Using Movement Behavior Of Adult Female Moose To Estimate Survival And Cause-Specific Mortality Of Calves In A Declining Population

Abstract

Low calf production and survival can contribute to ungulate population declines and sustained low numbers. Continuing research on cause-specific mortality and annual survival of moose (Alces alces) calves in northeastern Minnesota is critical to understanding the long-term trajectory of the population. Beginning in late April 2016, 35 global positioning system (GPS)-collared adult females were computer-monitored for calving movements, a long distance movement followed by an intense localization. I observed 28 of 33 (85%) females make a calving movement, and with additional field observations of calves, determined 31 of the 35 (89%) females were pregnant. Mean birth-date was 12 May 2016 (median = 11 May 2016, range = 24 Apr ̶ 10 June. In 2013–2014, I observed an unusual behavior by dams, a “mortality movement,” associated with the death of their neonate. The dam made a rapid, long-distance movement (“flee”), followed by a return to the calf mortality site. In 2016, using this behavior as a calf mortality indicator, I documented evidence of 15 mortalities at a mean age of 30.6 days (± 15.5 [SE], range = 3–243 days). Based on observations of mortality movements, 21 investigations were launched and resulted in confirmation of 11 of the 15 calf mortalities. Specific causes of mortality included 9 wolf (Canis lupus)-kills, 3 black bear (Ursus americanus)-kills, 1 unknown predator-kill, and 2 deaths following vehicle collisions. Predation accounted for 87% of all mortalities. The cumulative probability of calves being killed by wolves and bears was 33% and 11%, respectively. The mean distance dams fled after a mortality was 1,873 m (± 412, range = 126–5,805 m, n = 14). Dams that made return visits returned a mean 2.8 times (± 0.5, range = 1–5, n = 8) to the mortality site. Dams returned to within a mean 106 m (± 22, range = 34–230 m, n = 8) from the mortality site. Calf survival to 30 days of age was 67% (± 8, 95% CI = 53–84%, n = 36). Survival declined to 33% (± 8, CI = 20–53%, n = 36) at almost 1 year of age, associated with an estimated calf:cow ratio of 0.35. I developed 2 population-level movement models to improve efficacy of the mortality movement for identifying calf mortalities during the first summer of life. The first approach, a temporal-based model, used a single, estimated movement velocity threshold (164 m/hr) for the dam to indicate calf mortality and accurately predicted survival status 51% of the time. My second approach, an age-specific model, using different thresholds (28–135 m/hr) for the dam relative to calf age, was 80% accurate. Using movement behavior of dams to assess calf mortality can yield important insights into mechanisms related to the decline of the population in northeastern Minnesota and aid in future management decisions.