DeBow, J., J. Blouin, E. Rosenblatt, K. Gieder, J. Murdoch, and T. Donovan. 2023. Birth Rates and Calf Survival in a Parasite Rich Moose Herd in Vermont, USA. Alces 58: 51–73.
Abstract
Moose (Alces alces) populations have recently declined across the southern portion of their geographic range in North America. In Vermont and other northeastern U.S. states, declines have been attributed to low survival rates driven primarily by winter tick (Dermacentor albipictus), which may also affect birth rate and survival of new calves. We radio-collared and monitored 75 female moose (38 calves and 37 adults) in Vermont from 2017 to 2020 and examined the effects of physiological conditions and spatial and temporal factors on adult birth rates. Physiological measures included fecal glucocorticoid metabolites concentrations (fGCM), which reflect stress, and urine urea nitrogen:creatinine ratio (UN:C), which proxies nutritional state. The pregnancy rate at capture across years was 0.67 (95% CI = 0.50 – 0.80), and negatively influenced by the presence of lungworm (Dictyocaulus spp.). Birth rates, calculated as the average number of offspring delivered per adult female, were < 1.0 overall, did not differ among years (2017-2020, LCI = 0.22, UCI = 0.86), but differed by adult age class, where rates increased with age. We further evaluated daily calf survival rates to age 60 days, the point at which calves become independent of direct care. Logistic exposure models indicated that daily calf survival increased as Julian birth date and days since birth increased (log odds = 0.0819, SE = 0.0215). The per capita independence rate (the rate at which adult females add independent calves to the population) was negatively influenced by average UN:C ratios and positively influenced by fGCM levels. Further, this rate was related to the habitat conditions in home ranges of adult females during the fall, when ticks attach to moose. Specifically, female adults whose fall home ranges were characterized by high levels of mature (canopy) evergreen forests and wetland habitats, and low levels of mixed forests and elevation, had a higher, average binomial success rate in adding an independent calf into the population than those whose home ranges consisted of high levels of mixed forest at high elevation. Our results suggest that winter ticks negatively affect fecundity, and that efforts to reduce parasite loads on individual moose (e.g., directly by reducing moose density or indirectly through habitat alteration) may improve productivity and recruitment.