River Birds Research Articles

Shared environmental responses drive co‐occurrence patterns in river bird communities

Positive or negative patterns of co‐occurrence might imply an influence of biotic interactions on community structure. However, species may co‐occur simply because of shared environmental responses. Here, we apply two complementary modelling methodologies – a probabilistic model of significant pairwise associations and a hierarchical multivariate probit regression model – to 1) attribute co‐occurrence patterns in 100 river bird communities to either shared environmental responses or to other ecological mechanisms such as interaction with heterospecifics, and 2) examine the strength of evidence for four alternative models of community structure. Species co‐occurred more often than would be expected by random community assembly and the species composition of bird communities was highly structured. Co‐occurrence patterns were primarily explained by shared environmental responses; species’ responses to the environmental variables were highly divergent, with both strong positive and negative environmental correlations occurring. We found limited evidence for behaviour‐driven assemblage patterns in bird communities at a large spatial scale, although statistically significant positive associations amongst some species suggested the operation of facilitative mechanisms such as heterospecific attraction. This lends support to an environmental filtering model of community assembly as being the principle mechanism shaping river bird community structure. Consequently, species interactions may be reduced to an ancillary role in some avifaunal communities, meaning if shared environmental responses are not quantified studies of co‐occurrence may overestimate the role of species interactions in shaping community structure.

Changing habitat use associated with distributional shifts of wintering raptors

There is widespread evidence that multiple drivers of global change, such as habitat degradation, invasive species, and climate change, are influencing wildlife. Understanding how these drivers interact with and affect species may be difficult because outcomes depend on the magnitude and duration of environmental change and the life history of the organism. In addition, various environmental drivers may be evaluated and managed at different spatial scales. We used a historical dataset from 1991 to 1994 and current information from 2010 to 2012 to examine whether occupancy patterns of wintering raptors were consistent with regional changes in distribution or habitat conditions within a local management unit, the Morley Nelson Snake River Birds of Prey National Conservation Area (NCA). We predicted that if local populations reflected regional shifts, then rates of raptor occupancy within the NCA would be higher compared to historical estimates and birds would use different habitats compared to historical use. Alternatively, if local populations were determined by habitat conditions, then we predicted that occupancy rate of raptors within the NCA would be lower compared to historical estimates and current habitat use would be consistent with historical use. Results support the hypothesis that northward distributional shifts influenced wintering raptor populations in southwest Idaho to a greater extent than local habitat conditions. Wintering raptors had higher occupancy rates in 2010–2012 compared to 1991–1994, whereas invasive grasses have increased and native shrubs have decreased suggesting that habitat suitability for raptors has declined over time. On the species level, changes in habitat use were associated with greater increases in occupancy rates in 2010–2012 compared to 1991–1994. Organisms flexible in their habitat use may be better able to respond to continental forces driving distribution shifts. Conversely, habitat or prey specialists may be poorly equipped to handle such rapid, large-scale global change. Further, Grinnellian niche models predicting species response to change by mapping current habitat use to forecasted vegetation maps should consider plasticity in habitat use and changes in the cost-benefits of life-history strategies. © 2015 The Wildlife Society.

River birds’ response to hydrological extremes: New vulnerability index and conservation implications

Abstract There is growing evidence that as a consequence of climate change the frequency of extreme hydrological events will increase. Predicting the impacts of these extreme events on ecological systems is a major research challenge. It is predicted that change in future river flows, characterised by greater occurrence of floods and droughts, will have profound impacts on aquatic invertebrate communities by removing sensitive species and restructuring food networks. However, it remains unclear how an increase in these hydrological extremes will impact on riparian communities and species at higher trophic levels. Here, we describe a new methodology that facilitates the integration of quantitative outputs of species’ distribution models with the expert knowledge of conservation practitioners to produce a species’ vulnerability index (SVI). Using our SVI framework, we assessed and ranked the vulnerability of 16 river bird species to a potential climate-induced shift in the frequency, duration and magnitude of flood and drought events. Vulnerability was associated primarily with ecological traits that restrict species to in-channel riverine habitat. Whilst the SVI was developed to assess species’ vulnerability to hydrological extremes on rivers, it is equally applicable to other environmental domains as well as a range of avian and non-avian taxa. Furthermore, this original methodological approach provides researchers and managers with a valuable conservation tool that allows them to identify the species most vulnerable to climate change impacts and plan mitigation and adaptation strategies.

Recreational Fishing-Related Injuries to Australian Pelicans (Pelecanus Conspicillatus) and Other Seabirds in a South Australian Estuarine and River Area

seabirds treated over 5.5 years had 132 fishing-related injuries that included entanglement with line only (N=35/132; 26.5%), entanglement with line and an associated hook (N=47/132; 35.6%), embedded hooks only (N=34/132; 25.7%) and foreign body ingestion (N=16/132; 12.1%). The percentage of fishing-related injuries ranged from 0.9% for banded stilts ( Cladorhynchus leucocephalus), pacific gulls ( Larus pacificus) and masked lapwing plovers (Vanellus miles), to 59.3% for Australian pelicans (Pelecanus conspicillatus). Entanglement and/or embedded hooks were present more often than injuries from ingestion; i.e. 97% (70/72) of pelicans had entanglement and/or embedded hook injuries; of these 35/72 (48.6%) were entangled with line and hooks, 24/72 (33.3%) had embedded hooks alone and 11/72 (15.3%) were entangled with lines only, with only 3% (2/72) having injuries from ingestion. A count of sea and river birds in close proximity to fishers revealed that the major - ity were pelicans (33.9%), compared to pied cormorants (28.6%), silver gulls (21.4%) and black swans (16.1%). Regular removal of discarded fishing material along local shores resulted in no reduction in the numbers of entangled or hooked seabirds. It appears likely, therefore, that such injuries may result from seabird proximity to active recreational fishing, rather than from entanglement in discarded material.

Avian community responses to variability in river hydrology.

River flow is a major driver of morphological structure and community dynamics in riverine-floodplain ecosystems. Flow influences in-stream communities through changes in water velocity, depth, temperature, turbidity and nutrient fluxes, and perturbations in the organisation of lower trophic levels are cascaded through the food web, resulting in shifts in food availability for consumer species. River birds are sensitive to spatial and phenological mismatches with aquatic prey following flow disturbances; however, the role of flow as a determinant of riparian ecological structure remains poorly known. This knowledge is crucial to help to predict if, and how, riparian communities will be influenced by climate-induced changes in river flow characterised by more extreme high (i.e. flood) and/or low (i.e. drought) flow events. Here, we combine national-scale datasets of river bird surveys and river flow archives to understand how hydrological disturbance has affected the distribution of riparian species at higher trophic levels. Data were analysed for 71 river locations using a Generalized Additive Model framework and a model averaging procedure. Species had complex but biologically interpretable associations with hydrological indices, with species’ responses consistent with their ecology, indicating that hydrological-disturbance has implications for higher trophic levels in riparian food webs. Our quantitative analysis of river flow-bird relationships demonstrates the potential vulnerability of riparian species to the impacts of changing flow variability and represents an important contribution in helping to understand how bird communities might respond to a climate change-induced increase in the intensity of floods and droughts. Moreover, the success in relating parameters of river flow variability to species’ distributions highlights the need to include river flow data in climate change impact models of species’ distributions.

Bioaccumulation of Trace Mercury in Trophic Levels of Benthic, Benthopelagic, Pelagic Fish Species, and Sea Birds from Arvand River, Iran

In this study, concentration of mercury was determined in the trophic levels of benthic, benthopelagic, pelagic fish species, and river birds from Arvand River, located in the Khuzestan province in the lowlands of southwestern Iran at the head of the Persian Gulf. The order of mercury concentrations in tissues of the fish species was as follows: liver>gill>muscle and in tissues of the kingfisher species was as follows: feather>liver>kidney>muscle. Therefore, liver in fish and feather in kingfisher exhibited higher mercury concentration than the other tissues. There was a positive correlation between mercury concentrations in fish and kingfisher species with size of its food items. We expected to see higher mercury levels in tissues of female species because they are larger and can eat larger food items. The results of this study show that the highest mean mercury level were found in the kingfisher (Anas crecca), followed by benthic (Epinephelus diacanthus), benthopelagic (Chanos chanos), and pelagic fish (Strongylura strongylura). Mean value of mercury in fish species, S. strongylura were (0.61 μg g−1 dry weight), C. chanos (0.45 μg g−1 dry weight), E. diacanthus (0.87 μg g−1 dry weight), and in kingfisher species A. crecca was (2.64 μg g−1 dry weight). Significant correlation between mercury concentration in fish and kingfisher may be related to high variability of mercury in the fish.

Fragile Wetland Will Test Turkey's Resolve in Protecting Biodiversity

Scientists are racing to document the biodiversity of Turkey's Aras River Bird Paradise before construction starts on a dam that will flood the wetlands.

Relationships between yolk androgens and nest density, laying date, and laying order in Western Burrowing Owls (Athene cunicularia hypugaea)

Increases in yolk androgens within and among avian clutches have been correlated with decreased incubation time, increased aggression within a nest, increased begging behaviour, decreased immune response, and decreased life span. Although the mechanisms that lead to variability in yolk androgens within and between clutches are not completely known, yolk androgens can be a function of both social and environmental conditions. We were interested in if and how nesting density, laying date, and laying order influenced yolk androgens in Western Burrowing Owls ( Athene cunicularia hypugaea (Bonaparte, 1825)) in which nest density varies considerably. In 2006 and 2007, we used radioimmunoassay to quantify the concentrations of testosterone, 5α-dihydrotestosterone, and androstenedione in the egg yolks from one early and one late-laid egg in 47 nests of Burrowing Owls located in the Morley Nelson Snake River Birds of Prey National Conservation Area in southern Idaho. Nesting density had no detectable effect on yolk androgens. Yolk androgens varied temporally and peaked in the middle of the laying season while being low before and after this time period. Within nests, late-laid eggs had higher testosterone and dihydrotestosterone than early-laid eggs; adrostendione exhibited a similar pattern in one but not both years of our study. It is possible that the seasonal pattern in yolk androgens that we observed is related to aspects of mate quality for females or declining chances of fledging success for later nesting females, whereas rises in egg androgens between early and late eggs within clutches could reflect a mechanism to assist nestlings from late-laid eggs that hatch one to several days after their siblings to better compete for resources within the nest or promote survival in the presence of larger siblings.

American Dippers Indicate Contaminant Biotransport by Pacific Salmon

Migrating salmon can increase productivity in Pacific Northwestern streams and lakes through the deposition of nutrients from their decomposing carcasses after spawning. Several studies also report simultaneous biotransport of persistent organic pollutants that have contaminated lake food webs, although no similar effect has been shown conclusively in rivers. We tested the prediction that salmon enhance contaminants in river food webs using the American dipper (Cinclus mexicanus), an aquatic songbird and a recognized indicator of stream quality. Over 3 years, we analyzed 29 dipper eggs and aquatic invertebrate samples from 14 different rivers in 10 catchments in southern British Columbia, Canada to assess whether variations in autumn spawning density of Pacific salmon were reflected in dipper egg contamination or stable carbon and nitrogen isotopes. δ(13)C isotope signatures, but not δ(15)N, in aquatic invertebrates and dipper eggs increased among catchments in proportion to the average density of spawning salmon. Concentrations of brominated flame retardants (PBDEs), dichlorodiphenyltrichloroethane metabolites (DDTs), and chlordane compounds were related in part to the δ(13)C measure of salmon density, but mercury, chlorobenzenes, and polychlorinated biphenyls (PCBs) were explained better by dipper trophic level. We conclude that spawning Pacific salmon result in the increased availability of salmon fry as dipper prey and salmon are a significant source of PBDEs, DDTs, and chlordanes to river ecosystems. However, contrary to lake studies, postspawn concentrations of legacy PCBs in river birds, even in salmon-rich rivers, were not significantly higher than would be expected from atmospheric deposition alone. We recommend using δ(13)C isotopes to trace salmon-derived lipids which may persist over winter particularly in rivers, and are potentially a better reflection of lipophilic contaminant transfer.

Ecological strategies successfully predict the effects of river floodplain rehabilitation on breeding birds

ABSTRACTTo improve the ecological functioning of riverine ecosystems, large‐scale floodplain rehabilitation has been carried out in the Rhine–Meuse Delta since the 1990s. This paper evaluates changes in abundance of 93 breeding bird species over a period of 10 years in response to rehabilitation, by comparing population changes in 75 rehabilitated sites with 124 non‐rehabilitated reference sites. Such quantitative, multi‐species, large‐scale and long‐term evaluations of floodplain rehabilitation on biodiversity are still scarce, particularly studies that focus on the terrestrial component. We try to understand the effects by relating population trends to ecological and life‐history traits and strategies of breeding birds. More specifically, we try to answer the question whether rehabilitation of vegetation succession or hydro‐geomorphological river processes is the key driver behind recent population changes in rehabilitated sites. Populations of 35 species have significantly performed better in rehabilitated sites compared to non‐rehabilitated floodplains, whereas only 8 have responded negatively to rehabilitation. Differences in effects between species are best explained by the trait selection of nest location. Reproductive investment and migratory behaviour were less strong predictors. Based on these three traits we defined eight life‐history strategies that successfully captured a substantial amount of variation in rehabilitation effects. We conclude that spontaneous vegetation succession and initial excavations are currently more important drivers of population changes than rehabilitation of hydrodynamics. The latter are strongly constrained by river regulation. If rehabilitation of hydro‐geomorphological processes remains incomplete in future, artificial cyclic floodplain rejuvenation will be necessary for sustainable conservation of characteristic river birds. Copyright © 2010 John Wiley & Sons, Ltd.

Local to Continental Influences on Nutrient and Contaminant Sources to River Birds

Stable isotope and contaminant analyses revealed differences in nutrient sources and contaminant pathways in two species of dipper, Cinclus mexicanus and Cinclus cinclus in western Canada and western Britain. We collected dipper prey and eggs from relatively pristine British Columbian rivers contrasting in the presence of salmon-derived nutrients, and rural Welsh rivers with varying food-web complexity associated with stream acidity. Enriched delta(13)C and to a lesser extent delta(15)N in American dipper eggs and prey confirmed that streams with migrating Pacific salmon were enriched with marine-derived nutrients, but overall contaminant concentrations did not differ strongly in dipper eggs between rivers with and without salmon. However dipper contaminant profiles, particularly PBDEs, mercury and SigmaDDTs, were related to delta(13)C, reflecting the marine influence and greater fish consumption at salmon sites. Irrespective of catchment influences and despite feeding at a higher trophic level, American dipper eggs (n = 17) contained lower levels of organohalogens than Eurasian dippers (n = 37), but with similar PCB (153 and 138) and PBDE (47 and 99) congeners dominanating. Eurasian dipper eggs from circumneutral streams contained more dieldrin, SigmaDDT and Sigmahexachlorocyclohexanes while Sigmachlordanes, mirex, SigmaPBDEs and SigmaPCBs predominated at acid sites. Our data reveal how dippers indicate contaminant levels and sources under contrasting conditions at scales ranging from local to intercontinental, but local environmental conditions apparently alter feeding ecology and exposure pathways even in these closely related species.

Interactions between freshwater input, light, and phytoplankton dynamics on the Louisiana continental shelf

We examined the effects of freshwater flow and light availability on phytoplankton biomass and production along the Louisiana continental shelf in the region characterized by persistent spring–summer stratification and widespread summer hypoxia. Data were collected on 7 cruises from 2005 to 2007, and spatially-averaged estimates of phytoplankton and light variables were calculated for the study area using Voronoi polygon normalization. Shelf-wide phytoplankton production ranged from 0.47 to 1.75 mg C m −2 d −1 across the 7 cruises. Shelf-wide average light attenuation ( k d ) ranged from 0.19–1.01 m −1 and strongly covaried with freshwater discharge from the Mississippi and Atchafalaya Rivers ( R 2=0.67). Interestingly, we observed that the euphotic zone (as defined by the 1% light depth) extended well below the pycnocline and to the bottom across much of the shelf. Shelf-wide average chlorophyll a (chl a) concentrations ranged from 1.4 to 5.9 mg m −3 and, similar to k d , covaried with river discharge ( R 2=0.83). Also, chl a concentrations were significantly higher in plume versus non-plume regions of the shelf. When integrated through the water-column, shelf-wide average chl a ranged from 26.3 to 47.6 mg m −2, but did not covary with river discharge, nor were plume versus non-plume averages statistically different. The high integrated chl a in the non-plume waters resulted from frequent sub-pycnocline chl a maxima. Phytoplankton production rates were highest in the vicinity of the Mississippi River bird's foot delta, but as with integrated chl a were not statistically different in plume versus non-plume waters across the rest of the shelf. Based on the vertical distribution of light and chl a, a substantial fraction of phytoplankton production occurred below the pycnocline, averaging from 25% to 50% among cruises. These results suggest that freshwater and nutrient inputs regulate shelf-wide k d and, consequently, the vertical distribution of primary production. The substantial below-pycnocline primary production we observed has not been previously quantified for this region, but has important implications about the formation and persistence of hypoxia on the Louisiana continental shelf.

Evidence of a highly complex phylogeographic structure on a specialist river bird species, the dipper ( Cinclus cinclus)

This study details the phylogeographic pattern of the white-throated dipper ( Cinclus cinclus), a Palearctic, temperate, passerine bird that is exclusively associated with flowing water. Our results reveal a complex phylogeographic structure with at least five distinct lineages for the Western Palearctic region. As for many species of the Western Palearctic fauna and flora, this genetic structure is probably linked to the isolation of populations in different southern refuges during glacial periods. Furthermore, the isolation of populations in Scandinavia and/or Eastern regions, but also in Morocco and probably in Corsica, was accentuated by ecological and biogeographic barriers during Quaternary interglacial periods. During glacial periods, Italy, Sicily and the Balkano-Carpathian region acted as major refuge zones for the dipper. At the end of the last ice age, Western Europe was repopulated by dippers from an Italian refuge, while Eastern Europe was recolonised by Balkano-Carpathian birds. A large contact zone between these two lineages was evidenced and extends from Luxembourg to Hungary. Finally, our results indicate the need to clarify the taxonomic status of the dipper, especially concerning the European subspecies whose validity appears uncertain.

Niche segregation of Himalayan river birds

The Himalayan Mountains have more specialist river birds than anywhere on earth, but factors permitting their coexistence have not been evaluated. We examined potential ecological segregation of five sympatric insectivores based on diet, morphology, and foraging behavior during the breeding and nonbreeding seasons in central Nepal. Little Forktails {Enicurus scouleri) and Brown Dippers (Cincluspallasii) were highly aquatic in habitat use and diet (89-96% of prey of aquatic origin), but took prey that differed in size and composition. Spotted Forktails (E. maculatus) foraged primarily along channel margins, and took more aquatic than terrestrial prey (59% vs. 40%). White-capped (Chaimorrornis leucocephalus) and Plumbeous (Rhyacornis fuliginosas) water redstarts took primarily terrestrial or aerial prey by gleaning marginal surfaces and flycatching over channels. Two species were similar morphologically (Little Forktail and Plumbeous Water Redstart), but had different diets. Our results illustrate clear ecological separation among this guild of co-occurring river birds on one or more niche dimensions that we suggest is mediated by the physical complexity of mountain, riverine habitats. SINOPSIS. La segregaci?n del nicho de las aves ribere?as en las Himalayas Las monta?as Himalayas tienen mas aves especializadas en r?os que cualquier otro lugar del planeta, pero los factores que permiten su co-existencia no han sido evaluados. Examinamos la potencial de segregaci?n ecol?gica de cinco especies insect?voras que viven en simpatria basado en su dieta, morfolog?a y comportamiento de b?squeda de alimento. Enicurus scouleri y Cinclus pallasii usaron un habitat y dieta altamente acu?tico (89-96% de las presas fueron de origen acu?tico), pero las presas capturadas fueron de distintos tama?os y taxa. E. maculatus busc? su alimento principalmente en las orillas de los canales y captur? mas presas acu?ticas que terrestres (59 y 40%, respectivamente). Chaimorrornis leucocephalus y Rhyacornis fuliginosas capturaron principalmente presas terrestres o a?reas, mediante b?squedas sobre las superficies en ?reas marginales y mediante vuelos cortos sobre los canales. Dos especies tuvieron una morfolog?a similar (E. scouleri y R fuliginosus), pero presentaron dietas diferentes. Aunque las causas evolutivas no son claras, nuestros resultados presentan una separaci?n ecol?gica en una o mas dimensiones del nicho. Debido a que existen habitats espec?ficos de los r?os cuales son importantes en el uso de los recursos dentro y entre especies, nuestros datos proveen evidencia adicional que las modificaciones a lo largo del habitat ribere?o en las Himalayas pueden afectar a la distribuci?n de las aves.

Aspects of Distribution and Ecology of Harlequin Ducks on the Torrent River, Newfoundland

Abstract Harlequin Ducks (Histrionicus histrionicus) breeding on the upper Torrent River of the northern peninsula of Newfoundland were studied from 1993 to 2002. This reach of the watershed was proposed for hydro-electrical development in the 1990s. Harlequin Ducks arrive on the Torrent River as soon as ice-out permits in early to late May. Pairs concentrate in the upper watershed that drains the Long Range Barrens ecoregion. Nesting and brood rearing appear to take place primarily in proximity to spring-pair activity, although movements of more than ten km were observed. Young remain in the upper watershed until fledging in late August to mid-September. Counts of Harlequin Ducks breeding on the upper Torrent River increased throughout the 1990s, and the estimated population growth rate for the Torrent River birds was very similar to the growth rate for birds wintering at Cape St. Mary's, Newfoundland (1.14 vs. 1.13) during the same period. Paired females spent ∼ 40% of the day in feeding activities wher...

Combining surveys of river habitats and river birds to appraise riverine hydromorphology

Summary1. Biological monitoring is vital to river conservation. Aside from providing census data, regular monitoring may detect population trends that reflect the degradation or remediation of riverine environments. Birds are major candidates for this purpose because of their connections to riverine food webs and river habitat features. However, much information on factors affecting river bird distributions is qualitative.2. River bird populations have been surveyed annually in the U.K. since 1974. The value of this monitoring could be increased if links between population trends and changes in riverine landscapes or channel hydromorphology were better understood. We modelled the relationships between bird species’ distributions recorded by the British Trust for Ornithology's Waterways Breeding Bird Survey (WBBS) and hydromorphology recorded by the Environment Agency's River Habitat Survey (RHS).3. Regression models were built for 20 bird species associated with the river channel and neighbouring floodplains. This provided one of the first illustrations in ecology of generalized estimating equations (GEEs) for extending generalized linear models to instances where individual observations are not statistically independent. GEEs could offer a range of benefits to freshwater biologists, as they are a simple approach to analysing hierarchical, correlated data while maintaining the ease of interpretation with a range of link functions and data types.4. Upland, fast water species and reedbed specialists showed the closest links with RHS, followed by lowland, slow water species, where correlations to RHS varied widely in magnitude. The distributions of floodplain wading birds were poorly modelled by RHS, suggesting that floodplain land uses as quantified by RHS were poor predictors of distributions. Marked preferences between upland and lowland channels were evident for most species. Specific features were also important, including hydraulics, bank/channel vegetation, depositional features and anthropogenic structures.5. Monitoring the abundance and distribution of a range of bird species, with a diversity of life histories, could be a valuable tool for assessing trends in riverine landscapes and whole river basins. RHS captures useful information about channel hydromorphology and vegetation, but needs supplementing with additional environmental information to describe (i) the wider riverine landscape (e.g. more detailed floodplain variables, proximity of other waterbodies) and (ii) water chemistry. Although further work is required, this study suggests that WBBS and RHS add mutual value in appraising the river environment. We advocate the further development of birds as indicators in riverine landscapes that could convey the importance of the ecological integrity, biological production and conservation value of river systems to a wider audience.

POPULATIONS AND HABITAT RELATIONSHIPS OF PIUTE GROUND SQUIRRELS IN SOUTHWESTERN IDAHO

Abstract Piute ground squirrels (Spermophilus mollis idahoensis) are normally above ground from late January until late June or early July in the Snake River Birds of Prey National Conservation Area in southwestern Idaho. In 2002 they were rarely seen above ground after early May. Because of the ecological importance of ground squirrels for nesting raptors and other species, we sought to determine the reasons for their early disappearance. We sampled 12 sites from January 2003 through March 2003 to determine if a population crash had occurred in 2002. Tests indicated that Piute ground squirrels had not been exposed to plague within the past year. The presence of yearlings in the population indicated that squirrels reproduced in 2002 and that at least some yearlings survived the winter. Both yearling and adult squirrels appeared to be reproducing at or above normal rates in 2003. The most plausible explanation for the early disappearance of Piute ground squirrels in 2002 is that squirrels entered seasonal ...

Association between Wildlife and Agriculture: Underlying Mechanisms and Implications in Burrowing Owls

Western burrowing owls (Athene cunicularia hypugaea) are endangered in Canada and several western U.S. states. Population declines have been linked with control measures aimed at burrowing mammals and loss of nesting habitat. The owls frequently associate with irrigated agriculture throughout portions of their western U.S. range. To determine potential factors driving the association of burrowing owls with agriculture, we examined availability of suitable nest burrows (burrow availability hypothesis), abundance of potential prey (prey availability hypothesis), and predation of nest burrows (predation hypothesis) for owls nesting in the Snake River Birds of Prey National Conservation Area in southwestern Idaho during 2001–2002. Nest burrow availability did not differ between agricultural and nonagricultural habitats, and occupancy rates of owls in artificial burrows were greater near agriculture. More rodent prey species were live-trapped in agricultural habitat compared with nonagricultural habitat, and there was no difference in relative abundance of prey between habitat types. Pellet remains indicated greater abundance and biomass of prey being consumed in agricultural habitat compared with nonagricultural habitat. Finally, predation rates of dummy nests in agricultural and nonagricultural habitat did not differ. These findings allow us to reject the burrow availability and predation hypotheses, while the prey availability hypothesis remains tenable. Thus, burrowing owls may nest near irrigated agriculture in southwestern Idaho because of increased diversity or availability of prey. We suggest that research is needed to determine how widespread this prey availability relationship may be and how management of burrowing owls in agricultural landscapes may take advantage of this apparent rich prey resource.

Size Dimorphism, Molt Status, and Body Mass Variation of Prairie Falcons Nesting in the Snake River Birds of Prey National Conservation Area

VARIACIONES EN EL DIMORFISMO DEL TAMANO, ESTADO DE LA MUDA Y PESO CORPORAL DE INDIVIDUOS DE FALCO MEXICANUS QUE NIDIFICAN EN EL AREA DE CONSERVACION NACIONAL DE AVES DE PRESA SNAKE RIVER Los adultos de Falco mexicanus capturados y medidos durante la estacion reproductiva en el sudoeste de Idaho mostraron un alto nivel de dimorfismo sexual. El peso corporal y el largo de la cola pueden ser las mejores caracteristicas para distinguir los sexos de F. mexicanus en el campo. El peso promedio de las hembras disminuyo significativamente desde el inicio de la incubacion hasta la cria de los pichones, pero el peso de los machos no mostro cambios significativos durante el mismo periodo. El 63% de las hembras y el 34% de los machos estaban mudando las plumas del ala (P4 o P5) en el momento que fueron capturados. Las hembras presentaron mayor probabilidad de estar mudando que los machos durante los estadios tempranos de la incubacion. La mayoria de los machos no mostro signos de muda hasta que eclosionaron sus pichon...

Effects of Radiomarking on Prairie Falcons: Attachment Failures Provide Insights About Survival

From 1999–2002, we attached satellite-received platform transmitter terminals (PTTs) to 40 adult female prairie falcons (Falco mexicanus) on their nesting grounds in the Snake River Birds of Prey National Conservation Area (NCA) in southwest Idaho. We used 3 variations of a backpack harness design that had been used previously on raptors. Each radiomarked falcon also received a color leg band with a unique alphanumeric code. We monitored survival of birds using radiotelemetry and searched for marked birds on their nesting grounds during breeding seasons after marking. Because 6 falcons removed their harnesses during the first year, we were able to compare survival rates of birds that shed PTTs with those that retained them. We describe a harness design that failed prematurely as well as designs that proved successful for long-term PTT attachment. We resighted 21 marked individuals on nesting areas 1–5 years after they were radiomarked and documented 13 mortalities of satellite-tracked falcons. We used a Cormack-Jolly-Seber model to estimate apparent survival probability based on band resighting and telemetry data. Platform transmitter terminals had no short-term effects on falcons or their nesting success during the nesting season they were marked, but birds that shed their transmitters increased their probability of survival. Estimated annual survival for birds that shed their transmitters was 87% compared to 49% for birds wearing transmitters. We discuss possible reasons for differences in apparent survival rates and offer recommendations for future marking of falcons.