Jennifer A. Gervais

Space use and pesticide exposure risk of male burrowing owls in an agricultural landscape

We estimated home-range size and habitat selection in a population of burrowing owls (Athene cunicularia) living within an agricultural landscape in the Central Valley of California, USA, in 1998 and 1999. We modeled home-range size and habitat selection of breeding male owls (n = 33) as a function of biological and physical factors. Biological factors included number of young fledged and diet, and physical factors included cover-type composition around the nest. We also examined patterns of space use in conjunction with agricultural pesticide application records for evidence of secondary poisoning risk to the owls. Owl home ranges varied in size within (but not between) years, and not in conjunction with any of the biological factors we measured. Foraging versus random locations were differentiated most strongly by distance from the nest, with 80% of nocturnal foraging observations falling within 600 m of the nest burrow. No single cover type was selected when distance to nest was also included in the model. Owls did use agricultural fields recently treated with pesticides, although we did not find evidence of owls selectively foraging in these fields.

The Potential for Seed Dispersal by the Banana Slug (Ariolimax columbianus)

Abstract We observed wild banana slugs (Ariolimax columbianus) eating fruits of several Pacific Northwest plant species. Slime trails and direct observations indicated that slugs are capable of reaching the fruits of many wild plants. This motivated us to test the hypothesis that slugs may act as seed dispersers, provided that they defecate viable seeds. We fed captive slugs the fruits of Rubus spectabilis, R. discolor, Vaccinium ovatum, V. parvifolium, Gaultheria shallon and Disporum smithii to determine the effects of slug ingestion on seed germination. At least some seeds of each species germinated after the fruits were consumed by the slugs, but the effects on germination were species-specific. Seeds of Rubus spectabilis were less likely to germinate after passage through the guts of slugs, and we found significant evidence that the two fruit color morphs reacted differently over time. Disporum smithii seeds did not statistically differ in germination behavior between treatments, although the trend suggested possible germination enhancement following rasping of the seeds by slugs. All other species of seeds tested germinated following consumption by slugs, but results could not be tested statistically. Gut passage times of R. discolor seeds were determined (x̄ = 25.4 h, se = 3.6 h). We conclude that despite the short distances slugs are likely to disperse seeds, their generalist habits and ubiquity suggest that they may have complex and ecologically significant effects on seed dispersal in Pacific Northwest forests.

Avian selection of the color-dimorphic fruits of salmonberry, Rubus spectabilis : a field experiment

Although the mutualism between frugivorous animals and the fruiting plants whose seeds they disperse has been noted for over a century, it remains unclear whether animal selection has had any part in the evolution of fruit characteristics. We conducted field experiments using the color-dimorphic fruits of salmonberry (Rubus spectabilis) to determine whether free-ranging birds choose fruits on the basis of color, and if so, whether these patterns indicated the potential for birds to exert evolutionarily significant selective pressure on this fruit trait. Birds consistently selected red over orange fruits in experimental displays in the field despite wide geographic variation in fruit-color frequencies, fruit-crop densities, and numbers and species composition of avian frugivores in Oregon and Alaska. This is the first field study demonstrating significant and consistent fruit-trait selection by birds at a scale relevant to coevolutionary processes. These results indicate that forces other than animal selective pressure are also shaping the occurrence of fruit color traits in bird-dispersed fruiting plants.

CHRONIC ORGANOCHLORINE CONTAMINANTS, ENVIRONMENTAL VARIABILITY, AND THE DEMOGRAPHICS OF A BURROWING OWL POPULATION

We studied a population of Burrowing Owls whose eggs contained the or- ganochlorine compound p,p9DDE and traces of other organochlorine contaminants to de- termine if the levels of contamination were associated with survival or reproduction when nonanthropogenic environmental and biological variables were also considered. Demo- graphic data were collected in conjunction with sampling eggs for contaminants and an- alyzing pellets for dietary information. Levels of p,p9DDE in eggs varied over four orders of magnitude during the study but were not by themselves associated with reproductive failure. However, contaminant concentrations in combination with low rodent abundance in the diet were related to reduced productivity. The variation within and among years in egg contaminants suggests that patterns of egg contaminants are the result of immigrating owls from more contaminated sites, and to a lesser extent, to annual patterns in prey availability. Even low levels of chronic pesticide exposure may be detrimental when com- bined with other stressors, and documentation of the existence of a persistent pesticide in a biotic system is not enough to either infer the origin of the contamination or its potential

Estimation of reproductive rates of burrowing owls

Obtaining reliable estimates of absolute and relative reproductive rates is challenging for avian species whose nests are difficult to observe, such as the burrowing owl (Athene cunicularia). We compared methods for estimating reproductive rates of burrowing owls, defined as the number of 21- to 28-day-old young per successful nest. We compared observations using (1) the mean and (2) the maximum number of young observed during 5 30-min observation periods, and (3) the maximum number of young videotaped during 2-hr video surveillance. We evaluated the reliability of these methods with the known number of young present in nest boxes. All 3 methods performed poorly as estimators of absolute reproductive rates (absolute bias >23%, root mean square error [RMSE] >42%). Video surveillance performed most poorly of the 3, with a high incidence of failing to detect any young at successful nests. The maximum number of young observed from direct nest observations was correlated with the known number of young (r=0.82 ± 0.13, n = 21) and provided more reliable estimates of relative than absolute reproductive rates. The mean number of young observed from direct observations was correlated with the known number of young (r = 0.64 ± 0.18, n = 21), but had both higher bias and lower precision than the maximum number observed for estimation of relative reproductive rates. Our results suggest that using counts of young observed outside of the nest burrow may lead to incorrect conclusions on factors affecting reproductive rates. When counts are the basis of inference, the effort researchers use at each nest should be standardized and reported. Further work on field methods that allow estimation of detection probability, or ensuring that all young are observed, will be imperative in providing reliable estimates.

Radiotransmitter Mount Type Affects Burrowing Owl Survival

JENNIFER A. GERVAIS, Department of Forest, Range, and Wildlife Sciences, Utah State University, Logan, UT 84322, USA DANIEL H. CATLIN, Department of Forest, Range, and Wildlife Sciences, Utah State University, Logan, UT 84322, USA NATHAN D. CHELGREN, Department of Forest, Range, and Wildlife Sciences, Utah State University, Logan, UT 84322, USA DANIEL K. ROSENBERG, Department of Forest, Range, and Wildlife Sciences, Utah State University, Logan, UT 84322, USA

Effects of gray-tailed vole activity on soil properties.

Abstract Voles are well-known crop pests, especially when peak populations are present, but their role in soil fertility and impacts on agricultural sustainability are not well understood. Five months after the abrupt disappearance of a peak in a gray-tailed vole (Microtus canicaudus) population, we examined burrow structure, determined concentrations of trace elements, carbon and nitrogen in the soil immediately surrounding vole burrows, and compared soil chemical properties to a depth of 90 cm between areas with prior vole activity and areas of no activity. Vole tunneling activity was confined to the top 10 cm of the soil profile and was coincident with the majority of root biomass. Soil NH4+, NO3-, extractable organic carbon, and soil organic matter were greater below vole tunnels than above; however, due to small sample sizes, differences were not significant. There were no differences in trace elements with respect to position around vole tunnels. Vole activity was associated with increased soil nitrate concentrations and decreased soil pH to a depth of 90 cm, indicating that nitrification might be enhanced by vole activity, and that this effect continues after vole populations crash. Greater inorganic nitrogen could have long-term effects on ecosystem productivity. The effects voles have on soil processes that influence carbon and nutrient cycle requires further investigation.

Testing sign indices to monitor voles in grasslands and agriculture.

Abstract I evaluated the use of sign indices as indicators of relative vole population abundances in grasslands and agricultural systems in western Oregon grasslands. The development of a reliable index based on vole sign that does not rely on the repeated use of traps would greatly aid rapid assessment of relative population densities, and allow greater flexibility in both research and management. I tested the presence and number of burrows, runways, droppings, and damaged vegetation along transects and in quadrats to evaluate each metrics correlation to estimated population size. Vole population size was estimated with mark-recapture techniques. None of the indices performed well, particularly in situations where plots were mowed. The number of animals captured on the first trapping occasion was most correlated with estimated population size. Indices for vole abundance should be tested in the system and with the species of interest prior to their use in research or management.

Winter Fruit Removal in Four Plant Species in Maine

We monitored individual plants of winterberry (Ilex verticil/ala), highbush cranberry (Viburnum opulus), wild lily-of-the-valley (Maianthemum canadense), and partridgeberry (Mitchel/a repens) during two reproductive seasons to measure the disappearance of fruits (presumably removed by fruit-eating animals) in plant species whose fruits ripen in early fall but persist through the winter. Patterns of fruit removal were variable between species, individuals, and years, but in general removal rates for all species were slow and relatively constant. Only Ilex fruits showed visible deterioration during the winter. Exclosures placed around Maianthemum plants in the field and preliminary feeding trials suggested that mice (Peromyscus sp.) and other small mammals may remove most fruits and disperse many viable seeds of Maianthemum, as well as other low-growing plants with persistent fruits, such as Mitchel/a. In the temperate zone the overwhelming majority of vertebrate-dispersed plant species ripen their fruits during the late summer and early autumn, a time which corresponds with the peak of fall migration of fruiteating birds (Burger 1987; Sherburne 1972; Skeate 1987; Stiles 1980; Willson 1986). As a consequence, in many species most fruits are removed over a short period (Sargent 1990). However, a small number of plant species retain their fruits through much of the late fall and winter (Borowicz and Stephenson 1985; Burger 1987; Jones and Wheelwright 1987; Sallabanks 1992; Stiles 1980). Such fruits can persist in the face of microbial attack and damage from freezing because of their low lipid content and the presence of secondary compounds (Cipollini 1993; Herrera 1982; Stiles 1980), but those same compounds presumably reduce the attractiveness of the fruits to birds and slow their removal rates (Herrera 1982; Jones and Wheelwright 1987; Sorensen 1983). Moreover, the diversity and abundance of active seed dispersers are relatively low during the winter; plants must rely on resident birds and mammals for seed dispersal, or maintain fruits until spring migrants reappear. The disadvantages of presenting fruits during the winter months are presumably outweighed by the benefits of avoiding the season of heaviest damage to fruits and seeds by microbes, fungi and insects (Herrera 1982), and minimizing competition with other fruiting plants for a limited number of avian seed dispersers (McKey 1975; Wheelwright 1985). In this study we examined disappearance rates of fruits of four wood* Department of Biology, Bowdoin College, Brunswick, ME 04011 (current address: Department of Wildlife, Humboldt State University, Arcata, CA 95521 (for reprints, write to N. T. W.)); ** Department of Biology, Bowdoin College, Brunswick, ME 04011. 16 Maine Naturalist Vol. 2, No.1 land plant species in southern Maine. The species -winterberry (/lex verticillata), highbush cranberry (Viburnum opulus), wild lily-:of-thevalley (Maianthemum canadense), and partridgeberry (Mitchella repens) -differ in habitat, secondary compounds, and fruit presentation patterns, but each produces persistent fruits eaten by vertebrates. Exclosures were used to determine the relative importance of birds, small rodents, and natural abscission in accounting for fruit disappearance in Maianthemum canadense. Preliminary feeding trials were conducted to clarify the role of mice (Peromyscus sp.) as potential seed dispersers or seed predators. METHODS AND STUDY SPECIES Our main study site was a mixed deciduous forest in North Yarmouth, Maine. Viburnum opulus was studied at a second site, in Brunswick, Maine, 20 km (12 mi) to the east. Censuses were conducted approximately weekly from late September, 1987 until low plants were covered by snow in December, and resumed at less frequent intervals from snow-melt until late April, 1988. Censuses resumed in early September, 1988 and continued at biweekly intervals until snowfall in mid-December. We refer to the September, 1987 to April, 1988 season as 1987, and the September, 1988 to December, 1988 season as 1988. Viburnum opulus L. (Caprifoliaceae), a shrub found along forest edges and in the understory, produces 50-500 bitter, bright red drupes on terminal panicles which persist through the winter (Sherburne 1972; Jones and Wheelwright 1987). The fruit, whose diameter averages 11.0 mm (SD [:t] = 0.5; N = 25), encases a single flattened seed (7.8 x 7.1 x 1.8 mm, N= 10). We found five shrubs of similar height (1.5-2.0 m) and fruit crop (300-400 fruits) growing along a dirt logging road in the forest. On each plant four infructescences, one from each side of the plant, were marked at the base of the panicle with a 1 x 5 cm piece of blue flagging. An average of 7.8 fruits were plucked from each infructescence to reduce it to 20 fruits and standardize infructescences (initial crops sizes of marked inflorescences in 1987: 27.8:t 7.9). Fruit crop sizes were smaller in fall 1988 (initial crops sizes: 17.6:t 5.5). Accordingly, in that period some infructescences were used with fewer than 20 fruits to repeat observations on the same shrubs (1987: N = 400 fruits; 1988: N = 313). Censuses began in both years before many fruits had been removed (empty pedicels per inflorescence: 1987: 1.3:t 1.8; 1988: 0.6:t 1.6). Ilex verticillata (L.) Gray (Aquifoliaceae), a dioeciouswetland shrub, grows to a height of 2.5 m and produces 10-1000 sessile orange-red drupes along its branches. The fruits, which are relatively dry, measure 7.7 mm in diameter (:t 0.5; N = 25), with .47 seeds 3.3-4.1 mm in length. Ten shrubs were haphazardly selected along a 200 m stretch of a swampy power line roughly 75 m wide, edged by mixed coniferous and deciduous 1994 J. A Gervais and N. T. Wheelwright 17 trees. On each plant, two branches bearing at least 20 fruits each were chosen. The number of fruits on each branch was reduced to 20, and branches were inconspicuously marked with twined grass at the base. In fall 1988, the same shrubs were censused (1987: N = 400 fruits; 1988: N = 411). Mitchella repens L. (Rubiaceae), a low-growing woody creeper of the forest floor, presents its red fruits singly in leaf axils. Its drupes, which have low water content, measure 7.8 x 6.4 mm (.:t 0.5; N= 6) and contain 4-7 seeds 2.9 mm in length (.:t 0.2; N = 20). Six plants growing beneath mixed hardwoods and conifers with little understory growth were haphazardly chosen and marked by blue flagging 1 m away. Plants were similar in size, covering about 0.5 m2 of ground, and each bore 2328 fruits (1987: N =157 fruits; 1988: N = 170). Once snow covered the plants in December, plants were not censused to avoid artificially exposing fruits to foraging animals. Maianthemum canadense Desf. (Liliaceae) is a clonal herb 4-15 cm in height (Worthen and Stiles 1986). Fruits are presented on a single raceme per plant (technically, a ramet or physiologically independent portion of the clone) and number 6-15 per infructescence. The mean fruit diameter was 5.9 mm (.:t 0.8; N = 12) with 1-2 seeds pe}:fruit averaging 3.5 mm in length (.:t 0.4; N = 16). In 1987, ten singly occurring fruiting ramets were located roughly 10 m apart along an old logging road in mixed forest. Each plant had 8-11 fruits (N= 89). In 1988,25 ramets (6-15 fruits each, N = 239) were monitored at the same site. Plant locations were marked by tying 8 cm of cotton string to sticks placed 20 cm away. Temporal patterns of disappearance of fruits as well as the firm attachment of fruits (periodically tested by shaking branches) and the general lack of fruits, seeds or pedicels beneath plants suggested that most fruits were removed by animals rather than simply abscised by plants. To determine the relative contributions of birds, small mammals, and natural abscission by the plant to fruit disappearance, in 1987 we constructed exclosures of 0.64 x 0.64 cm wire netting cut and folded into cubes 10 cm on a side. Ten exclosures had no access holes, and 10 had 4.5 cm square holes cut into each side but not the top. A large dense patch of fruiting Maianthemum was located and 30 plants haphazardly selected. The plants were divided into three fruit crop size categories (6-10, 11-13, and >13 fruits) then randomly spit into three groups. The first groups plants were covered by the complete exclosures (N = 86 fruits), the second group covered by the partial exclosures (N = 90 fruits), and the final group left as controls (N = 91 fruits). A mouse (Peromyscus sp.), captured in December, 1987 and maintained on an ad libitum diet of millet and sunflower seeds, was used in two feeding trials to explore the role of small mammals as seed dispersers of Maianthemum. Trials consisted of leaving only Maianthemum