Background
White-tailed deer are widely distributed, inhabiting a wider range of latitudes and diversity of habitats than most mammals¹.  In general, wide ranging, generalist species should be more resistant to the impacts of climate change².  Computer simulations suggest that white-tailed deer would not be very sensitive to the effects of a doubling of atmospheric CO2 (greenhouse gas emissions)³.  However, these simulations looked only at survival and did not include potential effects on reproduction.

Climate Impacts
The severity of winter can affect reproduction and behavior of white-tailed deer.  In more northern regions, breeding is limited to a certain time in the fall, commonly called the rut.  This narrow breeding window is tied to fawn survival, as fawns born too early are less likely to survive extreme winter conditions, while fawns born too late will have less time to gain fat reserves needed to survive the winter.  The breeding season becomes longer further south, until it is nearly continuous near the equator¹.  It is possible that milder winters in Massachusetts could impact the length of the breeding season, but potential impacts on population dynamics are unknown. 

Deer also avoid areas with permanent heavy snow cover and northern populations will migrate to wintering areas that are typically sheltered from extreme weather^1,4.  Severe winters have been documented to increase mortality^1,5,6.  Deer fecundity (reproductive capacity) is linked to winter malnutrition and warm season recovery⁷.  Studies have shown that severe winters have delayed negative impacts on deer abundances^5,7.  As winters become less severe, white-tailed deer may already be replacing moose as the dominant herbivore in some locations, such as the Hubbard Brook Experimental Forest in New Hampshire⁸.

In Massachusetts, deer populations located in the Berkshires, where winter is more severe, produce fewer fawns that are typically smaller than deer at lower elevations⁹.  This suggests that milder Massachusetts winters might be expected to result in increased numbers of deers in the state.  However, deer at higher elevations also have less access to human-related foods, and the mature forests commonly found there lack large supplies of winter deer food⁹.  It is not clear that milder winters alone will compensate for this decreased food availability relative to low elevation, suburban locations.  In addition, deer in suburban areas, such as in eastern Massachusetts, have shorter movements, smaller home ranges, and much higher densities than deer in more rural western Massachusetts because of a higher concentration of resources such as food and shelter.¹⁰  The role that suburban environments may play in mitigating changes in climate on species, such as white-tailed deer, is largely unknown.