. That's speculation though, and I couldn't find any articles to confirm/deny that.