Academic journal article The American Midland Naturalist

Sex Ratio of Rodents as Barn Owl (Tyto Alba) Prey

Academic journal article The American Midland Naturalist

Sex Ratio of Rodents as Barn Owl (Tyto Alba) Prey

Article excerpt


Research on the sex ratio of rodents has included examination of sex ratios of rodents identified in owl pellets (e.g., Temían and Sassaman, 1967; Myers and Krebs, 1971; Clark and Wise, 1974; Holt and Williams, 1995). Some authors have suggested owls select one sex of prey over the other (e.g., Dickman el al., 1991), whereas other authors have suggested unequal sex ratios of prey in owl pellets may be due to sexual differences in susceptibility to predation (e.g., Boonstra, 1977; Longland and Jenkins, 1987). Herein we describe the sex ratios of the two most abundant prey taxa in a sample of - 120 barn owl (Tyto alba) pellets.

Pellets were collected from a corrugated tin farm-equipment shed enclosed on three sides and the top with an opening approximately 15 m wide and 5 m high. Located in southeastern Washington state (46°19.591'N, 118°00.182'W), the shed sat on an artificially leveled area in the bottom of a shallow, steep-sided, south-draining canyon. Vegetation in the canyon included a dense stand of ~2 m tall grass (Agropyron sp., Bromus sp., Poa sp.) with a few scattered trees (Pinus sp.), shrubs (Rosa sp.), and forbs. Most of the canyon slopes were used as horse pasture from the 1970s through 2010. All ridges defining the canyon edges were wheat fields in production from the 1970s through the 1990s. For purposes of soil conservation, in the autumn of 2000 the northwest-ridge field, approximately 20% of the land area (25 ha) in the immediate environment of the shed, was placed in a soil bank and remaining wheat stubble was seeded to bluebunch wheatgrass (Pseudoregneria spicata), slender wheatgrass (Agropyron trachycaulum), big bluegrass (Poa ampia), and Indian ricegrass (Oryzopis hymenoides). By July 2001, this field had a patchy stand of grass. Areas of partially deteriorated field residue, without green vegetation cover, existed on ~30% of the field. By 2006, most of the field residue had deteriorated or been removed by wind and relatively continuous grass cover existed over -95% of the field.

Pellets were collected in 1999, 2000, 2001, 2009, and 2011. We refer to the combined 1999 and 2000 pellets (n - 63) as the "agricultural" sample and the combined 2001, 2009, and 2011 pellets (n - 57) as the "soil bank" sample. Previous analyses of the fauna in the owl pellets found (i) the most abundant prey were deer mice (Peromyscus maniculatus) and microfines (Microtus spp.) (Table 1), (ii) most of these two taxa were skeletally immature individuals (Lyman et ai, 2001; Lyman, 2012), and (iii) the difference between the agricultural sample and the soil bank sample involved a shift from deer mice as the most abundant taxon in the former to voles in the latter (Lyman, 2012). This aligns with local habitat usage by voles and deer mice (e.g., Randall and Johnson, 1979; Kaufman et ai, 1988; and Falls et ai, 2007, respectively). It also reflects the fact that barn owls are opportunistic feeders (Smith et ai, 1972; Marti, 1988; Marra et ai, 1989), therefore remains of prey in their pellets tend to reflect the small mammal fauna on the landscape (Hadly, 1999; Terry, 2010; Heisler et ai, 2016).

We did not know what the rodent population on the landscape around the shed was like. We initially suspected females would be more abundant than males in the general populations of both taxa as a result of agricultural tillage destroying nests, cover, and burrow systems (e.g., Witmer et ai, 2007) and surviving females adjusting their reproduction to favor females in order for the population to increase (or at least remain static). Previous research showed that microtines tend to produce more females than males when population density is low (Lambin, 1994; Bond el al, 2003; Bryja et al, 2005). After tillage was reduced by converting some land to soil bank, and population density increased, we suspected the sex ratio would become more even.

Methods and Materials

Upon collection, pellets were placed in plastic bags and transported to the Zooarchaeology Laboratory at the University of Missouri-Columbia (MU), where they were stored at room temperature and numbered. …

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