Avian Community Composition Research Articles

Stochastic and deterministic drivers of spatial and temporal turnover in breeding bird communities

AbstractAimA long‐standing challenge in ecology is to identify the suite of factors that lead to turnover in species composition in both space and time. These factors might be stochastic (e.g. sampling and priority effects) or deterministic (e.g. competition and environmental filtering). While numerous studies have examined the relationship between turnover and individual drivers of interest (e.g. primary productivity, habitat heterogeneity, or regional – ‘gamma’ – diversity), few studies have disentangled the simultaneous influences of multiple stochastic and deterministic processes on both temporal and spatial turnover. If turnover is governed primarily by stochastic sampling processes, removing the sampling effects of gamma diversity should result in non‐significant relationships between turnover and environmental variables. Conversely, if deterministic processes govern turnover patterns, removing sampling effects will have little influence on turnover gradients. Here, we test these predictions.LocationThe United States.MethodsContinental‐scale, multidecadal data were used to quantify spatial and temporal turnover in avian community composition within 295 survey routes. A series of regression and structural equation models were coupled with a null model to construct statistical models describing turnover patterns.ResultsExamining explanatory variables alone or in combination showed that spatial and temporal turnover increased together, decreased with primary productivity and increased with habitat heterogeneity. The relationships between turnover and all variables became weaker when sampling effects were removed, but relationships with primary productivity and habitat heterogeneity remained relatively strong. In addition, spatial turnover increased strongly with spatial gamma diversity after sampling effects were removed.Main conclusionsOur results show that spatial and temporal turnover are related to each other through a stochastic sampling process, but that each type of turnover is further influenced by deterministic processes. The relative influence of deterministic processes appears, however, to decrease with primary productivity and increase with habitat heterogeneity across an east–west longitudinal gradient in North America.

Bird community structure in natural and urbanized habitats along an altitudinal gradient in Pauri district (Garhwal Himalaya) of Uttarakhand state, India

In the Indian subcontinent there is hardly any study that compares the bird community structure of urban/suburban areas with those of forest habitat. The present survey identified diverse assemblages of birds in the Pauri district at different elevations. A total of 125 bird species belonging to 40 families including two least count species (Lophura leucomelanos and Pucrasia marcolopha) were recorded during this survey in the forest and urbanized habitats of Pauri District (Garhwal Hiamalaya) of Uttarakhand state, India. The high elevation (Pauri 1600–2100 m a.s.l.), mid elevation (Srikot-Khanda 900–1300 m a.s.l.) and low elevation (Srinagar 500–900 m a.s.l.) contributed 88.8%, 63.2% and 58.4% of the total species respectively. Rarefaction analysis and Shannon diversity index showed that the high elevation forest habitat had highest bird species richness (BSR) and bird species diversity (BSD) followed by the mid and then the low elevation forests. BSR and BSD fluctuated across seasons at all elevations but not across habitat types. Present study provides a base line data about avian community composition in urbanized and natural habitats along altitudinal gradient in the study area. This information may be useful to the conservation biologists for the better management and conservation of the avifauna in the Western Himalaya, a part of one of the hot biodiversity spots of the world.

The Influence of Summer Climate on Avian Community Composition in the Eastern Boreal Forest of Canada

DesGranges, J., and M. LeBlanc. 2012. The influence of summer climate on avian community composition in the eastern boreal forest of Canada. Avian Conservation and Ecology 7(1): 2.http://dx.doi.org/10.5751/ACE-00512-070102

Grassland Bird Communtiy Response To Large Wildfires

We studied breeding season communities of grassland birds on short-grass and mixed-grass prairie sites during the second and third breeding seasons following two large wildfires in March 2006 in the Texas panhandle, USA. There was an apparent temporary shift in avian community composition following the fires due to species-specific shifts associated with life-history traits and vegetation preferences. Species that prefer sparse vegetation and bare ground on short-grass sites, such as Horned Lark (Eremophila alpestris), benefited from wildfires, while others, such as Western Meadowlark (Sturnella neglecta), that prefer more dense vegetation, were negatively impacted. Mixed-grass sites had species-specific shifts in 2007, two breeding seasons after the fires; grassland bird communities on burned plots were similar by 2008 to those on unburned plots. Avian communities appeared to return to pre-burn levels within 3 years following wildfires. Many of the responses in our study of wildfire were similar to those reported following prescribed fires elsewhere. Prescribed fires appear to have similar effects on the avian community despite differences in intensity and environmental conditions during wildfires.

Effects of oak barrens habitat management for Karner blue butterfly (Lycaeides samuelis) on the avian community

The federally endangered Karner blue butterfly (Lycaeides samuelis) is the focal species for a conservation plan designed to create and maintain barrens habitats. We investigated whether habitat management for Karner blue butterflies influences avian community structure at Fort McCoy Military Installation in Wisconsin, USA. From 2007 through 2009 breeding bird point count and habitat characteristic data were collected at 186 sample points in five habitat types including two remnant barrens types, barrens habitat restored from woodland and managed specifically for the Karner blue butterfly, and two woodland habitat types. Although the bird community of managed barrens was not identical to the communities of remnant barrens, the Field Sparrow (Spizella pusilla), a species of conservation concern, and sparse canopy associated bird species, such as the Baltimore Oriole (Icterus galbula) and Eastern Bluebird (Sialia sialis) were predicted to occupy managed barrens and remnant barrens in similar proportions. Adjacent habitat was the most influential factor in determining the community of bird species using the managed barrens. In Wisconsin, and likely throughout the range of the Karner blue butterfly, management for the butterfly creates habitat that attracts a bird community similar to that of remnant barrens, and benefits several avian species of conservation concern. Additionally, the landscape context surrounding the managed habitat influences avian community composition. Managed barrens that are adjacent to remnant barrens, rather than adjacent to woodland habitats, have the highest potential for conserving barrens breeding birds.

Applying a territory mapping method to census the breeding bird community composition in a montane forest of Taiwan.

Territory mapping is considered one of the most accurate methods for estimating forest bird densities and studying the subtle relationships between birds and their habitats. Nevertheless, the territory mapping method has rarely been applied to estimate entire bird communities in tropical and subtropical regions of Asia. We conducted territory mappings in an area of around 40 ha at a mid-elevation site in Taiwan for a period of 20 consecutive weeks (40 census days in total) from early March to late July 2005 to establish field protocols of territory mapping and examine the effectiveness of this method in detecting the avian community composition. Fifty-seven bird species from 5719 registrations were recorded during the 40 field censuses. The results suggested that the optimal period for territory mapping at the study site was during 10~12 wk from early April to late June, because during this period, the prevalences of most breeding species were highest. Within this period of time, we registered 88% of the breeding species recorded historically. Repeatedly playing 2 repetitions of the territorial songs of the 14 commonest species did not significantly affect the total number of breeding species or registrations recorded on each census. These results indicate that with a good trail system, the territory mapping method is suitable for censusing bird communities in forests of Taiwan and similar habitats in Asia. However, due to the non-territorial behavior, asynchronous breeding season, and multiple broodings of birds in tropical and subtropical regions, the efficiency of territory mapping method is not as good as that in temperate regions.

Bird assemblages in natural and urbanized habitats along elevational gradient in Nainital district (western Himalaya) of Uttarakhand state, India

Abstract The Indian subcontinent is amongst the biologically better known parts of the tropics and its bird fauna has been well documented. However, avian community composition and diversity along elevational gradients and amongst habitat types remains unclear in India. We attempted to estimate bird assemblages in terms of diversity, species composition, status and abundance in urban and forest habitats of Nainital district of Uttarakhand (350–2450 m asl; 29°N), Western Himalayas. We sampled different elevational gradients and to understand the effect of urbanization and season on avian community composition. Field studies were conducted during January 2005 to January 2007. Results indicated that the forest had more complex bird community structure in terms of higher species richness (14.35 vs 8.69), higher species diversity (Shannon’s index 4.00 vs 3.54), higher evenness (0.838 vs 0.811) and more rare species (17 vs 5) as compared to urban habitat. However, the abundance of 11 species was higher in urban habitats. Bird Species Richness (BSR) varied considerably among study areas (91 to 113 species), was highest (113 species) at mid elevation (1450–1700 m asl) and decreased (22 species) at high elevation (1900–2450 m asl). It seems that high BSR at mid altitudes is not caused by the presence of a group of mid altitude specialists but rather that there is an overlap in the distribution of low land and high elevation specialists at this altitude. BSR and Bird Species Diversity fluctuated across seasons but not habitat type.

Birds as marine–terrestrial linkages in sub-polar archipelagic systems: avian community composition, function and seasonal dynamics in the Cape Horn Biosphere Reserve (54–55°S), Chile

Marine environments are known to affect adjacent terrestrial biotic communities. In South America’s sub-Antarctic archipelago, birds are the most abundant and diverse terrestrial vertebrate assemblage. We hypothesized that birds would reflect a marine influence that would gradually decrease inland, expecting to find greater species richness, abundance, and biomass near the sea with decreases toward the island interior. We seasonally compared these parameters, with identified indicator species and assessed functional groups at 0, 150, and 300 m from the coast. Unexpectedly, we found a marked marine (0) and terrestrial (150–300) patterns for avian assemblages, rather than a gradient. In addition, seasonal patterns were warm (spring–summer) and cold (autumn–winter). The only parameter that displayed a true gradient was avian biomass in spring. During the cold season, higher values were observed in all variables for coastal assemblages, compared to inland sites. In the warm season, abundance and richness of coastal and terrestrial assemblages were similar, owing to migratory species. Milvago chimango was the only species abundant and frequent in both terrestrial and coastal systems, thereby indicating potential as a marine–terrestrial vector. Functionally, coastal assemblages were conformed of herbivores, carnivores, and scavengers, while terrestrial communities were made up of omnivores and insectivores. We conclude that the sea coast is a unique habitat in this archipelago, providing refuge for both marine and terrestrial sub-Antarctic birdlife particularly in the cold season. The relevance of the land/sea ecotone is poorly known, but important is given to high demand for the installation of salmon aquaculture facilities along the southern Chilean coastline.

Effects of artificial afforestation and successional stage on a lowland forest bird community in southern China

To examine the relationship between forest succession after severe logging forestry practices and the composition of avian communities, we investigated how forest bird composition and guild structure change as a function of structural properties along a successional gradient, including a climax mature forest (>400 years), a rehabilitated mixed forest (50–70 years), and a disturbed Masson pine forest (70 years) of the Dinghushan Nature Reserve, Guangdong Province, China. Of a total of 51 resident species recorded, mixed forests hosted the highest numbers of individuals and species, reflecting the high species richness of both forest and non-forest species. For forest-dependent species, however, mature stands had the highest observed and estimated species richness. Of 6 habitat-use guilds identified, vertical-profile generalists and understory-birds formed the two dominant guilds, accounting for 54.0% and 38.7% of all individuals respectively. The results of canonical correspondence analysis (CCA) clearly showed that most forest-dependent species were associated with high proportions of native canopy cover and the mean density of dead trees and large trees, which are characteristic of old-growth mature forests (horizontal heterogeneity) at stand level. Accordingly, conservation efforts should focus on the specialized requirements of the most habitat-restricted species in the future, especially for understory insectivores (Babblers) and large-tree users in mature subtropical monsoon forests of southern China. Moreover, since regenerating mixed forests are very similar to mature forests in both vegetation structure and bird community composition, we recommend that logging cycles (>50 years) be increased to a minimum of 50 years in southern China, so that a balance between economic and ecological interest can be reestablished.

Do fire and rainfall drive spatial and temporal population shifts in parrots? A case study using urban parrot populations

Populations of several species of native parrots have been increasing in many Australian cities since the 1980s contributing to a shift in the composition of urban avian communities. Anecdotal evidence suggests that some species of parrot may move into the urban landscape during environmental disturbances, such as wild fires or periods of decreased rainfall. This study seeks to determine the extent to which fire and rainfall explain changes in the abundance of parrots in urban Sydney. Multiple regression using the Akaike Information Criterion was used to analyse a 26-year data set, beginning in 1981, to measure the change in abundance of 13 species of parrot in response to wild fire and rainfall. Wild fire, within a radial distance of 100 km, significantly predicted changes in abundance of five species of parrot in urban Sydney. Local and/or inland rainfall significantly predicted changes in abundance of six parrot species in the urban landscape, with decreases in inland rainfall resulting in an increase in abundance in the urban landscape of parrots that traditionally inhabited inland areas.

The Effects of Cropping Systems on Avian Communities in Cacao and Banana Agro-Forestry Systems of Talamanca, Costa Rica

Multispecies agro-forestry is generally lauded for providing ecosystem services, especially in tropical environments. Avian communities contribute to services such as biodiversity and pest management. Characterizing and evaluating avian community composition in similar cropping systems will help optimize management for ecosystem services. We examined the relationship between cropping system vegetation and avian communities in four shaded agro-forestry systems common to the Limprovince of Costa Rica: abandoned and managed systems of cacao, cacao with banana, and banana. During two field seasons, we detected 2605 birds from 106 species and identified 2791 trees and shrubs from 62 morphospecies. We compared vegetation and avian species richness across systems with mixed-effects linear models. Canopy, understory, and groundcover vegetation differed among agro-forestry systems. More ground- and understory-foraging forest species were detected in agro-forestry systems lacking banana, whereas richness of agricultural generalist species was highest in systems with banana. Richness of understory- and ground- foraging species correlated with understory tree species richness and leaf litter. Our results indicate that shaded cacao and banana systems can have similar canopy- foraging species richness that includes both agricultural and woodland generalist species, but that interspersing banana with cacao can adversely influence understory forest bird community composition. Agro-forests with diverse understory vegetation support more understory-foraging bird species that have proven valuable in pest management. Abstract in Spanish is available at http: //www.blackwell-synergy.com/loi/btp.

Short-Term Effects of Thinning and Burning Restoration Treatments on Avian Community Composition, Density, and Nest Survival in the Eastern Cascades Dry Forests, Washington

Abstract We monitored the short-term (<3 years) response of land birds to restoration treatments (thin, burn, and thin-burn) in dry forests located on the eastern slope of the Cascade Range in Washington. Overall avian community composition did not change among the treatments. However, individual species responses varied with the chipping sparrow showing lower density in treatments, whereas hermit thrush, mountain chickadee, white-headed woodpecker, western bluebird, American crow, and common raven increased in treatment units. Daily survival rates of nesting guilds were similar in treated versus control stands; however, burn-only showed lower daily survival rates compared with other treatments. Additional research is needed to validate this result. Cavity-nesters (mountain chickadee, red-breasted nuthatch, and house wren) and foliage-nesters (chipping sparrow) used trees that were larger in diameter than available regardless of the treatment. Our results, in combination with other results from this study area, provide important implications for managers: (1) thin-burn treatments were effective at restoring habitat for several avian focal species; (2) spring burn treatments should be carefully designed to achieve desired restoration objectives; (3) large trees provide important habitat functions and are a key component for maintaining or restoring the viability of focal avian species; and (4) additional research is needed to better understand the effects of spring burning and the long-term effects of dry forest restoration treatments.

Changes in Vegetation Structure through Time in a Restored Tallgrass Prairie Ecosystem and Implications for Avian Diversity and Community Composition

Grassland birds are one of the most endangered taxa in temperate North America. Because many species declines have been linked to habitat fragmentation and loss, large-scale prairie restoration projects have the potential to provide critical habitat for these declining species. We examined how the structure of restored grassland habitat changes through time and how diversity and community composition of grassland birds respond to these changes. Our study was completed at Neal Smith National Wildlife Refuge, a large-scale prairie restoration in central Iowa. Vegetation composition and structure were measured at 42 restored grassland plots throughout the refuge in 2007. Birds were surveyed at these locations from 1994 to 2007. Survey points were sorted into five categories (out of crop rotation for 1, 2, 3, 4–6, and > 6 y). In the initial phases of restoration, species such as horned larks, red-winged blackbirds, and killdeer were abundant. Other species such as common yellowthroats and dickcissels were more common in established restored points. Henslow’s sparrows appeared only at survey points that were out of crop rotation for more than 6 years. Diversity peaked in survey points that were 2–3 years out of crop rotation and points that were more than 6 years out of rotation. Community composition shifted through the chronosequence of prairie plantings. Changes in diversity and shifts in community composition can be explained by changes in vegetative structure. Our results suggest that managing for a variety of restored prairie stages will best maintain the highest levels of avian diversity and abundance.

Composition versus physiognomy of vegetation as predictors of bird assemblages: The role of lidar

Whether diversity and composition of avian communities is determined primarily by responses of species to the floristic composition or to the structural characteristics of habitats has been an ongoing debate, at least since the publication of MacArthur and MacArthur (1961). This debate, however, has been hampered by two problems: 1) it is notoriously time consuming to measure the physiognomy of habitat, particularly in forests, and 2) rigorous statistical methods to predict the composition of bird assemblages from assemblages of plants have not been available. Here we use airborne laser scanning (lidar) to measure the habitat (vegetation) structure of a montane forest across large spatial extents with a very fine grain. Furthermore, we use predictive co-correspondence and canonical correspondence analyses to predict the composition of bird communities from the composition and structure of another community (i.e. plants). By using these new techniques, we show that the physiognomy of the vegetation is a significantly more powerful predictor of the composition of bird assemblages than plant species composition in the field and as well in the shrub/tree layer, both on a level of p < 0.001. Our results demonstrate that ecologists should consider remote sensing as a tool to improve the understanding of the variation of bird assemblages in space and time. Particularly in complex habitats, such as forests, lidar is a valuable and comparatively inexpensive tool to characterize the structure of the canopy even across large and rough terrain.

Recommendations for design and management of golf courses and green spaces based on surveys of breeding bird communities in Montreal

It has been suggested that golf courses can serve as wildlife habitat, provided key landscape features are present. Breeding bird communities were characterized on six golf courses and six green spaces in the Montreal area, Quebec, Canada. Landscape variables were measured to see which were the best correlates of avian species richness within and across sites. While there was no difference in the mean number of bird species (golf: 31 ± 2.4 SE; green: 33 ± 4.5 SE), golf courses supported fewer total species (golf: 55; green: 66). However, most of the species contributing to the green spaces’ higher cumulative total were found at one site. Species composition differed between golf courses and green spaces, and was most correlated with site size, housing density surrounding the site, and extent of coniferous tree, grass, and vegetated water cover. Our results support the work of others, and suggest that golf course architects can improve habitat quality by: (1) increasing deciduous and coniferous tree cover; (2) increasing native vegetation within and surrounding all water bodies; (3) reducing the amount of highly managed grass area; (4) ensuring large, undeveloped buffers are maintained to prevent a shift in avian community composition with increasing urban sprawl; and (5) ensuring sites are planned as large as possible.

Toward increasing avian diversity: urban wildscapes programs

Residential development in urban and suburban settings is a major cause of habitat change affecting avian communities. The effects from land-clearing prior to initiation of construction followed by landscaping focused on traditional aesthetics has reduced habitat diversity at multiple levels. These residential developments generally emphasize the use of ornamental, nonindigenous vegetation with little to no concern for native species and natural vegetation parameters. As a result, avian community composition and diversity is changed. We compared avian communities at three sites in Bexar County, Texas; two residential neighborhoods in the city of San Antonio and an unincorporated undeveloped site, Government Canyon State Natural Area. The residential neighborhoods were a Texas Parks and Wildlife Department certified wildscapes development and a traditionally developed neighborhood. Bird abundance and species richness in the neighborhoods were greater than at Government Canyon State Natural Area (P < 0.01), but no differences existed between the neighborhoods. Bird diversity was greatest at the wildscapes neighborhood (P < 0.01). Further, bird diversity did not differ between the traditional neighborhood and the natural area. Evenness differed among sites (P = 0.006) with the natural area having the greatest value. Bird diversity measures correlated with woody plant density and vertical cover (r = 0.358 to 0.673, P < 0.05) at residential sites. Residential areas incorporating natural landscapes into their design attracted a greater variety of birds than traditionally landscaped residential areas.

Turkey fecal microbial community structure and functional gene diversity revealed by 16S rRNA gene and metagenomic sequences

The primary goal of this study was to better understand the microbial composition and functional genetic diversity associated with turkey fecal communities. To achieve this, 16S rRNA gene and metagenomic clone libraries were sequenced from turkey fecal samples. The analysis of 382 16S rRNA gene sequences showed that the most abundant bacteria were closely related to Lactobacillales (47%), Bacillales (31%), and Clostridiales (11%). Actinomycetales, Enterobacteriales, and Bacteroidales sequences were also identified, but represented a smaller part of the community. The analysis of 379 metagenomic sequences showed that most clones were similar to bacterial protein sequences (58%). Bacteriophage (10%) and avian viruses (3%) sequences were also represented. Of all metagenomic clones potentially encoding for bacterial proteins, most were similar to low G+C Gram-positive bacterial proteins, particularly from Lactobacillales (50%), Bacillales (11%), and Clostridiales (8%). Bioinformatic analyses suggested the presence of genes encoding for membrane proteins, lipoproteins, hydrolases, and functional genes associated with the metabolism of nitrogen and sulfur containing compounds. The results from this study further confirmed the predominance of Firmicutes in the avian gut and highlight the value of coupling 16S rRNA gene and metagenomic sequencing data analysis to study the microbial composition of avian fecal microbial communities.

Fuel‐Reduction Treatment Effects on Avian Community Structure and Diversity

Abstract: We assessed responses of the breeding bird community to mechanical thinning and prescribed surface fire, alone and in combination, between 2000 and 2006 in ponderosa pine (Pinus ponderosa) forests in northern Arizona, USA. Fuel‐reduction treatments did not affect species richness or evenness, and effects on density of 5 commonly detected species varied among species. Populations of some species, such as the western bluebird (Sialia mexicana), increased following burning treatments, whereas others, such as the mountain chickadee (Poecile gambeli), decreased in response to thinning treatments. Our results also identified a temporal response component, where avian community composition and structure changed synchronously on all treatments over time. Given the modest effects these small‐scale fuel‐reduction treatments had on avian composition and the specific density responses of particular species, our results suggest that land managers should consider implementing prescribed surface fire after thinning projects, where appropriate.

Landscape-scale disturbance and boreal forest birds: Can large single-pass harvest approximate fires?

Boreal forest birds have adapted to changes caused by natural disturbances such as fire and this adaptation forms the basis for the Natural Disturbance Paradigm (NDP) underlying recent proposed changes in forest harvesting practices in western Canada. To date, this paradigm has been evaluated primarily at the stand level and within conventional harvesting systems. The potential for improvements in avian conservation at the landscape scale by adopting the NDP approach is largely unknown. We examined the effects of landscape-scale disturbances on forest bird communities by contrasting richness and abundance of birds in (1) 16 single-pass harvest sites with residual forest patches, (2) 29 multi-pass harvest sites with residuals; and (3) 15 salvage-logged post-fire sites with variable harvest intensity. We contrasted bird communities in these treatments with those in unsalvaged post-fire sites of similar age. Post-fire sites were used to provide a metric of the Natural Range of Variation (NRV) to be expected in bird communities. Sites were surveyed for avian community composition and abundance 1–5 years post-disturbance. Redundancy analysis indicated that bird communities differed from the NRV in all of the harvest treatments. However, single-pass harvests provided a somewhat better fit to NRV than did multi-pass harvesting. Avian community similarity was influenced by non-linear responses to area harvested, amount of residual retention, residual composition and pre-disturbance forest composition. An optimization routine created from a General Linear Model, suggests that community similarity to NRV can be maximized by using single-pass harvests over multi-pass harvests, harvesting 66–88% of the timber in the planning unit, and retaining 5–19% of the disturbance area as live residual patches, with 50% of harvests having at least 9% of the area in residuals.

Lean birds in the city: body size and condition of house sparrows along the urbanization gradient

1. Urbanized habitats differ from natural ones in several ecological features, including climate, food availability, strength of predation and competition. Although the effects of urbanization on avian community composition are well known, there is much less information about how individual birds are affected by these human-generated habitat differences. 2. In this study we investigated the relationships between the morphological characteristics and the degree of habitat urbanization in house sparrows, Passer domesticus (Linne 1758) . We collected data for more than 1000 non-breeding adult birds in Hungary between 1997 and 2006, from seven sites including farmlands, suburban areas and city centres. 3. We found that the body mass, tarsus length and body condition of free-living sparrows differed among the sites: birds in more urbanized habitats were consistently smaller and in worse condition than birds in more rural habitats. A composite measure of habitat urbanization (based on building density, road density and vegetation cover) explained over 75% of variance between sites in the studied traits, after we controlled for the effects of sex, year, season and time of capture. 4. The difference in body mass between rural and urban sparrows was significant when birds were kept in aviaries under identical conditions, with constant ad libitum food availability. It is therefore unlikely that the reduced body size and condition of urban sparrows are a consequence of reduced access to food for adults (e.g. due to strong competition), or their short-term responses to high food predictability (e.g. by strategic mass regulation). 5. We suggest that habitat differences in nestling development or adaptive divergence of sparrow populations due to distinct environmental conditions (such as differing predation pressure) may account for the differences along the urbanization gradient.