Management Strategies: White-tailed Deer

WHY ARE THERE SO MANY DEER?

The increase in the White-tailed Deer herd in Sifton Bog is evident from increased reports of deer sightings, deer-vehicle accidents on adjacent roads and damage to trees during late winter. This species has increased steadily in abundance since the early 20th century (McCabe and McCabe 1984) and currently occupies a larger geographic range than any other terrestrial mammal in North America (Pagel et al. 1991). There are many reasons why deer populations have increased during the last 20 to 30 years.

The highly adaptable behaviour of deer enables them to effectively exploit new situations and habitat created by humans in suburban residential areas. It has been shown that deer become increasingly approachable and less fearful of humans in urban and suburban areas with repetitive close contact, the absence of harassment, and supplemental feeding (Witham and Jones 1987, Swihart et al. 1991).
The absence of mechanisms to regulate the deer population, such as natural predators, combined with the naturally high reproductive rate of deer (Haugen 1975, McCullough 1979) has led to lower annual mortality rates, a rapid increase in survivorship and higher population growth rates for suburban deer. Suburban environments provide year-round protection from hunters and natural predators. The low mortality rate for adult deer and the favourable habitat conditions for reproduction mean that urban deer herds can double in size every two to five years. Lack of hunting-induced mortality can result in a strong skew toward older age classes (Witham and Jones 1992).
Expanding suburban communities have created excellent deer habitat with an abundance of food for deer, including ornamental shrubs and garden plants. Being a forest edge rather than a forest interior species, deer thrive in transitional areas and suburban landscapes (McCaffery 1987). Deer populations will likely remain abundant due to the preservation of open spaces in the form of parks, nature preserves, natural areas and conservation areas that function as sanctuaries and important foraging sites for deer and other wildlife. In addition to providing aesthetic and recreational value to urban environments, refuges for native plants and animals, and remnant examples of pre-settlement assemblages of plants and animals, most of these areas are also potential deer habitat. This trait is evident in Sifton Bog, where deer trails radiate out from the denser black spruce stands on the edge of the sphagnum mat toward the outer ring of hardwood trees and shrubs that surround the residential houses. Since deer are relatively large, highly mobile and highly adaptable in their behaviour, they become very familiar with their home range and seldom leave it if they are able to fulfill all their requisites of life (Thomas et al. 1964, Beier and McCullough 1990, Nixon et al. 1991, VerCauteren and Hygnstrom 1998, Cornicelli 1992, Bertrand et al. 1996, Kilpatrick and Spohr 2000).
The patchy distribution of suitable deer habitat surrounded by unsuitable areas restricts local movements and can lead to deer activity being concentrated in isolated pockets of habitat (Witham and Jones 1992). This isolation is augmented by berms constructed along major traffic arteries, numerous municipal parks, nature centres and minimally developed private and commercial holdings as sound and visual barriers. The increase in suburban feeding sites, such as fields and lawns, interspersed with wooded areas may reduce the movements needed to meet daily energy requirements. It is believed that the deer at Sifton Bog were displaced from the nearby Thames River valley into Sifton Bog as a result of increased habitat fragmentation and will remain in the bog because of the high quality of feeding sites.
Supplemental feeding by urban residents throughout the winter as well as the planting of ornamental trees and shrubs for screening or aesthetics may increase survival by increasing the food supply. In the winter, the deer in Sifton Bog appear to be using the conifers at the edge of the bog for cover and moving out to feed on the hardwood edge at night, with ornamental hedges of white cedar and fruit tree of the local residences being the principle food sources. With the exception of some light browsing on maple saplings, there appears to be little impact of winter browsing on the native Sifton Bog species.

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WHITE-TAILED DEER BIOLOGY

Carrying capacity is defined as the number of deer that an area of land can support over an extended period of time. The number of deer that can be sustained in a given area is a function of food resources and the availability of winter cover. When deer numbers approach the carrying capacity, they have a negative impact on the habitat quality and the physical condition of the herd declines, increasing the likelihood of winter mortality due to poor nutrition and/ or disease (Eve 1981, Swihart et al. 1998). When over-browsing persists, a long term reduction in the carrying capacity can occur. Neither the herd health nor the habitat quality will improve unless deer densities are reduced.

White-tailed deer populations are organized into matrilineal (female led) groups in which related females are accompanied by their immediate offspring (Hirth 1977). Deer often travel in family groups, so motorists should be cautious if one or more deer are seen on the roadside. Female deer often remain in the area where they were born (natal range), with young females establishing home ranges that overlap the home range of their mothers (Marchinton and Hirth 1984, Porter et al. 1991, Marchinton and Jeter 1966, Nelson and Mech 1984, Tierson et al. 1985, Nixon et al. 1991). Males tend to disperse from their mother’s home range (Kammermeyer and Marchinton 1976, Holzenbein and Marchinton 1992, Nixon et al. 1994). The minimum home range size for suburban deer is approximately 80 hectares (200 acres) (Cornicelli 1992, Bertrand et al. 1996, Grund 1998, Kilpatrick and Spohr 2000).

Mating behaviour occurs primarily from mid-October through December. Deer have a high reproductive potential, with yearling does typically producing one fawn. Adults 2.5 years of age and older commonly produce twins and even triplets when conditions are favourable (Verme and Ullrey 1984). Since sex and age ratios are often skewed in favour of older females because of the poor physical condition for bucks after the breeding season (Hamilton et al. 1995, Jacobson and Guynn 1995, Gavin et al. 1984, McCullough 1979, Mech 1984) and the high mortality for yearling males associated with the spring and fall dispersal periods (Nixon et al. 1994, Rosenberry et al. 1999), deer populations can increase rapidly (McCullough 1979, 1984).

What is the population/ density estimate for the deer in Sifton Bog?

A baseline population estimate is helpful, but very difficult and expensive to obtain. The actual number of deer in a community is nearly impossible to determine and will change seasonally. Therefore, it is often sufficient to estimate the minimum size or approximate the density. According to pellet counts by Tom Purdy, Natural Heritage Education Specialist - Ministry of Natural Resources, it is estimated that more than 20 deer lived in Sifton Bog this past winter (2000). This estimate would equate to a density of over 60 deer/ square kilometre.

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