Showing papers by "Keith A. Hobson published in 2001"

Influence of chemical and biological factors on trophic transfer of persistent organic pollutants in the northwater polynya marine food web.

Abstract: Persistent organic pollutants (POPs) and stable isotopes of nitrogen (δ15N) were measured in zooplankton (6 species), a benthic invertebrate (Anonyx nugax), Arctic cod (Boreogadus saida), seabirds (6 species), and ringed seals (Phoca hispida) collected in 1998 in the Northwater Polynya to examine effects of biological and chemical factors on trophic transfer of POPs in an Arctic marine food web. Strong positive relationships were found between recalcitrant POP concentrations (lipid corrected) and trophic level based on stable isotopes of nitrogen, providing clear evidence of POP biomagnification in Arctic marine food webs. Food web magnification factors (FWMFs), derived from the slope of the POP−trophic level relationship, provided an overall magnification factor for the food web but over and underestimated biomagnification factors (BMFs) based on predator−prey concentrations in poikilotherms (fish) and homeotherms (seabirds and mammals), respectively. Greater biomagnification in homeotherms was attribute...

Use of stable-carbon and -nitrogen isotopes to assess weaning and fasting in female polar bears and their cubs

Abstract: In some species, stable-isotope techniques can provide insights into dietary regimens where there are temporal shifts in trophic level or feeding frequency. We determined stable carbon (δ13C) and nitrogen (δ15N) isotope values for plasma and milk proteins and δ13C values for milk lipids from female polar bears (Ursus maritimus) and cubs to (i) ascertain whether cubs are at a higher trophic level than their mothers as a result of nursing and whether we can determine when weaning occurs, and (ii) determine the impact of seasonal fasting on δ13C and δ15N values. The plasma δ13C values for mothers and cubs were similar to milk-protein δ13C values and were significantly enriched in 13C compared with those for milk lipid. Plasma from cubs of the year (COYs) in spring, when milk was their only diet, was isotopically enriched in 15N by 1.0‰ over that of their mothers (δ15N = 21.5 ± 0.8‰ (mean ± SD) for cubs and 20.5 ± 0.5‰ for mothers) and depleted in 13C by 0.8 ‰ (δ13C = –19.6 ± 0.5‰ for cubs and –18.8 ± 0.8‰ fo...

Hexachlorocyclohexane (HCH) Isomers and Chiral Signatures of α-HCH in the Arctic Marine Food Web of the Northwater Polynya

Abstract: Concentrations of hexachlorocyclohexane (HCH) isomers (α, β, and γ) and enantiomer fractions (EFs) of α-HCH were determined in the Northwater Polynya Arctic marine food web. Relative food web structure was established using trophic level models based on organic δ15N values. Concentrations of HCH in the samples collected, including water, sediment, benthic invertebrates (four species), pelagic zooplankton (six species), Arctic cod, seabirds (seven species), and ringed seal, were in the range previously reported for the Canadian Arctic. The relative proportion of the HCH isomers varied across the food web and appeared to be related to the biotransformation capacity of each species. For invertebrates and fish the biomagnification factors (BMFs) of the three isomers were >1 and the proportion of each isomer and the EFs of α-HCH were similar to water, suggesting minimal biotransformation. Seabirds appear to readily metabolize γ- and α-HCH based on low BMFs for these isomers, high proportions of β-HCH (62−96%),...

Stable carbon, nitrogen, and sulfur isotope ratios in riparian food webs on rivers receiving sewage and pulp-mill effluents

Abstract: We examined spatial trends in ratios of stable isotopes of nitrogen (δ15N), carbon (δ13C), and sulfur (δ34S) in riparian food webs leading to a riparian, insectivorous bird species, the tree swallo...

A stable-isotope approach to delineate geographical catchment areas of avian migration monitoring stations in North America.

Abstract: Migration monitoring stations (MMSs) were established to provide information on population trends of North American passerines. However, apart from inferring general origins of birds, there has been no way to delineate geographical catchment areas sampled by MMSs. The ability to resolve MMS catchment areas would greatly enhance our ability to link and constrain population declines to specific geographical areas and thereby focus conservation efforts. Here, we utilize stable-hydrogen (δD) and carbon (δ13C) isotope values of rectrices of fall hatch-year (HY) and spring adult after-second year (ASY) Swainson's Thrush (Catharus ustulatus) moving through two MMSs in Canada to determine natal and molting catchment areas for those stations. Stable-carbon isotope signatures provided no information on geographical origins of birds. Conversely, δD signatures provided good latitudinal geographical control, and this was also supported by analysis of feathers from two other species, Gray-cheeked Thrush (Catharus minim...

Effects of habitat fragmentation on pairing success of ovenbirds: importance of male age and floater behavior

Abstract: Between 1996 and 1998, we compared pairing success of territorial male Ovenbirds (Seiurus aurocapillus) in forest fragments created by forestry (n = 3) and agriculture (n = 10) to contiguous forest plots (n = 3) in the southern boreal mixedwood forest of central Saskatchewan. The percentage of Ovenbird males paired per site was lower in fragments created by agriculture (86 ± 3%) and forestry (87 ± 3%) than in contiguous forest (97 ± 3%). At the individual level, second-year males (82%) were less likely to be paired than after-second-year males (94%), whereas males closer to edges were less likely to be paired than those in forest interiors. Although pairing success differed among landscapes, those differences were smaller than reported in studies conducted in eastern North America. The high density of birds in our study area may have resulted in intense intraspecific competition, which could have prevented unpaired individuals from maintaining territories. Removal experiments in 1997 and 1998 dem...

Linking breeding and wintering grounds of bicknell's thrushes using stable isotope analyses of feathers

Abstract: Previous studies have shown that natural abundance of deuterium (D), and to a lesser extent 13C, in feathers of migrant songbirds in North America can be used to infer geographic origins of molt. We used that approach to investigate whether Bicknell's Thrush (Catharus bicknelli) populations could be segregated on the breeding grounds at six sites in eastern North America to evaluate contributions from the breeding areas to wintering populations. Further, we tested our isotopic model using feathers from hatching-year migrant Swainson's Thrush (Catharus ustulatus) and Gray-cheeked Thrush (Catharus minimus) moving through southern Manitoba and Ontario and found that δD values were consistent with northerly boreal forest natal sites in northwestern and central Canada. Despite a strong latitudinal gradient in δD of average growing season precipitation over the breeding range, we found considerable overlap in δD values of Bicknell's Thrush among all breeding sites. No strong pattern in δ13C values of f...

Diet of ringed seals (phoca hispida) on the east and west sides of the north water polynya, northern baffin bay

Abstract: In conjunction with the International North Water Polynya Study (NOW) in northern Baffin Bay, we examined the diets of ringed seals (Phoca hispida)' on the west (Grise Fiord, Nunavut) and east (Qaanaaq, Greenland) sides of the polynya, using conventional stomach content analysis, as well as inferences from stable isotope ratios in seal muscle. Between May and July 1998, stomach and muscle tissue samples were collected from 99 ringed seals taken near Grise Fiord and 100 taken near Qaanaaq. The amphipod Themisto libellula was the dominant prey type in the diet of immature ringed seals from Grise Fiord, whereas arctic cod (Bweogadzls saida) and polar cod (Arctogadus glacialis) predominated in the diet of adults. Both immature and adult seals collected near Qaanaaq fed predominantly on arctic cod. Overall, seals collected near Grise Fiord had significantly higher a13C values than those collected near Qaanaaq (P < 0.001), but there was no statistical separation in 6'5N values between the two samples (P = 0.06). Differences in diets of ringed seals from

Sources of protein in two species of phytophagous bats in a seasonal dry forest: evidence from stable-isotope analysis

Abstract: The relative importance of plants and insects as sources of protein through the year was evaluated in 2 bat species in a seasonal forest in the neotropics using stable-isotope analysis of carbon and nitrogen. Artibeus jamaicensis, a frugivore, met almost all of its protein requirements from plants with no seasonal or sexual variation. In contrast, Glossophaga soricina, a nectarivore, relied mostly on insects, but in females, plants and insects were equally important in the early rainy season and in the mid dry season. Evidence of changes in food origin of protein associated with reproductive activity was not detected in either species.

The Role of Fruits and Insects in the Nutrition of Frugivorous Bats: Evaluating the Use of Stable Isotope Models1

Abstract: We estimated the relative contribution of fruits and insects as sources of dietary protein in two species of Neotropical frugivorous bats (Artibeus jamaicensis and Sturnira lilium) using stable carbon and nitrogen isotope analyses. An insectivorous species (Pteronotus parnellii) was also included for comparison. We found constant patterns in stable carbon and nitrogen isotope composition in blood that separated the two species of frugivorous bats from the insectivorous bat. When we used these isotopic values (combined with those of dietary fruits and insects) to estimate the percent contribution of fruits and insects to the diet of the bats, we obtained different results, depending on assumptions and model adopted. We tested models using both δ15N and δ13C results simultaneously and separately and further used diet-tissue fractionation factors of 3‰ for nitrogen and 1 and 3.5‰ for carbon. We found that a carbon-based model with a diet-blood enrichment factor of 3.5‰ produced the most parsimonious...

Isotopic delineation of north american migratory wildlife populations: loggerhead shrikes

Abstract: The ability to link breeding and wintering sites of declining or threatened species is fundamental to their effective conservation, but previous conventional methods such as banding have proved largely unsuccessful at the population level. New approaches to tracking migratory birds in North America, using stable isotope measurements of feathers, can assist in determining where feathers were grown and, hence, the origins of migrants on wintering grounds or stopover sites. Moreover, this approach is applicable to the use of metabolically inactive tissues of migratory animals other than birds. Here, we demonstrate how stable isotopes can be used to investigate the origins of a wintering population of a threatened species. The Loggerhead Shrike (Lanius ludovicianus) is declining throughout most of its range, but various factors may operate on different subpopulations. We used the stable-carbon (813C) and hydrogen (8D) isotope values in the outer tail feather of Loggerhead Shrikes wintering in Texas (n = 70), Florida (n = 121), and Georgia (n = 24) in 1997-1998, and northern Mexico (n = 72) in 1999, in order to ascertain the minimum percentage of wintering birds that were northern migrants. For comparative purposes, we also measured stable isotopic compositions of feathers from nestling shrikes from Manitoba (n = 6) and from known resident shrikes from Texas (n = 16). As expected, shrikes from Manitoba were more depleted in deuterium than shrikes from Texas, providing a clear isotopic gradient with which to compare the wintering population structure. Of wintering shrikes in Florida, 10% were northern migrants, compared with 8% for the Mexican sample and only 4% of the Texas population. We detected no northern migrants in the Georgia population. Most U.S. wintering shrikes had AD values around-30%0, indicative of southern latitudes, but a broader range in AD values was found for shrikes in northern Mexico. Shrikes wintering in Mexico may be composed largely of individuals originating from the eastern Rockies and central regions of the breeding range. The considerable variation in bl3C values indicated a broad range of C3- to C4-based food webs used by shrikes. Our study indicates that the stable isotopic technique answers a number of long- standing and fundamental questions concerning the breeding or natal origins of wintering populations of Loggerhead Shrikes and thereby provides a template for isotopic approaches to ecological investigations of other migratory species.

Movement patterns of adult male ovenbirds during the post-fledging period in fragmented and forested boreal landscapes

Abstract: Movement of forest songbirds among isolated forest patches following breeding represents an important but poorly understood component of landscape ecology and metapopulation theory. Using radio-telemetry, we followed 44 male Ovenbirds (Seiurus aurocapillus) during the post-fledging period to determine if movement patterns differed in landscapes dominated by agriculture versus those dominated by forest. No differences in home-range size, mean distance moved per day, or maximum distance moved were observed for males captured in a forested landscape vs. those captured in forest fragments in an agriculturally dominated landscape. Male Ovenbirds observed with young moved less than males without young and rarely crossed open gaps. Individuals that failed to breed moved more extensively than successful breeders, possibly in an effort to find new territories for use in future breeding seasons.

Patterns of arthropod abundance, vegetation, and microclimate at boreal forest edge and interior in two landscapes: Implications for forest birds

Abstract: The effects of anthropogenic edges on leaf-litter and shrub arthropods were examined in both agriculture and forestry-dominated landscapes in central Saskatchewan, Canada. Microclimate, vegetation, and shrub and leaf-litter arthropods were sampled at south-facing forest edges and interiors of three sites within each landscape. Forest edges in the agricultural landscape were older and had vegetation structures that differed from those in the forestry landscape. This resulted in a sharper contrast in microclimate (temperature and humidity) between edge and interior in forests abutting agriculture than in stands abutting recent clearcuts. Arthropods from the leaf-litter and shrub-layer samples exhibited different responses to edges. In both landscapes, total shrub-arthropod abundance was not significantly affected by edges, while total leaf-litter arthropod abundance was lower at edges than in forest interiors. This reduced abundance of leaf-litter arthropods could negatively affect birds and other i...

Renal structure in neotropical bats: using stable isotopes to explore relationships between diet and morphology.

Abstract: The relationship between kidney structure and diet was examined in several species of bats from the tropics The trophic level of the bats was determined by their stable nitrogen isotopic composition (δ 15N) The results indicate that the relative thickness of the renal medulla is directly related to the trophic level of the bats Additionally, we also determined the relationship between body mass and kidney structure Relative medullary thickness was reciprocally related to body mass in animalivorous bats but no relationship was found in phytophagous bats Stable isotope analysis provided a quantitative approximation of diet to test its relationship with kidney structure

Predator responses to similarity and dispersion of artificial nest sites: implications for the structure of boreal forest songbird communities

Abstract: Relatively little is known about the role of predation in shaping patterns of coexistence and nest dispersion of songbird species. It has been hypothesized that predators diversify songbird communities by preying more heavily on individuals and species with greatest similarity in nest-site use. To investigate the importance of predation, we tested how predators responded to assemblages of artificial songbird nests that varied in nest-site placement, vegetation features, and nest dispersion patterns in boreal forest of west-central Alberta, Canada. Variability among nest sites was achieved by deploying wicker nests throughout a gradient of vegetation cover and by deploying nests to simulate two- and three-species assemblages. Two-species assemblages, comprising 20 simulated White-throated Sparrow (Zonotrichia leucophrys) and 3 simulated Hermit Thrush (Catharus guttatus) nests, and three-species assemblages, comprising 10 simulated White-throated Sparrow, 9 simulated Hermit Thrush, and 4 simulated ...