Comparing the diet of Great Horned Owls (Bubo virginianus) in rural and urban areas of southwestern British Columbia

We investigated the diet of Great Horned Owls (Bubo virginianus) in southwestern British Columbia. Our objective was to compare the diets of owls in urban and rural areas and determine whether urban owls consume a higher proportion of commensal rodents to understand possible pathways of secondary rodenticide poisoning of Great Horned Owls. Among 546 prey items identified at seven sites, Townsend’s Vole (Microtus townsendii [Bachman, 1839]) and rats (Rattus G. Fischer, 1803) were the two main prey items, making up 65.9% and 13.1% of the diet, respectively. The proportion of rats in the diet was positively correlated with the degree of urban development in the owls’ home range (rp = 0.83, P < 0.05, df = 5).

Risk of parasite-induced predation: an experimental field study on Townsend's voles (Microtus townsendii)

Studies of causes of mortality in rodents, snowshoe hares, and red grouse show that many individuals succumb to predation and few die from starvation or parasite infestation. Predation may be the proximate cause of death and factors like parasite infestation and starvation could facilitate predation. Our aim was to test whether Townsend's voles, Microtus townsendii, experimentally rid of parasites are less likely to be killed by predators than are control voles. During two breeding seasons we treated half of the adults caught at Boundary Bay, British Columbia, Canada, with the anthelmintic Ivomec® and left the other half as controls. Voles were randomly assigned to the treatment and control groups. Predators killed 5 of 23 treated voles and 11 of 26 control voles in 1998. In 1997 predators killed 3 of 17 treated voles and 2 of 18 control voles. Vole density was higher in 1998 than in 1997 and predation was the main cause of mortality in both years. Survival of control voles was lower in 1998 than in 1997, indicating lower predation pressure in 1997 than in 1998. Botfly prevalence and intensity were low and similar in both years. We speculate that the lack of treatment effect in 1997 could have been due to lower overall parasite prevalence in that year than in 1998, resulting in relatively higher susceptibility of voles to predation. Parasite-induced predation reduced monthly survival by 17% in 1998.

Predation on seabird eggs by Keen's mice (Peromyscus keeni): using stable isotopes to decipher the diet of a terrestrial omnivore on a remote offshore island

We used stable isotope techniques to analyze tissues of Keen's mice (Peromyscus keeni) and Townsend's voles (Microtus townsendii cowani) and a subset of prey items at Triangle Island, British Columbia, western Canada's largest seabird colony. Isotope analysis allowed us to investigate the importance of seabird prey in rodent diets in a system where seabirds and non-introduced rodents occur sympatrically. The δ15N values for terrestrial plants and terrestrial invertebrates on Triangle Island exceeded levels found in many terrestrial biomes and are typical of localities with high inputs of marine-derived N. We used multiple-source mixing models to estimate the relative inputs of potential prey items to vole and mouse diets. The δ13C and δ15N values of liver and muscle tissues of voles indicate that voles on Triangle Island derived their protein primarily from terrestrial plants, with some contribution by terrestrial invertebrates. In contrast, isotopic values of liver and muscle tissues of mice on Triangle Island indicated that mice prey primarily on seabird eggs and terrestrial invertebrates. Our results show that egg predation on Triangle Island is a general phenomenon in the mouse population, rather than occurring in only a few specialist feeders. Mice appear to feed on eggs once they become available and continue to utilize seabird prey, likely in the form of abandoned eggs or carcasses of chicks and adults, throughout the breeding season.