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White-tailed deer form large herds in the winter. These herds are made up of several family units that had been relatively independent of each other throughout the spring, summer and fall months of food abundance. It may seem paradoxical that when food resources become limited the deer bunch up into large groups, but by confining themselves to the most sheltered and protected sub-sections of their broader summer ranges they reduce some of the stress of the cold season and also gain some security from predators. The entire herd may also gain some benefit from the leadership and knowledge of the older does (which I will talk about below).
Mostly, deer rely on their fat deposits for their metabolic energy in the winter. At the start of the winter a deer in prime condition may have as much as 30% of its body weight in fat! The fat is subcutaneous (which also adds to body insulation) and is also found extensively around the internal organs of their bodies. This is their “fuel tank” carefully filled through the spring, summer, and fall. In normal years these fat deposits represent enough calories to carry the individuals through to the next, bountiful spring.
Deer, though, also consume low caloric browse throughout the winter. Preferred winter browse include cedar (like the arbor vitae on the east side of my property!), sassafras, apple, most types of maples, basswood, and flowering dogwood. Secondary browse includes hemlock, honeysuckle, mountain ash, willow, white oak, and many other deciduous trees. Last resort choices for browse (sometimes referred to as “starvation food” because if you see these trees being browsed by deer you know that the herd is in trouble!) include pines, mountain laurel, beech, aspens, poplars, black locust and birches. (This browse data is from the New York State Department of Environmental Conservation).
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I have observed some very interesting aspects of deer behavior in the large, composite herd that crisscrosses through my yard and field throughout the winter. The group is made up of four older does, four yearlings and this year’s now well grown fawns. In the afternoon or early evening the largest doe (who I assume to be the oldest in the group) leads the other deer in single file around the yard and field sampling a great variety of plants. The lead doe browses on the vegetation (the low hanging apple tree branches, the bushy crabapple tree twigs, the arbor vitae, the oak saplings, the hemlocks, the honeysuckle bushes, and so on) and one by one the members of the group walk up to the same spot after she has moved on, feed for a few minutes, and then move on to the next vacated feeding station. The orderliness of the process is amazing. It has occurred to me that the lead doe possesses the “group knowledge” of what browse is best to take or possibly what order of browse is most digestible. She may also have some knowledge about what intensity of browsing is suitable for the long-term, sustainable productivity of the habitat (but, probably, that is hoping for too much!). Possibly these feeding behaviors have been habituated in the lead doe by example and repetition in her youth, and, hopefully, they are being drilled into the younger deer. The winter diet of this group (significantly augmented by the sunflower seeds from my bird feeders, of course) has maintained a good number of generations of these animals very well over the twenty-eight winters that I have observed them!
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The deer that live in my fields and woodlots are very accustomed to people. They are examples of the “city deer” I wrote about a year or so ago (see Signs of Spring 1, March 1, 2018). These city deer are larger than their “country” cousins and have a much shorter “flight initiation distance” (FID) in response to human approach. The city deer also eat different foods than their more rural counterparts. A study at Georgia found that city deer consumed a diverse array of often non-native landscaping plants from their urban and suburban habitats. These plants provide them with a diet that is richer in calories and possibly even richer in nutrients than the wild foods of the surrounding rural ecosystems. In a study in Massachusetts city deer were shown to have ranges that were one tenth of those of rural deer, indicating a much higher quality of habitat and, possibly, a reduced probability of chance encounters with either cars or predators.
City deer in Georgia had low mortality impacts from both natural predators and also human hunters, and a study in Minnesota indicated that city deer not only have a very high survival rate but also that city does have a 93% pregnancy rate typically with twin and even triplet fawns. In Indiana fawn survival rates to 32 weeks were 70% in urban areas (primary cause of fawn deaths were cars) but only 44% in rural areas (primary cause of fawn deaths were coyotes).
In light of all of these benefits to city life it is not surprising that Clark Adams (in Urban Wildlife Management (3rd edition, 2016)) states that 82% of the white-tailed deer in the United States live in urban areas.
Photo by D. Sillman
Some recent research by wildlife biologists at Penn State also looked into the influence of human modified habitats on fawn survival rates (see Penn State News, May 1, 2018). They found that overall only 41% of fawns survive to six months of age, but the more the habitat in which the fawns are reared has been changed by humans, the greater the rate of fawn survival. In fact, for every ten percent increase in human landscape alteration there was a five percent rise in fawn survival rates! Primary factors causing fawn mortality were predators (black bears and coyotes) with collisions with vehicles or farm machinery taking on a much less significant role. Vehicle and machinery deaths rose in more human modified landscapes, but this rise was more than offset by the precipitous decline in losses to predators. Further, these researchers noted that pregnant female deer were actually observed moving from forested, non-human modified habitats into more human modified landscapes where they then had their fawns in the less predator-intensive environment.
Humans are changing so many aspects of the natural word. Cities are sites of intense adaptation, selection and evolution! White-tailed deer (like many other mammals, birds, insects, plants and more!) are taking on distinctive urban traits as they try to live in the developing Anthropocene!