Showing papers by "Keith A. Hobson published in 1997"

Linking breeding and wintering grounds of neotropical migrant songbirds using stable hydrogen isotopic analysis of feathers.

Abstract: Recent studies have shown that stable hydrogen isotope ratios (δD) in the tissues of animals often correlate with δD of local precipitation. Here we examined the relationship between δD in feathers and growing season precipitation for neotropical migrant songbirds breeding over a continent-wide isotopic gradient. δD values were determined on feathers of 140 individuals of 6 species of wild insectivorous forest songbirds (Setophaga ruticilla, Empidonax minimus, Vermivora peregrinus, Catharus ustulatus, Seiurus aurocapillus, Hylocichla mustelina) taken from 14 breeding locations across North America. The δD of feathers was strongly correlated with the δD of growing season precipitation at breeding sites across North America. As feather hydrogen is metabolically inert after growth, this relationship was then used to assess the breeding origins of wintering migrants. Deuterium values of feathers from 64 individuals representing 5 species of migrants (Helmitheros vermivorus, Wilsonia citrina, Hylocichla mustelina, Dumetella carolinensis, Seirus aurocapillus) at a wintering site in Guatemala were consistent with those predicted from the known breeding ranges of these species. This study demonstrates hydrogen isotopes may become a powerful tool for linking breeding and wintering grounds of neotropical migrant songbirds, as well as other migratory species moving between isotopically distinct regions.

Preservation of blood and tissue samples for stable-carbon and stable-nitrogen isotope analysis

Abstract: Researchers engaged in collecting animal material for stable-carbon and stable-nitrogen isotope analysis are frequently faced with the need to preserve tissues prior to transportation to the labora...

Comparing the effects of landscape fragmentation by forestry and agriculture on predation of artificial nests.

Abstract: Fragmentation of forested landscapes can reduce the reproductive success of birds occupying residual forest patches. Previous studies, however, have focused primarily on how nest predation can change when landscapes become fragmented by agriculture rather than by logging. To determine if predation on artificial nests in forest patches was influenced by the surrounding landscape, we placed ground and shrub nests along the edge and interior of forest patches located in agricultural, logged, and contiguous forest landscapes within a single region of the southern boreal mixedwood forest of central Canada. Nest predators were identified using remotely triggered cameras and by marks left in plasticine eggs, whereas the relative abundance of nest predators such as corvids and small mammals was estimated by surveys and live-trapping. The percentage of ground nests destroyed at the edge and interior of patches in the logged and contiguous forest landscapes was significantly lower than at the edge or interior of patches in the agricultural landscape. No differences in predation rate were observed for shrub nests among landscapes. We attributed higher rates of predation in the agricultural landscape to higher densities of red squirrels ( Tamiasciurus hudsonicus) in the interior and a greater diversity of predators along edges. The similarity in predation rate between logged and contiguous forest landscapes suggests that fragmentation caused by logging may result in little change in predator communities and in this respect, might be similar to fragmentation caused by natural disturbance regimes such as fire. In contrast, fragmentation by agriculture seems to have more serious consequences for nesting birds. Policy changes that limit the growth of agriculture in the southern boreal mixedwood forest are required to conserve the diverse avifauna of this region. Comparacion de los Efectos de la Fragmentacion del Paisaje Causada por la Accion Forestal y Agrico sobre la Depredacion de Nidos Artificiales La fragmentacion de paisajes forestales puede reducir el exito reproductivo de aves que ocupan los parches residuales del bosque. Sin embargo, estudios previos se han enfocado principalmente al modo en que puede cambiar la depredacion de nidos cuando los paisajes son fragmentados para agricultura en lugar de para foresteria. Para determinar si el paisaje circundante influyel sobre la depredacion de nidos artificiales, en parches de bosque colocamos nidos sobre el suelo y en arbustos a lo largo del borde y en el interior de parches de bosque, localizados en paisajes agricolas, talados, de bosque continuo, en una sola region del bosque boreal mixto de Canada central. Los depredadores de nidos fueron identificados mediante camaras disparadas remotamente y por marcas dejades en huevos de plastilina, mientras que la abundancia relativa de depredadores de nidos como corvidos y mamiferos pequenos se estimo mediante reconocimientos y trampeos. El porcentaje de nidos sobre el suelo destruidos en el borde e interior de parches en los paisajes talados y continuos fue significativamente menor que en el borde o interior de parches en el paisaje agricola. No se observo diferencia en las tasas de depredacion de nidos en arbustos en los paisajes estudiados. Atribuimos las mayores tasas de depredacion en el paisaje agricola a la mayor densidad de ardillas rojas ( Tamiascuirus hudsonicus) en el interior y a una mayor diversidad de depredadores en los bordes. La similitud de tasas de depredacion en paisajes talados y continuos sugiere que la fragmentacion causada por la foresteria puede producir cambios pequenos en las comunidades de depredadores y en este sentido puede ser similar a la fragmentacion causada por regimenes de perturbacion natural tales como fuego. En contraste, la fragmentacion por agricultura parece tener consecuencias mas serias para aves anidantes. Se requieren cambios politicos para limitar la expansion de la agricultura en el bosque boreal mixto para conservar la diversidad de la avifauna de esta region.

Investigating trophic relationships of pinnipeds in Alaska and Washington using stable isotope ratios of nitrogen and carbon

Abstract: We measured stable-nitrogen (δ15N) and stable-carbon (δ13C) isotope ratios in muscle and hair from 7 northern fur seals (Callorhinus ursinus) from the Pribilof Islands, Alaska, and 27 Steller sea lions (Eumetopias jubatus), and 14 harbor seals (Phoca vitulina) from the Gulf of Alaska and coast of Washington State, in order to contrast dietary information derived from isotopic vs. available conventional dietary studies. Stable-nitrogen-isotope analysis of muscle revealed that harbor seals were enriched over sea lions (mean δ15N = 18.6‰vs. 17.5‰) which were in turn enriched over northern fur seals (mean δ15N = 16.6‰). Trophic segregation among these species likely results primarily from differential reliance on herring (Clupea harengus), Atka mackerel (Pleurogrammus monopterygius), and large vs. small walleye pollock (Theregra chalcogramma). According to their δ15N values, adult male Steller sea lions showed a higher trophic position than adult females (mean δ15N: 18.0‰vs. 17.2‰), whereas adult female northern fur seals were trophically higher than juvenile male fur seals (mean δ15N: 16.5‰vs. 15.0‰). Each of these observed differences likely resulted from differential reliance on squid or differences in the size range of pollock consumed. Three northern fur seal pups showed higher δ15N enrichment over adults (mean 17.7‰vs. 15.8‰) due to their reliance on their mother's milk. Stable-carbon isotope measurements of hair revealed a cline toward more negative values with latitude. Segregation in hair δ13C between Steller sea lions and harbor seals off the coast of Washington (mean δ13C: −13.6‰vs.−15.0‰) reflected the greater association of harbor seals with freshwater input from the Columbia River. Our study demonstrates the utility of the stable isotope approach to augment conventional dietary analyses of pinnipeds and other marine mammals.

Trophic Relationships among Seabirds in Central California: Combined Stable Isotope and Conventional Dietary Approach

Abstract: We used stable isotope analysis (SIA) and conventional techniques of diet assessment to determine marine trophic relationships in the Gulf of the Farallones, California, with an emphasis on marine birds. Stable-carbon (δ 13 C) and nitrogen (δ 15 N) isotopes were obtained from 98 tissue samples of 16 species representing primary and secondary consumers in 1993-1994. The values of δ 13 C ranged from -20.1‰ in whole euphausiids (krill) to -15.0‰ in muscle of northern sea lions. Values of δ 15 N showed step-wise trophic enrichment and ranged from 11.2‰ in euphausiids to 19.8‰ in sea lions. SIA of egg albumen from birds indicated reliance on zooplankton by Cassin's Auklet, Common Murre, and Western Gull, and on fish by Brandt's and Pelagic Cormorants, Rhinoceros Auklets, and Pigeon Guillemots during egg formation (April-May). However, analysis of prey brought to chicks during summer indicated the prevalence of fish in the diet of most seabirds, except Cassin's Auklet which fed primarily on krill. Results suggest a shift in trophic level and diet between spring and summer from krill to fish for Common Murres. δ 13 C analysis confirmed that Brandt's Cormorants and northern sea lions feed in neritic habitats, whereas Cassin's and Rhinoceros Auklets foraged in epipelagic offshore waters. Our approach demonstrates the utility of combining both SIA and conventional dietary assessments to understand trophic relationships in dynamic marine ecosystems.

Using stable-isotope analysis to identify endogenous and exogenous sources of nutrients in eggs of migratory birds: Applications to great lakes contaminants research

Abstract: -Stable-isotope analyses (carbon [13C/12C] and sulfur [34S/32S]) were performed on eggs of two migratory fish-eating birds, Caspian Terns (Sterna caspia) and Doublecrested Cormorants (Phalacrocorax auritus) collected from Lake Ontario, Canada, to examine the extent to which nutrient reserves acquired on marine wintering grounds are transferred to eggs laid on freshwater breeding grounds. In order to establish isotopic patterns typical of eggs of birds using marine and freshwater C-3 biomes, eggs of Herring Gulls (Larus argentatus), a year-round resident on the Great Lakes, and those of Caspian Terns and Herring Gulls, breeding resepectively in the Gulf Coast of Texas and Atlantic Coast of Canada, were analyzed isotopically. Individual egg components showed distinct isotope values that were similar for both migratory and nonmigratory birds breeding in a freshwater biome and significantly lighter than those breeding in a marine biome. Hence, there appeared to be little evidence for significant nutrient transfer between the two biomes. The intermediate isotope values shown for egg components of Herring Gulls breeding on the Atlantic Coast suggest nutrient input from terrestrial as well as marine sources. Our results indicate the utility of stable-isotope analysis for tracing endogenous and exogenous contributions to reproduction in birds and further validate the use of migratory birds as indicators of breeding area contaminant levels and their effects on the Great Lakes. Received 16 October 1996, accepted 14

Isolation of microsatellite DNA markers from a passerine bird, Dendroica petechia (the yellow warbler), and their use in population studies.

Abstract: We describe the isolation and genetic characterization of five microsatellite loci in a passerine bird, the yellow warbler Dendroica petechia, and assess their use for various types of population-level analysis using data from two breeding populations. All five loci show levels of variability comparable to those observed in other vertebrates (Hexp = 0.388-0.989). One locus, Dp mu 05, is highly variable with 46 alleles detected in 41 individuals. All loci appeared to segregate in a Mendelian fashion as judged by patterns of inheritance in known families. However, one locus showed a significant heterozygote deficiency in one population suggesting the possible presence of null alleles at this locus. These markers provide a highly accurate system for determination of parentage in this species: the probability of detecting extrapair fertilization by males given known maternity was 0.999 in each of two separate populations. Comparison of allele frequencies and genetic distances between the two populations showed no evidence for significant differences in allele frequencies at individual loci, whereas the overall genetic distance and FST-value are significantly different from zero suggesting weak differentiation. Finally, cross-species amplification experiments showed that at least one locus appears to amplify products in a wide range of birds including nonpasserine species. Thus, our results demonstrate that these loci will provide a useful set of genetic information for addressing a wide range of population-level analyses in this and other bird species.

Predation on artificial nests in relation to forest type: contrasting the use of quail and plasticine eggs

Abstract: Previous studies of avian nest predation have focused on how human-induced changes in the landscape influence the frequency of predation. However, natural variation in the abundance of predators due to their choice of habitat can also influence predation rate. To determine if predation on artificial nests was influenced by forest stand type, we placed ground and shrub nests containing quail and plasticine eggs in contiguous coniferous, mixedwood and deciduous stands in the southern boreal mixedwood forest of central Canada. Nest predators were identified using remotely triggered cameras and marks left in plasticine eggs, while the relative abundance of nest predators such as squirrels and corvids were estimated using acoustic-visual surveys. Using the fate of quail eggs to calculate predation rate, we found that predation was significantly higher in coniferous (67%) than in deciduous (17%) or mixedwood (25%) forest, with similar predation on ground (37%) and shrub (29%) nests. Using plasticine eggs to calculate predation rate, nests in coniferous forest still suffered higher rates of predation, although predation rates were 15-20% higher, and ground nests suffered significantly higher rates of predation than shrub nests. Quail eggs seemed to suffer lower rates of predation because small mammals were unable to penetrate the shell, but could leave marks on plasticine eggs. The higher predation rate in coniferous forest was likely caused by higher abundance of red squirrels Tamiasciurus hudsonicus, the presence of fishers Martes pennanti and a simplified understory which may have made it easier for predators to find nests relative to the deciduous and mixedwood forest. Plasticine eggs provide new insights into nest predation by identifying predation events by smaller predators such as mice that are missed when using quail eggs.

Low variation in blood δ13C among Hudson Bay polar bears : Implications for metabolism and tracing terrestrial foraging

Abstract: We investigated the use of stable-carbon isotope analysis of serum and cellular fractions of blood to detect the extent of terrestrial feeding in polar bears on land during the ice-free period in western Hudson Bay. We compared blood in bears that were restricted entirely to coastal areas, who showed no evidence of terrestrial feeding, with blood in bears sampled at inland locations and who were known to have fed on berries of Vaccinium uliginosum and Empetrum nigrum. Despite a separation of approximately 9‰ between terrestrial and marine foods, we found no statistical difference in blood 613C values between these two groups of bears. This suggests that (1) carbon pathways associated with feeding on berries result in minor incorporation of terrestrial-based carbon into bulk plasma or cellular fractions of blood, (2) bears feed insignificantly on berries despite observational evidence to the contrary, or (3) carbon mobilized from endogenous lipid reserves overwhelmed the terrestrial signal or could not be segregated isotopically from carbon derived from berry carbohydrates. We discuss evidence for each of these scenarios and suggest that a more effective approach to using stable-carbon isotope analysis to delineate the importance or use of terrestrial foods to polar bears on land in Hudson Bay during the ice-free period might be through the isotopic analysis of exhaled carbon dioxide rather than blood components.

Temporal patterns of predation on artificial nests in the southern boreal forest

Abstract: We examined temporal and spatial patterns of nest predation using artificial nests containing a timing device. On a daily basis, predation was bimodally distributed with peaks of activity occurring 3 hours after sunrise and 2 hours before sunset. The average times when nests were destroyed by mice, squirrels, and corvids differed significantly. Daily nest survival over a 12-day incubation period was not constant, as 58% of all predation occurred within the first 3 days. Experimenter visits to nests may have influenced predation, because 8% of all predation occurred less than 1 hour after observers left nests. The probability that nearestneighbor nests were destroyed within 1 hour of each other was significantly greater than expected if nests were destroyed randomly. Artificial nests containing timing devices provide useful data on patterns of nest predation that cannot be obtained if nests are checked infrequently.

Relationship of polychlorinated dibenzo-p-dioxin and polychlorinated dibenzofuran levels to stable-nitrogen isotope abundance in marine birds and mammals in coastal california†

Abstract: Levels of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) were determined in common murre (Uria aalge), Brandt's cormorant (Phalacrocorax penicillatus), rhinoceros auklet (Cerorhinca monocerata), and pigeon guillemot (Cepphus columba) eggs, and Steller sea lion (Eumetopias jubatus) blubber collected from the Gulf of the Farallones National Marine Sanctuary in 1993. In addition, the samples were analyzed for stable-nitrogen isotopes (δ15N). Of the PCDDs and PCDFs, the 2,3,7,8-TCDD (TCDD) and 2,3,7,8-TCDF (TCDF) congeners were the most prominent in the birds. The levels of TCDD in the eggs ranged from 0.2 to 6.6 ng/wet kg in the pigeon guillemot and Brandt's cormorant, respectively. The TCDF ranged from 0.30 to 2.25 ng/kg in the pigeon guillemot and Brandt's cormorant eggs, respectively. Other prominent PCDD and PCDF congeners detected in all bird species were 1,2,3,6,7,8-HxCDD, 2,3,4,7,8-PeCDF, 1,2,3,7,8-PeCDD, and 1,2,3,4,6,7,8-HpCDD. In the Steller sea lion the most prominent congeners were 1,2,3,7,8-PeCDD at 3.2 ng/kg, 2,3,7,8-TCDD at 2.9 ng/kg, OCDF at 2.2 ng/kg, 1,2,3,6,7,8-HxCDD at 1.92 ng/kg, and 1,2,3,4,7,8-HxCDF at 1.3 ng/kg. Stable-nitrogen values ranged from 16.9‰ in the pigeon guillemot and rhinoceros auklet to 19.8% in the Steller sea lion.

Artificial nest predation dynamics along a forest fragmentation gradient: a preliminary analysis

Abstract: Studies in eastern North America suggest that nest predation on forest songbirds increases with habitat fragmentation However, the majority of these studies have been conducted in highly fragmented suburban/rural deciduous forest habitat, making generalization of the results difficult The objective of this study was to examine artificial nest predation dynamics along a fragmentation gradient (farm woodlots, logged forest stands and contiguous forest) in the conifer dominated Boreal Mixedwood Predation was significantly higher in farm woodlots (788% edge and 785% interior) than the forest interior in contiguous and logged areas (428% and 419%, respectively) Predation at logged edges (600%) was not significantly different from either the woodlots or the forest interiors All land-uses showed a variety of avian and mammalian predators Using timer nests, we determined that predation was highest immediately after sunrise, with a second peak around sunset Census results suggest that farm woo

Erratum : breeding bird communities of western and northern canadian boreal forest : relationship to forest type

Abstract: To illustrate the problem I have wed the statistical model derived by Bartsh et al. (1992) to compare predicted ages with actual ages of male Weddell seals, using a sample that includes many more large males (Table I). The length -age model used by Bartsh et al. (1992) performed poorly as a predictor of age. Accurate prediction for males 5 -9 years old was only 25 %, and age estimates were commonly off by 3 -5 years and more. The modal predicted value was usually not the same as the actual age, and most predicted ages were younger than the known age. In an otherwise valuable contribution, Bartsh et al. (1992, p. 698) reached the conclusion that the youngest age at which males become sexually active in the breeding colonies was 7 years, with first breeding likely, on average, in year 8. While this is plausible, it was premature to reach that conclusion on the basis of the available data (5 seals). Bartsh et al. (1992) did not comment on the interesting observation that, age aside, male body size in Weddell seals was correlated with male territorial success. This implies that there should be selection pressure for sexual size dimorphism in Weddell seals, yet that dimorphism does not occur (Hill 1987).