The Division of Wildlife’s research projects focusing on the decline of the state’s mule deer herds are beginning to provide clues that may help scientists better understand and perhaps even reverse the long-term trend.
Two ongoing studies investigating fawn survival and the number of fawns that make it to their second year are providing new information on one of the most critical aspects of mule deer research.
Information compiled from fawn survival studies on the Uncompahgre Plateau, Middle Park and Red Feathers Lakes near Fort Collins has revealed many new factors that are helping researchers understand why relatively few fawns are being recruited into mule deer herds each summer and fall.
“Fawn production and survival is one of the most volatile population parameters,” said Tom Pojar, the Division’s lead researcher for the Fawn Survival Study. “Due to a variety of causes, they’re the first to die by Mother Nature.”
Mule deer herds declined sharply following the harsh winters of 1983-84 and 1992-1993 in Colorado and throughout the Western United States. While the decline is clear, the causes are not so obvious.
To find out why, biologists are conducting several studies, two of which are looking into factors contributing to fawn deaths. One study is designed to monitor how many newborns survive the first few tenuous weeks of life and throughout the remainder of summer. The second study involves monitoring how many fawns survive the winter.
The first phase of the Fawn Survival Study was completed last summer and the Division’s researchers will conduct the same study next summer to confirm their initial findings.
Fawn survival studies began in 1997 and are primarily intended to provide survival rates needed for the Division’s population modeling of deer herds. Colorado has more than 500,000 deer, which is only half the number of deer of 50 years ago.
Obtaining good estimates of mule deer survival rates is one of the most critical components of deer population models. Biologists are now studying the causes of winter and summer fawn mortality as part of the Division’s overall management of mule deer herds.
“We expect a certain amount of fawn mortality, but when and how the fawns are dying is important to our ongoing research,” Pojar said.
Last summer’s monitoring studies have provided the Division with many new clues as to why fawns are dying prior to their first winter.
Many possible causes for the mule deer decline have been discussed. Some people consider an increase in predation to be the primary problem. Others feel habitat is the main reason because the quality of forage may be inadequate for deer as the result of the loss and fragmentation of habitat and changes in plant communities. Poor habitat quality can also contribute to predation by lowering the physical condition of deer, making them less able to flee from predators.
There is also speculation that Colorado’s expanding elk population and competition with elk for wintering grounds or fawning and calving grounds could also be a factor. But available data fails to show a consistent relationship where increasing elk numbers have been associated with declining deer herds.
“We expected the summer fawn study to show what predators ate the fawns and when, but to our surprise at least half died from non-predator causes,” said Bruce Watkins, area wildlife biologist for the Uncompahgre Plateau.
Although biologists know predators such as coyotes, lions and bears contribute to deer mortality rates, many say the problem is much more complex.
Last spring, Division researchers captured and radio-collared 50 neonatal fawns on the Uncompaghre Plateau and 14 in Middle Park.
To track each fawn’s movements and survival, the Division’s researchers used a 65-gram radio collar fastened to the fawn shortly after birth.
Of the 64 neonatal fawns captured and monitored this past summer, 39 percent survived, 28 percent died of sickness or starvation, 25 percent died from predators, the cause of death was not determined for 6 percent, and 2 percent were poached.
“It is not uncommon for 50 percent of the young to perish,” Pojar said. “As with many species, there is an excess of young produced and if they all survived then there would be an over abundance of that species.”
Pojar added that if the mortalities from sickness or starvation were reduced or eliminated, then the overall survival rate would be considered excellent and one would expect the population to grow.
Coyotes represented a small portion of the 39 deaths recorded, 21 percent. “Coyotes have many alternate food sources during the summer such as ground squirrels and rabbits,” Pojar said. Cats (mountain lions or bobcats) and bears each represented about 6 percent of the deaths. From these initial findings, the Division’s biologists are now focusing on the fawns that died of sickness or starvation.
“In these cases, we found whole carcasses with no indication of predation and had the opportunity to do a thorough necropsy,” Pojar said.
Other clues for fawn mortality were found in the Division’s annual post hunt deer surveys that are used to determine the fawn/doe ratio going into winter. On the Uncompahgre Plateau, biologists found that the fawn/doe ratio has generally declined over the past 20 years but can vary considerably from year to year. In 1997, the ratio was 34 fawns per 100 does. But in 1998 it climbed to 52 fawns.
“We conducted the summer fawn survival study to find out why we are seeing the low fawn/doe ratio going into winter,” Watkins said. Over the winter survival rates also varied and were 49 percent in 1997 and 72 percent in 1998. “Winter fawn survival is a function of fawn size and condition and the severity of winter weather,” Watkins said. When a fawn is born small or late in the fawning season, that animal has less of a chance to achieve the growth and strength necessary to survive winter.
Ultra-sound studies conducted last winter on the Uncompahgre deer herd showed that 93 percent of the does were carrying fetuses, with an average of 1.7 fetuses per doe. These rates compare favorably with historic data when the deer herds were much more productive. This means there were enough bucks to breed with the does despite the relatively low buck/doe ratio of 10 bucks for every 100 does. Essentially, the Division’s biologists knew in advance how many fawns should have been produced during the fawning season, and that the does were getting bred.
What is still unknown, however, is why so many of the fawns died before they had a chance to make it from birth to winter, and of the ones that died from sickness, what was the cause. Results of the Fawn Survival Study suggest that many does were in less than optimal physical condition prior to giving birth and during lactation as evidenced by the weak and sick fawns found.
From this study biologists have found that many fawns are dying over the summer, and most importantly, the causes are not solely predation.
“One reason may be the doe is under some sort of stress condition, thus giving birth to a fawn that is in poor condition,” Pojar said.
“Many of the fawns we studied were not in good shape, which indicates that the does were also in poor condition.” Learning why does are in poor condition may provide a clue as to why there may not be a healthy recruitment of fawns going into a population, Watkins said. Coincidentally, many of the fawns studied also showed signs of stress. Necropsy reports revealed several fawns had a reduced thymus gland. “It’s a clue that the animal was in poor condition,” Watkins said.
“A reduced thymus gland can compromise an animal’s immune system,” Pojar said. When an animal’s immune system is compromised it is more susceptible to a variety of pathogens. “This may explain why fawns are dying of such a variety of sicknesses,” he said.
Does are better able to survive the variety of pathogens encountered because they have a more developed immune system.
In addition, fawns do not emit a scent during the first month after birth. But if the animal is sick and has diarrhea, a predator may be attracted to its scent and thus the animal is more susceptible to predation.
Mule deer populations may be cyclic. A similar decline was observed in the 1970s.
Some biologists feel the over-grazing of the late 1800s and early 1900s resulted in an increase in forbs and shrubs that set the stage for the explosion in deer numbers that occurred in western Colorado from the late 1930s to mid-1960s. As the 20th century progressed, plant communities became increasingly mature due to the suppression of fire; trees, especially pinyon and juniper, became more predominant.
These changes could have resulted in less nutritious forage available for deer and thus less capacity of the habitat to support large deer numbers. Elk may not have been negatively impacted because of their larger digestive system, which allows them to eat lower quality forage than deer and still derive the nutrients necessary for reproduction and winter survival.
The quality and quantity of sagebrush and other shrubs and forbs that deer depend on to gain energy reserves for breeding and winter survival are critical for the production of healthy, viable fawns. This may contribute to nutritional stress on does and thus on newborn fawns.
“The weak and unthrifty fawns that we saw this summer leads us directly to our best clue as to the cause -- the nutritional status of the does during key times of the reproductive cycle.” Pojar said.