Bird Community Composition Research Articles

Cattle Grazing Benefits Farmland Bird Community Composition in Oil Palm Plantations

Oil palm expansion has caused considerable biodiversity loss as rainforest has been converted to plantation. However, it is still possible to mitigate such biodiversity losses and increase agricultural sustainability by introducing sustainable oil palm farming practices. One such method is the use of biological control agents for understory weed management in place of herbicides. Integrating cattle into oil palm plantation management to control undergrowth is expected to improve biodiversity (including avian diversity) in oil palm plantations. This study investigated the association between cattle grazing and farmland bird species composition in oil palm plantations. We used point transect sampling to survey farmland birds in 45 oil palm plantations which were divided into systematically and non-systematically cattle grazed plantations and control plantations (without cattle grazing). We found that both oil palm plantations with systematic (P=0.001) and non-systematic (P=0.005) cattle grazing had greater avian diversity than plantations without cattle grazing. Based on feeding guild, avian insectivores made up the majority of farmland birds observed. Bird species composition was determined by four attributes in our model: number of cattle, selective weeding frequency, age of oil palm stand, and palm height. We conclude that systematic grazing can improve avian diversity in oil palm plantations. We stress, that cattle integration into oil palm agriculture can provide an excellent means of maximising agricultural land use efficiency as well as increasing biodiversity in agricultural landscapes by increasing habitat complexity.

Multi-century periods since fire in an intact woodland landscape favour bird species declining in an adjacent agricultural region

Habitat modification by fire and habitat loss via anthropogenic vegetation clearance and fragmentation both impact animal populations. Yet, there has been limited investigation as to whether animals that decline under one of these types of habitat change also decline under the other, and how their cumulative impacts affect the status of species and communities. Using a ~400-year chronosequence in the world's largest extant temperate woodland in south-western Australia, we examine how time since fire affects bird community richness, reporting rates and composition, and whether taxa grouped on the basis of responses to vegetation clearance and fragmentation in an adjoining agricultural landscape are associated with either recently-burnt or long-unburnt woodlands. Consistent with substantial changes in vegetation composition and structure after fire in obligate-seeder eucalypt woodlands, woodland bird communities were strongly affected by fire. Species richness and total reporting rates increased with time since fire, and community composition changed across the entire multi-century span of the chronosequence. Woodland birds most negatively impacted by vegetation clearance and fragmentation were strongly associated with long-unburnt woodlands. In a regional south-western Australian context, where extensive vegetation clearance has substantially reduced the range and populations of many woodland bird species, the ability of remaining unfragmented woodlands to support populations of these species will be strongly contingent on appropriate fire management. Specifically, as stand-replacement fires have affected 25–30% of extant woodland over recent decades, management to limit the extent of fire in remaining long-unburnt woodlands would appear a priority for conservation of woodland bird diversity.

Effects of sagebrush restoration on plant and bird communities in Grand Teton National Park

Approximately half of sagebrush steppe range-wide has been converted to non-native grasslands, which has contributed to population declines of sagebrush-associated songbirds. Removal of non-native grasses and restoration treatments are time-, resource- and energy-intensive, but could lead to the return of functional habitat for sagebrush wildlife. The extent to which restoration efforts repair the structure and functionality of sagebrush steppe for different types of wildlife, however, remains largely untested. To determine breeding songbird community responses to sagebrush restoration treatments, we are conducting a longitudinal study with sampling every 5 years within restoration units at different stages of restoration in the Kelly Hayfields restoration area in Grand Teton National Park, Wyoming. Thus far, in 2013 and 2018 we compared bird and plant communities in unrestored (largely smooth brome [Bromus inermis]) units to those in various stages of restoration treatments, and to areas of native sagebrush. The sagebrush plots will serve as desired comparators for the endpoints of restoration efforts. The in-progress and recently replanted units were either dominated by bare ground (following herbicidal application) or native forbs with very little shrub cover (< 0.1%). Native sagebrush units were dominated by shrubs and native bunchgrasses. Bird community composition was distinct among the different unit types. Abundance of grassland birds was highest in unrestored units, whereas the abundance of shrubland birds was highest in native sagebrush and positively associated with shrub cover. There were very few detections of birds in recently re-seeded units. Restored areas may initially provide little breeding bird habitat, especially prior to the establishment of native bunch grasses and a mature shrub layer. Plant and bird sampling efforts will continue every five years to document how plant and bird assemblages shift over time in response to restoration efforts.
 
 Featured photo by Matt Lavin on Flickr. https://flic.kr/p/fh7UJz

Urbanization Around an Airfield Alters Bird Community Composition, but not the Hazard of Bird–Aircraft Collision

SummaryBird strikes are a considerable hazard to the aviation industry, which has led airports to develop detailed management strategies to identify and reduce the risk of them. These strategies include plans to modify bird habitats in and around airports to limit the establishment of high-risk species. Although airports are generally located on the outskirts of towns and cities, they may be encroached on by urban sprawl. In this study, we examined how an urbanization gradient affects the bird community around an air force base in Cape Town (South Africa) and quantified how this could modify the hazard of bird–aircraft collision. We surveyed the bird community in and around the Air Force Base Ysterplaat across four seasons and categorized species based on their response to increased urbanization (urban avoiders, adapters and exploiters). By combining this with species-specific information on body size, flock size, flight behaviour and abundance, we showed that although urbanization affects bird composition, the overall hazard of bird–aircraft collision is consistent regardless of urban response. However, urbanization complicates mitigation in the matrix surrounding the airport and creates novel challenges for the management of bird hazards. Therefore, new bird hazard management strategies should be integrated within community-driven land-use planning to minimize the negative effects of bird–aircraft collisions.

Response to fragmentation by avian communities is mediated by species traits

AbstractAimThe hypothesis that habitat fragmentation negatively influences biodiversity stems from island biogeography and metapopulation theory which predict negative impacts of decreasing patch size on richness and distribution patterns. Empirical support of this idea is weak in terrestrial systems, though tests of fragmentation effects are typically confounded with landscape composition and potentially obscured by imperfect detection. Here, we used multispecies occupancy models and a mensurative experimental design to test competing hypotheses about how forest fragmentation influences distributions of breeding forest bird species and communities.LocationSouthern Indiana, USA.MethodsDuring the breeding seasons of 2011–2013, we recorded over 80,000 bird detections in 202 forest fragments using a sampling design that isolated the effects of patch size per se from the effects of forest amount within a 2 km radius, edge distance, local vegetation and sampling area. We modelled the effects of these covariates on distributions of individual species categorized by ecological trait groups (i.e., forest, forest interior or forest edge), and evaluated how forest loss and fragmentation impact species richness.ResultsThough our results indicated little effect of patch size on total species richness, decreasing patch size had a negative effect on interior species, and a positive effect on edge species. The effects of total forest amount were much more variable, and surprisingly had a negative influence on many species, particularly cavity nesters.Main conclusionsOur results do not support theoretical predictions that forest patch size should positively influence bird species richness. However, composition of bird communities shifted towards edge species from interior species with decreasing patch size. Maintaining large forest patches is thus critical for supporting forest interior species, which tend to be of the greatest conservation concern.

Using Camera Traps to Assess Mammal and Bird Assemblages in a Midwestern Forest

Abstract Ecologists are increasing the use of remote technologies in their research, as these methods are less labor intensive than traditional methods and oftentimes minimize the number of human errors. Camera traps can be used to remotely measure abundance and community composition and offer the potential to measure some phenotypic traits, such as body size. We designed a camera-trap setup that enabled us to capture images of both large and small animals and used our camera-trap design to investigate the community composition of mammals and birds and to estimate the biomass of mammals along two transects in a conservation reserve in Missouri. One transect ran from the edge of an agricultural field to an upland forest, and the other transect ran from the edge of a wetland to an upland forest. Over the 4.5-wk study, our cameras recorded 2,245 images that comprised 483 individuals of 16 species of mammals and birds. Coyotes Canis latrans and nine-banded armadillos Dasypus novemcinctus were unique to the riparian transect, as were several bird species. Fewer species used the forest immediately adjacent to the agricultural field, but more species used the forest immediately adjacent to the wetland. Biomass estimates from our camera-trap images were similar to those of published accounts. This is the first study to use camera traps to successfully estimate biomass. We showed that the value and utility of camera traps in wildlife studies and monitoring can be expanded by 1) using multiple cameras at different heights from the ground so as to capture different-sized animals and 2) obtaining phenotypic information of the captured animals.

Are Boreal Riparian Bird Communities Unique? Contrasting Riparian and Upland Bird Assemblages in the Boreal Plain of Western Canada

Riparian ecotones in arid regions often contain unique species and have higher species richness and abundance relative to upland habitats making them of higher conservation priority than other habitats. However, such differences in species richness and abundance may not be apparent in more mesic regions. We compared species frequency of occurrence and abundance, richness and turnover of bird assemblages in two aquatic ecotone types with those in nearby interior upland forests in the boreal forest of Alberta, Canada. Riparian edge habitats had higher species richness and more species with significant indicator values than lowland forest edge or interior upland forest and the greatest differences in bird community composition were between riparian edge and upland interio habitats. Species turnover was highest in riparian habitats attributable to the presence of rare species or higher spatial and structural heterogeneity of those systems. When contrasting the upland forested area adjacent to the waterbody versus a similar sized area in the upland forest interior, riparian areas still contained higher richness but had fewer indicator species. We suggest that riparian areas and the adjacent shoreline forest contribute considerably to the regional richness of bird communities in the boreal forest and management policies should reflect this characteristic.

Predicting the seasonal dynamics of bird communities along an urban-rural gradient using NDVI

The normalized difference vegetation index (NDVI) is a strong and widely used proxy for bird species richness in urban environments. However, its potential to predict composition and seasonal dynamics of bird communities in urban areas remains unexplored. Our main objective was to analyze the power of NDVI to predict the seasonal dynamics of bird communities along urbanization gradients. Birds were surveyed in the breeding and non-breeding seasons. Bird species richness was estimated using COMDYN, and bird community composition was summarized in two axes using Non-metric Multidimensional Scaling (NMDS). For each of the breeding and non-breeding seasons, bird richness and the NMDS axis scores were related to both the mean NDVI of the season and the annual seasonal variation of NDVI (mean NDVI of the breeding season – mean NDVI of the non-breeding seasons). Results showed that NDVI decreased towards the most urbanized areas, with the highest seasonal variation occurring in agricultural areas. Single-family houses surrounded by yards held a high NDVI in both seasons and the lowest seasonal variation. Bird richness increased at intermediate levels of NDVI and was negatively related to the seasonal variations of the NDVI. Seasonal variation in bird community composition increased with annual mean and seasonal variation of the NDVI predicting higher seasonal stability of assemblage composition in urban than in non-urban areas. The results suggest that urbanization alters ecosystem functioning by reducing seasonal dynamics and the amount of primary productivity, which in turn promotes seasonal homogenization of urban avifauna relative to non-urban areas.

Ecological Impacts of Emerald Ash Borer in Forests at the Epicenter of the Invasion in North America

We review research on ecological impacts of emerald ash borer (EAB)-induced ash mortality in the Upper Huron River watershed in southeast Michigan near the epicenter of the invasion of North America, where forests have been impacted longer than any others in North America. By 2009, mortality of green, white, and black ash exceeded 99%, and ash seed production and regeneration had ceased. This left an orphaned cohort of saplings too small to be infested, the fate of which may depend on the ability of natural enemies to regulate EAB populations at low densities. There was no relationship between patterns of ash mortality and ash density, ash importance, or community composition. Most trees died over a five-year period, resulting in relatively simultaneous, widespread gap formation. Disturbance resulting from gap formation and accumulation of coarse woody debris caused by ash mortality had cascading impacts on forest communities, including successional trajectories, growth of non-native invasive plants, soil dwelling and herbivorous arthropod communities, and bird foraging behavior, abundance, and community composition. These and other impacts on forest ecosystems are likely to be experienced elsewhere as EAB continues to spread.

Avian Community Response to Short-rotation Aspen Forest Management

Abstract In the upper midwestern US and parts of Canada, forests dominated by Populus tremuloides (Aspen) are increasingly being considered as a bioenergy feedstock for power plants. When used for bioenergy, these forests are harvested at much younger ages than when they are used for more traditional products, such as pulpwood and lumber. To better understand the potential consequences of a shift in shorter-rotation-harvest strategies on avian communities, we employed point counts to examine bird community composition in a chronosequence (10-45 y since harvest) of 12 coppiced, even-aged Aspen stands. Young (8-15 y old), middle (20-44 y old), and mature (45 y old) stands had no significant differences in species richness or relative abundance, but distinct avian community assemblages were associated with each stand-age class. Four bird species were significantly associated with a particular age class. Maintaining a wide range of Aspen stand-age classes in the landscape appears to be the best strategy for c...

Habitat quality is more important than matrix quality for bird communities in protected areas.

Protected areas are meant to preserve native local communities within their boundaries, but they are not independent from their surroundings. Impoverished habitat quality in the matrix might influence the species composition within the protected areas through biotic homogenization. The aim of this study was to determine the impacts of matrix quality on species richness and trait composition of bird communities from the Finnish reserve area network and whether the communities are being subject of biotic homogenization due to the lowered quality of the landscape matrix. We used joint species distribution modeling to study how characteristics of the Finnish forest reserves and the quality of their surrounding matrix alter species and trait compositions of forest birds. The proportion of old forest within the reserves was the main factor in explaining the bird community composition, and the bird communities within the reserves did not strongly depend on the quality of the matrix. Yet, in line with the homogenization theory, the beta‐diversity within reserves embedded in low‐quality matrix was lower than that in high‐quality matrix, and the average abundance of regionally abundant species was higher. Influence of habitat quality on bird community composition was largely explained by the species' functional traits. Most importantly, the community specialization index was low, and average body size was high in areas with low proportion of old forest. We conclude that for conserving local bird communities in northern Finnish protected forests, it is currently more important to improve or maintain habitat quality within the reserves than in the surrounding matrix. Nevertheless, we found signals of bird community homogenization, and thus, activities that decrease the quality of the matrix are a threat for bird communities.

How disturbances and management practices affect bird communities in a Carpathian river ecosystem?

We studied how interactions between disturbances, succession, human alterations and other habitat and landscape attributes affect bird community patterns in a lower reach of a large West Carpathian river Váh with complex disturbance and alteration histories. Breeding-bird communities, their habitats (54 variables) and surrounding landscapes (11 metrics) were sampled using standardized point counts with limited distances at 40 riparian sites divided among two transects along a 12.9 km river stretch. The most frequent and abundant birds were generalists typically associated with forest edge habitats, such as Parus major, Sylvia atricapilla, Fringilla coelebs, Oriolus oriolus, Phylloscopus collybita, Sturnus vulgaris, Turdus merula and Luscinia megarhynchos. Abundances show significant increase at the lower transect responding apparently to greater size and heterogeneity of riparian habitats and more abundant food supply linked to more diverse and intense human influences in a suburban zone. Both indirect (NMDS) and direct ordination (CCA) revealed remarkably large number of evenly important factors underlying riparian bird-habitat interactions. It suggests considerable environmental heterogeneity and complexity of these interactions as a likely outcome of long and complex disturbance and alteration histories of the area. Yet structure and relative importance of first two gradients (longitudinal and lateral linkages) remains simple and stable, complying well with predictions of river continuum concept and stream ecosystem theory. Of the nine statistically significant variables most strongly correlated with first two CCA axes, percentages of Helianthus tuberosus, footpaths, fields, Calystegia sepium and steep banks uphold our hypotheses predicting significant effects of invasive species, visitor disturbances, agricultural land use and unaltered river banks/bed on bird community composition and structure. A small but significant contribution of patch size standard deviation within a 150 m radius to the CCA model provides the sole evidence for a hypothesized effect of adjacent terrestrial landscapes on riparian bird communities over different scales of patch size variability. Percentages of fields, invasive species and vertical banks along with average tree diameter were also the most productive predictors across our generalized linear models of riparian bird species diversity, evenness and abundance. Adopting an integrated river basin management, maintaining or restoring both longitudinal and lateral connectivity, taking advantage of natural processes and traditional management practices to maintain or restore riverine biodiversity can be recommended by our data in support of more systematic and evidence-based decision-making.

A taxonomic, functional, and phylogenetic perspective on the community assembly of passerine birds along an elevational gradient in southwest China.

Integrating multiple facets of biodiversity to describe spatial and temporal distribution patterns is one way of revealing the mechanisms driving community assembly. We assessed the species, functional, and phylogenetic composition and structure of passerine bird communities along an elevational gradient both in wintering and breeding seasons in the Ailao Mountains, southwest China, in order to identify the dominant ecological processes structuring the communities and how these processes change with elevation and season. Our research confirms that the highest taxonomic diversity, and distinct community composition, was found in the moist evergreen broadleaf forest at high elevation in both seasons. Environmental filtering was the dominant force at high elevations with relatively cold and wet climatic conditions, while the observed value of mean pairwise functional and phylogenetic distances of low elevation was constantly higher than expectation in two seasons, suggested interspecific competition could play the key role at low elevations, perhaps because of relative rich resource result from complex vegetation structure and human‐induced disturbance. Across all elevations, there was a trend of decreasing intensity of environmental filtering whereas increasing interspecific competition from wintering season to breeding season. This was likely due to the increased resource availability but reproduction‐associated competition in the summer months. In general, there is a clear justification for conservation efforts to protect entire elevational gradients in the Ailao Mountains, given the distinct taxonomic, functional, and phylogenetic compositions and also elevational migration pattern in passerine bird communities.

Food preferences determine habitat selection at multiple scales: implication for bird conservation in tropical forests

AbstractForest degradation and fragmentation of quality habitat across the tropics underpins an urgent need for the conservation of area‐sensitive species. Management strategies adopted for small‐scale landscape patches may not be suitable for conservation of different communities across large landscapes. In this context, we examined the relationship of bird community composition and different bird functional groups with tree cover and normalized difference vegetation index (NDVI) at multiple landscape scales and also tested the effect of forest degradation on bird community composition. This study demonstrates the importance of core forest habitat for the conservation of area sensitive species. Forest degradation had negative impact on the bird community composition though some functional groups were associated with degraded areas. At the community level, bird density and richness was best explained by a combination of both NDVI at the patch level (50 m scale) and tree cover at the landscape level (500 m scale) where the patch level character was found to be more important than landscape level character. Most of the functional groups which preferred insects as primary or secondary diet items were sensitive to differences at the patch level (NDVI), whereas large frugivorous species were sensitive to differences at the landscape level (tree cover). High vegetation complexity should be conserved at the patch level (at least up to 50 m) for the conservation of insectivorous, nectarivorous and small to medium size frugivorous groups and tree cover should be conserved at the landscape level (at least up to 500 m) for the conservation of large‐bodied frugivorous birds.

Scale dependence of biotic homogenisation by urbanisation: a comparison of urban bird communities between central Argentina and northern Finland

AbstractRecent studies showed contrasting results about the homogenising force of urbanisation on bird community composition at large and regional scales. We studied whether urbanisation promotes the homogenisation of wintering bird communities and if this varies when comparing towns located within a specific region and towns located in two different biomes of two countries. We used both similarity indices based on the presence/absence data and the abundance data in comparing communities. Processes governing bird community dissimilarity between urbanisation levels were examined with the partitioning of Sörensen index in species turnover and nestedness. We made bird surveys in town centres and suburban habitats of three cities located in the Pampean region of Argentina and in the boreal region of Finland using a single-visit study plot method. Rarefacted species richness did not differ amongst the town centres between the countries, but it was higher in the suburban areas of Argentina than in Finland. At the country-level comparison, we found a higher similarity amongst the town centres than amongst the suburban areas; whereas at the regional comparison, similarity between town centres was comparable to the similarity between suburban areas. The use of an abundance-based index produced a higher similarity between town centre communities of both countries than when using a presence-based index. The dissimilarity between habitats in Argentina was related to nestedness and to species turnover in Finland. Our results indicate that urban-based biotic homogenisation of bird communities is dependent on the scale used, being more evident when comparing cities of different biomes where the same and abundant bird species, such as sparrows and doves, dominate. At the regional scale, quite a high beta-diversity can still be found within urban habitats. Processes of community dissimilarity between urban habitats may differ according to the regional pool of species, being more related to nestedness toward the tropics.

The figs of winter: Seasonal importance of fruiting fig trees (Ficus: Moraceae) for urban birds

Birds and figs are conspicuous members of the tropical and subtropical ecosystems. Because they are easily observed and very speciose, their relationships have been well studied in many areas, and the figs are considered a keystone resource for many bird species which are efficient fig seed dispersers. Taiwan has a relatively high endemism rate for many taxa (17% of bird species) but because of its high human population density, most lowland habitats are heavily developed, of which much of it covered by dense urban habitation. To establish the importance of urban figs for birds, we focused our surveys mostly on three common urban fig species (Ficus caulocarpa, F. microcarpa and F. subpisocarpa). We observed trees with ripening figs from July 2013 to December 2016 in order to determine the composition of the fig-consuming bird community. In addition, we added all the information available in the scientific literature and birdwatchers' observations which we could find. In total, we observed 42 bird species consuming 18 fig species. The bird diversity in urban areas was non-negligible even during winter. Therefore, there are two reasons why figs are important for Taiwan's bird avifauna: in cities, the tree diversity is generally low so that figs provide a stable food resource; and since figs are fruiting all year-round, they are one of the few reliable resources available during winter when many migrant birds overwinter in Taiwan. Already crucial for many species in tropical and subtropical forests, fig trees may also be essential for urban birds in tropical and subtropical regions.

Coffee management and the conservation of forest bird diversity in southwestern Ethiopia

Moist evergreen forests of southwestern Ethiopia host high levels of biodiversity and have a high economic value due to coffee production. Coffee is a native shrub that is harvested under different management systems; its production can have both beneficial and detrimental effects for biodiversity. We investigated how bird community composition and richness, and abundance of different bird groups responded to different intensities of coffee management and the landscape context. We surveyed birds at 66 points in forest habitat with different intensities of coffee management and at different distances from the forest edge. We explored community composition using detrended correspondence analysis in combination with canonical correspondence analysis and indicator species analysis, and used generalized linear mixed models to investigate the responses of different bird groups to coffee management and landscape context. Our results show that (1) despite considerable bird diversity including some endemics, species turnover in the forest was relatively low; (2) total richness and abundance of birds were not affected by management or landscape context; but (3) the richness of forest and dietary specialists increased with higher forest naturalness, and with increasing distance from the edge and amount of forest cover. These findings show that traditional shade coffee management practices can maintain a diverse suite of forest birds. To conserve forest specialists, retaining undisturbed, remote forest is particularly important, but structurally diverse locations near the forest edge can also harbour a high diversity of specialists.

Relative importance of the land-use composition and intensity for the bird community composition in anthropogenic landscapes.

Humans are changing the biosphere by exerting pressure on land via different land uses with variable intensities. Quantifying the relative importance of the land‐use composition and intensity for communities may provide valuable insights for understanding community dynamics in human‐dominated landscapes. Here, we evaluate the relative importance of the land‐use composition versus land‐use intensity on the bird community structure in the highly human‐dominated region surrounding Paris, France. The land‐use composition was calculated from a land cover map, whereas the land‐use intensity (reverse intensity) was represented by the primary productivity remaining after human appropriation (NPP remaining), which was estimated using remote sensing imagery. We used variance partitioning to evaluate the relative importance of the land‐use composition versus intensity for explaining bird community species richness, total abundance, trophic levels, and habitat specialization in urban, farmland, and woodland habitats. The land‐use composition and intensity affected specialization and richness more than trophic levels and abundance. The importance of the land‐use intensity was slightly higher than that of the composition for richness, specialization, and trophic levels in farmland and urban areas, while the land‐use composition was a stronger predictor of abundance. The intensity contributed more to the community indices in anthropogenic habitats (farmland and urban areas) than to those in woodlands. Richness, trophic levels, and specialization in woodlands tended to increase with the NPP remaining value. The heterogeneity of land uses and intensity levels in the landscape consistently promoted species richness but reduced habitat specialization and trophic levels. This study demonstrates the complementarity of NPP remaining to the land‐use composition for understanding community structure in anthropogenic landscapes. Our results show, for the first time, that the productivity remaining after human appropriation is a determinant driver of animal community patterns, independent of the type of land use.

Temperate grassland songbird species accumulate incrementally along a gradient of primary productivity.

Global analyses of bird communities along elevation gradients suggest that bird diversity on arid mountains is primarily limited by water availability, not temperature or altitude. However, the mechanism by which water availability, and subsequently primary productivity, increases bird diversity is still unclear. Here we evaluate two possible mechanisms from species-energy theory. The more individuals hypothesis proposes that a higher availability of resources increases the total number of individuals that can be supported, and therefore the greater number of species that will be sampled. By contrast, the more specialization hypothesis proposes that increasing resource availability will permit specialists to exploit otherwise rare resources, thus increasing total diversity. We used 5 years of surveys of grassland songbird communities along an elevational gradient in British Columbia, Canada, to distinguish between these hypotheses. Vegetation changed markedly in composition along the gradient and contrary to the expectations of the more specialization hypothesis, bird community composition was remarkably constant. However, both total abundance and species richness of birds increased with increasing water availability to plants. When we used rarefaction to correct species richness for differences in total abundance, much of the increase in bird diversity was lost, consistent with the expectations of the more individuals hypothesis. Furthermore, high species richness was associated with reductions in territory size of common bird species, rather than the fine-scale spatial partitioning of the landscape. This suggests that bird diversity increases when greater resource availability allows higher densities rather than greater habitat specialization. These results help explain a pervasive global pattern in bird diversity on arid mountains, and suggest that in such landscapes conservation of grassland birds is strongly linked to climate and hydrology.

Short-Term Response of Vegetation and the Riparian Bird Community to Dam Removal on the Rogue River, Oregon

Both establishment and subsequent removal of dams inevitably alter river systems and may impact aquatic and terrestrial organisms. Despite both stated and implicit restoration goals of dam removal, relatively few studies have examined ecological response. Using a before-after-control-impact framework one year before and two years after, I evaluated the short-term effect of a dam removal on the Rogue River in southwestern Oregon on vegetation, bird community composition, and abundance of eight riparian-associated bird species. No differences were detected for tree and shrub strata cover after dam removal for either treatments or controls. Within the three treatment habitat types (mainstem, slough, wetland), shrub and ground cover declined only in wetland areas, likely as a result of post dam removal vegetation management. Significant changes in relative abundance were found for only one of eight focal bird species. Chamaea fasciata (Wrentit) abundance declined on treatment points following dam removal, possibly a response to the decrease in cover of the shrub and ground vegetative strata in the wetland. Bird community composition did not differ between controls and treatments either before or after dam removal. In combination, the similarities in bird community composition and lack of detectable change in abundance for seven riparian associated species are consistent with the results for vegetative cover, suggesting shrub and tree components of riparian habitat did not change substantially in the two years following dam removal. Results from this study have important management implications for maintaining or restoring riparian vegetation in conjunction with future dam removals.