What’s Really Killing Delaware’s Fawns?
By: Justin Dion, Dr. Jacob Haus, and Dr. Jacob Bowman
Predators are one of the most widely discussed topics of deer management in the past few years, particularly in regards to fawn survival. If you were to ask any hunter what factor is most limiting to fawn recruitment nationwide, chances are they will say coyotes. It is no surprise why; nearly 90 percent of survival studies over the past 30 years have documented predation as the leading source of mortality for fawns. Many hunters have implemented predator control efforts with the common-sense logic that because predators kill fawns, less predators will mean greater fawn survival. While this appears to have worked in some areas, in other areas deer managers have seen little to no return on investment from these strategies.
Delaware offered a unique opportunity to examine fawn survival in a region where predation is all but non-existent. Small pockets of coyotes have been present in the state for some time but the coyote population is not yet large enough to potentially have large-scale impacts on fawn survival. Black bears and bobcats populations have not been established in the state for more than 100 years. If predation is limiting fawn recruitment, than fawn survival in Delaware should be through the roof!
We spent the summers of 2016 and 2017 capturing and collaring 109 newborn fawns throughout Sussex County. When a fawn died, we sent the carcass to veterinarians for necropsy to determine if any injuries to the carcass occurred before or after death. The timing of the injury was important; injuries occurring before death are from predators and injuries after death are from scavengers like foxes and vultures, which are plentiful in Delaware. What we found was somewhat surprising: even in the absence of predators, many fawns were still dying. In fact, our survival estimate after 90 days was only 45%; toward the lower end of survival estimates in the region from studies over the past 15 years, all of which listed predation as the leading cause of mortality.
More important was the discovery that all of our fawn deaths were linked to natural causes such as emaciation, disease, or birth defects. No fawns were killed by predators. What this showed us was that natural sources of mortality can cause low fawn survival similar to estimates seen in areas with predators. Our findings suggest that predators may simply be removing the “doomed surplus” – the individuals who would have died regardless of predator intervention. For example, if a predator kills a fawn that is dying of pneumonia, the event is recorded as predation because we have no way of knowing the animal was sick before it was eaten. This scenario might explain why many studies outside of Delaware have found predation to be the most important cause of mortality, as well as why predator management efforts have been largely unsuccessful.
If predation is not driving fawn mortality, then what is? What we found was that 3 factors noticeably influenced the probability of mortality for fawns in Delaware; birth weight, daily precipitation, and the age of their doe. Fawns weighing less than about 6.5 pounds at birth had a greater risk of death than fawns that were 6.5 pounds or heavier, and approximately one inch of rain doubles the risk of death for a fawn that day. Doe maturity was another important factor. We captured all the mothers to the fawns in our study, gave them a GPS collar and used their tooth development to estimate their age. We divided the does into two categories: mature and immature. As most hunters know, bucks grow larger antlers as they age because they can allocate more resources to antler growth once muscular and skeletal development is complete. Similarly, does have reached their peak body size by about 4 years of age, which means they are able to focus all nutritional resources on the growth of their fawns rather than their own body growth. We considered does older than 4 at the time they gave birth to be mature. These does are also experienced mothers and would therefore be more capable of caring for their offspring. What we found supported these ideas, with fawns of mature does experiencing lower rates of mortality when compared to immature does.
The connection between the maturity of the doe and survival of her fawns was logical, but we wanted to dig deeper to see if we could find a mechanism for the connection. First, we looked at the difference in home range size during the fawning season. What we saw was that mature does use a smaller home range than immature does in the weeks leading up to and following birth. Animals in areas of high quality resources usually maintain smaller ranges because they have to travel less to meet their nutritional demands. It appeared more mature, dominant does were occupying higher quality habitat than immature does.
What does this all mean for hunters in Delaware? Well, when coyote population do start to rise, it’s important to keep in mind that trapping and predator hunting can provide great recreation opportunities, but may not significantly increase fawn survival. If you want to protect your fawns, first hope we don’t get big rain showers in early June, then spend the rest of the year promoting quality habitat with fat old does. However, if you management goals are to reduce the deer population on your property than focusing harvest on mature (adult) does will have the greatest impact.
Several deer from the project are still carrying ear tags and radio collars. If you harvest or find a deer with a collar or ear tags, please call (302) 831-4621 to report the animal to University of Delaware researchers. We greatly appreciate the many landowners who allowed us access to their property to capture and track deer. This project would not have been possible without their generous support. This research was funded by the Delaware Division of Fish and Wildlife through a grant from the Wildlife and Sport Fish Restoration Program (Pittman-Robertson Act), University of Delaware, and the McIntire-Stennis grant from the U.S. Department of Agriculture.