CRITICAL MONTHS FOR DEER SURVIVAL

Written by: Dick Henry

Aside from the fall hunting season when half a million deer hunters are afield in New York State, the most critical months for deer survival are in late winter, especially if weather conditions have been harsh for an extended period of time.

Jack-Ballard_Whitetail-Deer_1Winter Fawn Kill

Deer rely heavily on their fat reserves to survive in northern climates. They are ruminants, similar to cows, and possess a four chambered stomach that progressively breaks down food as it passes from one chamber to the next. During the warmer months a deer’s diet consists of green, succulent vegetation and that allows them to build up fat levels in their bodies. However during the winter that food source is no longer  exists, and deer must rely on natural browse from trees and shrubs. Their winter diet has considerably less nutrition, and accordingly deer will then be forced to rely on their stored fat to survive. The quality of winter habitat varies considerably, and unfortunately much of our winter habitat in southeastern NY is mature forest and/or over-browsed landscape.

Deer store fat in three general locations in their body. The subcutaneous fat, found under the skin is utilized first, followed by the mesentery fat found in the gut cavity. As a last resource fat stored in the bone marrow is used for survival.

The femur, or upper back leg bone is one of the last sources of energy for a starving deer.   It is the leg bone that  attaches to the pelvis and is surrounded by meat tissue and blood vessels that serve to draw off the stored fat.  Pictured below are samples of deer femur fat levels in winter.  As stored femur fat is utilized for winter survival, the bone marrow changes from the normally solid white fat to a red gelatinous condition. Healthy deer going into the winter period will have solid, white marrow as shown in the photo below.  As fat is drawn off and utilized for survival the marrow changes color. When the marrow becomes reddish in color, it is a result of the fat having been heavily utilized by the deer for energy and survival. Once the marrow turns reddish and the last remaining stored fat is utilized,  it is pretty much “game-over” for the starving deer.

Samples of femur fat depicting the levels of fat remaining.

Body_hemmHow quickly are fat reserves burned during the winter?

It depends on a number of factors related to deer mobility, wind and cold temperatures and the overall severity of the winter conditions. Snow depth is very critical, especially if deer are forced to wallow through deep snow. Long-lasting deep snows can deplete fat reserves at a much faster rate, especially when deer are forced to drag their bellies through the snow for extended periods of time. Low temperatures and wind will also have a draining effect on fat reserves.

Typically young-of-the-year fawns are the first to perish from winter-kill, and that is because of their smaller body mass and the fact that  they never achieve the same level of fat depositions that adult deer have. The old, injured and crippled are typically next to succumb because of locomotion limitations. Surprisingly yearling bucks are often among the next group to perish because they have a  smaller overall body size and smaller amounts of stored fat as a result of having participated in the fall rut.  In severe winters, prime age does are the last group to succumb  from winter starvation. When the herd suffers wide scale losses in all age classes in really hard and long  winters, the deer population will be dramatically reduced in the upcoming  summer and fall. If starvation conditions occur for any measurable length of time it is critical for deer biologists to be in the field evaluating the impacts of winter. It is not necessary to specifically count the number dead deer but it is very crucial to know what age classes are being impacted by winter-kill in order for deer management permit quotas to be adjusted accordingly.

There are also other times of the year when deer experience increased levels of mortality from predation and roadkill, but none as long lasting or dramatic as the impacts from winter-kill. In our area a handful of past winters stand out as really severe winters for deer survival. Specifically the winters of 1976-77, 1987-88, 1992-93 and 2003-04 had extensive winter mortality.

If you should encounter a dead deer this spring while turkey hunting or trout fishing, you can perform a simple field check to determine if the deer died from starvation. Simply cut through the meat and tissues on the outside of the hind quarter and expose the upper back leg bone that attaches to hip.  Crack the that bone with a rock or small hand tool and note the color of the internal bone marrow.  If it’s deep reddish in color, it is in a good indicator that the deer that died from natural winter mortality.

 

 

 

 

 

 

Examing marrow from the femur(upper leg bone).

In spite of  all the other forms of non-hunting deer mortality, winter kill remains as the most potentially significant cause of  deer losses. After all of the losses throughout the year from predation, vehicle collisions and hunting season, winter mortality will reduce a deer population to its lowest level at any time throughout the year.

Inspite of the numbers of  deer we take during the hunting season,  how many deer  the predators remove, or how many we hit with our motor vehicles, the impact of Mother Nature are always be  the low hole in the bucket”.

7 Comments

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  1. steve menendez 12. Mar, 2013 at 5:18 pm #

    thanks for the informative article dick..i think the last few winters have been relatively mild and have not had a negative impact on our deer herds..do you believe that this has been the case?

  2. Brian O'Connor 12. Mar, 2013 at 5:47 pm #

    Once again another informative article by Dick Henry. Noting that our current winter conditions have been favorable for the herd all my observations show healthy animals going about their normal activities. Including last years mild winter, conditions are such that I have not witnessed any substantial herding activities. It appears the core groups of does stayed in their normal nucleus areas and not congregated to winter yards.
    I can remember back in the 1980’s collecting leg bones and checking marrow for you Dick. Those were some tough winters seeing the herds congregated along the frozen streams,because the snow was easier to walk through there, and seeing piles of deer carcass. Places like Kelly Hollow and Alder Lake or the Beaverkill Valley where the known winter yards showed the ugly side of deer mortality. These last couple of years certainly have given them a reprieve.

  3. Dick Henry 13. Mar, 2013 at 5:37 pm #

    I certainly agree with you that we’ve had some pretty mild winters and haven’t experienced any significant winter mortality in a number of years.

    However, I’ll always keep my fingers crossed until we get past the end of this month. Some may remember the huge snow storm that we had at the end of March in 1993. I can’t recall the exact amount but it was about 20 inches, and was belly-dragging snow for the previous years fawns.

    Green-up was late that spring and we were finding fresh dead deer well into late April. The fawns had used up their fat reserves and the exertion of going through the late deep snow put them beyond the point of no return.

    Thanks for your comment.

  4. Dick Henry 13. Mar, 2013 at 5:42 pm #

    Thanks Brian. We certainly had some real winters back then !

    You may remember that we referred to the Beaverkil Valley as the “Valley of Death”.

  5. Dick Henry 17. Apr, 2013 at 9:18 pm #

    I recently recieved a question from a guest about a young deer he harvested at West Point in 2009.

    Attached below is his question and my response:

    During the Bow Season in the Catskill Region in 2009 I was amazed to
    see a Fawn with faded spots and forked (4pt) antlers. The antlers were
    about four to five inches on each side with small but noticeable
    forks. I reported this to the Wildlife Biologist at West Point. He
    said that he had never heard of that before.

    “This is one of those cases where it would be interesting to see the jawbone. There are very distinct differences in the teeth and it’s easy to differentiate young of the year fawns because they still have milk teeth and fewer overall teeth on the lower jaw.

    Lacking the jawbone, there are a couple of possibilities about this deer’s age:

    In northern habitats, it is extremely rare for young-of-the-year fawns to ever grow anything more than small, polished knobs for their first antler growth. From a sample size of almost 20,000 deer that i have aged over the years, I’ve only seen a handful of the current year’s fawns that have had this rare, knob-like, mini-headgear.

    I’d venture a very likely guess that it’s actually a late drop fawn from last year that is now a runt yearling and because of an abnormal growth and hormone patterns has some retained spots from its fawn pelage. It can and does happen in rare instances. Late drop male fawns typically have smaller body sizes as yearlings and very small antler growth. Their body size doesn’t catch up with other bucks of their year class until they are 2 ½ years old.

    A second option, although extremely unlikely, is that it is indeed a true young of the year fawn that displays a very abnormal body coloration and even more abnormal antler growth patterns. However, for any fawn to grow the amount of branched headgear that your deer had is extremely doubtful in any northern climate. It is even less likely for a deer in the poor forest habitat in the Harriman Park/West Point complex.

  6. David Cinelli 24. Feb, 2015 at 7:14 am #

    Informative article. I wonder how the winter of 2014-2015 will be on the deer population?

  7. Dick Henry 26. Feb, 2015 at 3:26 pm #

    Thanks for your inquiry about this winter’s potential impact on deer in the Hudson Valley.

    Given the harsh winter that we’ve had so far, it’s likely we will see some winter mortality, especially among last year’s fawns. I was out on snowshoes in Stone Ridge a couple of days ago and there appeared to be 18+ inches of powder snow on my back woodlot. That’s belly height for last year’s fawns and if they’re dragging their belly through snow, they are burning up their fat reserves at a pretty rapid rate.

    Older deer may fare somewhat better, but it won’t take much more of a snow pack to put a hurt on them. The young ones typically have signifcantly less stored than than their elders and are the first to go down.

    Last year looked like it could have caused some problems, but the snow pack went pretty quickly and we were lucky. Given the forecasts and the continued snow and extreme cold we are experiencing, that may not be the case this year. We’ve still got all of March to endure and if the snowfalls continue at their present trend, we could quickly have both young-of-the-year deer in trouble.

    If anyone should find deer going down, please feel free to reach out to me; I’be glad to take a look at them with you.

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