MDF Projects
Mule Deer Fawn Survival in the Central Sierra Nevada
Kevin L. Monteith and Vernon C. Bleich
California Department of Fish and Game, 407 West Line St., Bishop, CA 93514
As recently as 1984, approximately 85% of the mule deer wintering in Round Valley (Inyo and Mono counties) migrated to the west slope of the Sierra Nevada to spend the summer. Since we began examining summer range use of deer captured on the Round Valley winter range, the proportion of deer that migrate to the west slope has declined to approximately 55%. This corresponds with lower survival rates experienced by fawns born to deer that summer west of the crest, when compared to fawns born east of the Sierra crest since 1997. Differential survival of fawns has been confirmed by recruitment data acquired from the winter range in Round Valley, which is used by deer that summer east and west of the crest. These results suggest that differential fawn survival is having a tremendous impact on the dynamics and behavior of mule deer in the central Sierra Nevada. These alterations in migratory patterns have occurred over a short period of time, and suggest that changes resulting in differential fawn survival have occurred very recently, and may reflect ecosystem-level changes. Nevertheless, it is essential to determine cause-specific mortality of neonates (fawns) in order to characterize relationships between population dynamics, habitat conditions, and influences of maternal body condition as related to fawn production among female deer that summer in the San Joaquin River drainage (west slope) and compared to nonmigratory deer that summer on the eastern slope of the Sierra Nevada.
The cause for the differential survivorship between these populations of mule deer is yet to be determined. As part of our investigations, we are conducting an in-depth study to determine cause-specific mortality of mule deer fawns. That information is essential to understanding the relationships between maternal costs of reproduction and its effects on female body condition, predators and their role in mule deer population dynamics, and the effects of habitat on female body condition. Additionally, understanding those factors will enable a better understanding of the effects of predation on the population dynamics of these deer.
To develop an understanding of these relationships, we must be capable of determining causes of mortality of neonates; we are currently employing the following methods to determine mortality. Adult does are captured on the winter range, evaluated for pregnancy and, if pregnant, are fitted with vaginal implant transmitters to facilitate our efforts to capture fawns and characterize birth sites. Fawns are then captured shortly after birth and are fitted with radio transmitters to allow monitoring of survival and habitat selection. These fawn collars emit a slow pulse signal when fawns are moving (i.e., alive). When the collar has not moved for >4 hours, it emits a signal at 2 times the normal rate to indicate mortality. Fawns are monitored daily for survival and mortalities are investigated immediately to ascertain cause of death.
During the summer of 2006, we began an exhaustive effort to capture and collar as many fawns as possible. A total of 37 adult females had been fitted with vaginal implant transmitters in March 2006, which facilitated capture of fawns. The vaginal implant transmitters, combined with observations of adult doe behavior, were employed to make possible the capture of 43 mule deer fawns. As of 18 September 2006, we have evaluated 16 fawn mortalities and were able to ascertain cause of death in 12 cases. Five of these fawns died immediately following the birthing process before we were able to collar them; 2 were killed by bear, 2 were related to malnutrition, and 1 was undetermined. One other mortality was also related to malnutrition; this fawn died within 2 days of birth. Three of the other mortalities were predation by coyotes, 2 by bobcat, 1 by mountain lion, and 1 other by bear. At this point we have been unable to determine cause of death in 3 cases. These 3 fawns were found dead with no signs of predation; tissue samples are currently being analyzed by California Fish & Game veterinarians to determine potential for disease related mortality. Two fawns also slipped their collars in thick vegetation resulting in loss of contact with these individuals. In addition, we have also documented 3 adult doe mortalities which subsequently resulted in death of their fawns. The 25 fawns that are still alive will be monitored for survival until they reach the winter range where animals are much more accessible. Some of the survivors will be captured in March and fitted with adult radio collars that will allow us to continue monitoring their survival and use of summer ranges. At this point we consider this summer a success, but look forward to fitting even more fawns with collars next year.
The major obstacle we face is the need for financial support. Vaginal implants and fawn collars are expensive, but are necessary to understand the aforementioned relationships. In addition, due to the formidable landscape of the Sierra Nevada, aircraft time for aerial telemetry is necessary to maintain daily contact with animals to determine date and location of fawn births and mortalities. Because of recent retirements, the unavailability of department pilots has necessitated the hiring of contract pilots to monitor survival of fawns. Inevitably, this has necessitated that funds previously approved for the purchase of collars and vaginal implant transmitters be applied to the aerial telemetry budget. Therefore, it is a logistical battle to balance the need for recurring flights and increased sample size of collared fawns to effectively determine cause-specific mortality of fawns. Fortunately, however, our long-term work in the eastern Sierra Nevada has been supported by numerous conservation organizations, including the Mule Deer Foundation (MDF). The California Department of Fish and Game has provided the base funding for over 10 years, but it is the support of organizations such as MDF that have made it possible to conduct meaningful investigations. Significant funds have also been provided by the Granite Bay Chapter of Safari Club International, California Deer Association, and other funds provided by organizations outside the department have been essential for the continuation of this project since its inception.