By John J. Ozoga, Deer & Deer Hunting Contributor
The whitetails’ reproductive cycle is geared to giving birth when conditions are best for newborn fawn survival. As you progress northward in the whitetail’s geographic range, a greater percentage of the fawns will be born during a relatively brief period in late May and early June. This is when new, lush vegetation provides excellent hiding cover for fawns and an abundant supply of nutritious forage, which is necessary if does are to produce their maximum amount of nourishing milk. Even fawns need nutritious vegetation to supplement their milk diet at an early age.
In the North, natural selection has minimized poorly timed births. Those fawns born too early likely die from exposure. Those fawns born too late seldom achieve favorable physical size and fatness necessary to survive the hardships of winter.
In the Midwest, late-born fawns are more often the result of ideal conditions and breeding among doe fawns. In rich farmland, where deer live at fairly low density and enjoy excellent nutrition year round, a high percentage of 1-year-old does will produce their first offspring. Because these precocious females tend to breed about a month or so later than adult does, their fawns usually are born in July or August.
Interestingly, fawns born late to 1-year-old mothers tend to be somewhat larger than normal at birth, as are those born to late-fawning adults. Given excellent nutrition, as is characteristic of Midwest farmland, these late-born individuals have a good chance of growing to respectable size before winter and surviving to be large and productive adults.
Late-born fawns are more prevalent in Southern states, where unbred adult does might re-cycle and come into estrus as often as seven times during one season. Further, many Southern states have very long hunting seasons, starting in August or September. As is usually the case, hunters prefer to harvest bucks, which skews adult sex ratios heavily in favor of females before peak breeding, increasing the chances of late breeding and birthing.
With unlimited nutrition, fawns born a few weeks late will catch up through time. However, in the wild, overbrowsing, poor nutrition and density stress generally go together. As a result, late-born buck fawns, in particular, might never grow large bodies or large antlers, regardless of their genetics.
In the North, few undersized fawns survive tough winters. In the South, researchers contend such trends are self-perpetuating, even with favorable nutrition, because it takes late-born deer longer to become sexually mature.