Experimental evaluation of bird movements in a fragmented Neotropical landscape

The potential negative effects of forest fragmentation on animal movement and dispersal, and its consequences for population persistence, require an understanding of how modified landscapes affect movement decisions of forest species. We used a dispersal challenge experiment, whereby we released individuals of six bird species in a cattle pasture at different distances from forest edge (0, 50, 100 and 150 m), in a fragmented tropical landscape in Mexico to investigate the gap-crossing abilities and movement behavior of six species of tropical forest birds. Gaps as narrow as 50 m affected movement behavior of tropical forest birds. A sharp change in movement behavior in gaps 100 m and larger suggested the presence of a threshold distance beyond which birds are less likely to attempt and successfully navigate during trans-gap flights. Bird responses varied with degree of forest dependence: three forest-restricted species showed greater latency to cross gaps, independent of gap width, as compared with forest-unrestricted species. Gap width had a stronger effect on the orientation and destination of forest-restricted species than that of forest-unrestricted species. The concordance of our results with those found in species-distribution and radio-tracking studies indicates that dispersal challenge experiments provide reliable predictive information about response of forest birds to gaps between isolated forest fragments. In the landscape we studied, reducing gaps to no more than 50 m and providing corridors or shade trees between fragments should facilitate movements of forest birds.     

Litter contribution to diurnal and annual soil respiration in a tropical montane cloud forest

Respiration of CO₂ from soils (R_s) is a major component of the carbon cycle of ecosystems, but understanding is still poor of both the relative contributions of different respiratory sources to R_s, and the environmental factors that drive diurnal variations in R_s. We measured total and litter-free R_s at half-hourly intervals over full 24 h periods, and thereafter twice a month for 10 months in a tropical montane cloud forest (TMCF) in Peru. Total R_s declined by about 61% during the night as a result of variations in respiration rate in the litter, which were partly correlated with the soil surface air temperature. Most of the diurnal variation of R_s in this TMCF appears to be driven by respiration in the litter layer, which contributed 37% to the total soil CO₂ efflux. Total R_s rates at this particular site would have been overestimated by 60% if derived from daytime measurements that had not been corrected for diurnal variations in R_s.     

Riparian zones provide for distinct bird assemblages in forest mosaics of south-east Australia

Riparian zones are a characteristic component of many landscapes throughout the world and increasingly are recognised as key areas for biodiversity conservation. Their importance for bird communities has been well recognised in semi-arid environments and in modified landscapes where there is a marked contrast between riparian and adjacent upslope vegetation. The value of riparian zones in largely intact landscapes with continuous vegetation cover is less well understood. In this study, birds were surveyed at 30 pairs of riparian and adjacent non-riparian sites in extensive mesic forests of the Victorian Highlands, Australia. Riparian sites were floristically distinct from non-riparian sites and had a more complex vegetation structure, including a mid-storey tree layer mostly absent from non-riparian sites. Bird assemblages at riparian sites had significantly greater richness, abundance and diversity of species than was recorded at adjacent non-riparian sites. Species composition also differed significantly between these habitat types. Compositional differences in assemblages were due to a suite of distinctive species in each habitat and to significant contrasts in the densities of species that occurred in both habitat types. Many species (36%) attained a significantly greater abundance in riparian habitats. The distinctiveness and richness of the riparian avifauna contribute to the diversity of continuous forest landscapes. The spatial patterning of the avifauna, the occurrence of complementary assemblages, the presence of rare species and the potential for riparian habitats to serve as refuges, all point to the value of riparian zones and highlight the importance of landscape-level planning and management for avifaunal conservation.     

Mixed-species bird flocks from Brazilian Atlantic forest: the effects of forest fragmentation and seasonality on their size, richness and stability

The effects of forest fragmentation on species richness, size and stability of bird mixed-species flocks were studied during rainy and dry seasons at nine Atlantic Forest fragments in Minas Gerais state, southeastern Brazil. Two distinct kinds of mixed-species flocks were identified at the study area: heterogeneous flock and understory flocks. The first type of association was observed in all forest fragments and was composed mainly of canopy and midstory species, but may include a few understory species. Understory flocks were composed mainly of understory species and were detected in only three forest fragments. Forest fragment area and season significantly affected species richness, size and stability of heterogeneous flocks. Moreover, species richness in heterogeneous flocks was correlated with total species pool in forest fragments. Species richness and size of understory flocks were different among the three forest fragments, but season influenced only flock size. Understory flock stability differed neither among forest fragments nor between seasons. The red-crowned ant-tanager Habia rubica was the nuclear and leader species of understory flocks, and determined the occurrence of this association in forest fragments. Our results show that forest fragmentation can affect species’ interactions in foraging associations, and that conservation plans for the region should preserve nuclear species of mixed-species flocks and their preferred habitats.     

Arthropod assemblages in vegetative vs. humic portions of epiphyte mats in a neotropical cloud forest

Epiphyte mats (contiguous pieces of live and dead epiphytes perched upon branches of trees) are a conspicuous component of tropical cloud forests and harbor diverse meso- and microarthropod communities. We investigated differences in arthropod assemblage structure between the vegetative (green) and humic (brown) portions of epiphyte mats in a lower montane forest in Monteverde, Costa Rica. Because of qualitative differences between the two substrates, we hypothesized that they would support different arthropod communities and that variation in community parameters would be linked to the quantity of brown material present in a mat sample. The green fraction contained twice as many individuals and species per gram dry mass than the brown fraction. Morphospecies composition was very similar between green and brown portions, but the relative abundance of several taxa differed significantly between the substrates. Contrary to our prediction, total arthropod abundance and richness in a sample were not correlated with the proportion of brown material present. In laboratory trials, the most common morphospecies of oribatid mite in this system showed a preference for brown substrates.     

Nest predation in a fragmented Afrotropical forest: evidence from natural and artificial nests

Nest predation accounts for a substantial share of nest failure and low reproductive success in most tropical songbirds. Normally, forest fragmentation leads to an increase in nest predation pressure due to reduced cover, fewer (and poorer) nest sites and predator influxes from the surrounding habitats. To test this hypothesis, we studied natural nesting behaviour and nest success of the white-starred robin (Pogonocichla stellata) in seven Afrotropical forest fragments differing in size and level of habitat disturbance. Based on data from 12 nests, we estimate that 29% of all natural nests initiated by the robins survive to produce fledglings across all fragments. We also conducted an experiment using artificial (plasticine) model-eggs to reveal potential predators and compare relative predation rates amongst fragments. This experiment revealed that small mammals might be the major predators on robin nests at the egg-stage. In addition, it showed that the highest incidences of nest disturbance during this stage were in the most heavily disturbed fragment. This was presumably attributable to an influx of mammalian predators from the surrounding habitats as forest degradation created suitable habitats for them. Such an infiltration was recently reported in this study site. Both nest placement and microhabitat did not significantly affect depredation levels in our experiment. This suggests that depredation was predominantly incidental (i.e., predators mainly encountered nests fortuitously while foraging for other food items), where the likelihood of encountering a nest largely depended upon the prevalence of the principal potential predators - the small mammals.     

Patch size and isolation effects on epiphytic and epiphyllous bryophytes in the fragmented Brazilian Atlantic forest

An inventory of the understory forest epiphyte and epiphyllous bryophyte floras was carried out in eight Atlantic rain forest fragments. The fragments were between 7 and 500 ha in size and belonged to two areas (lowland and submontane) of the state of Pernambuco, northeastern Brazil. The aim of this study was to investigate the effect of habitat fragmentation and changes in the natural landscape on community structure (composition, richness, diversity, and abundance). Although the influence of altitude was noted in this study by increasing richness, diversity, and abundance, it was clear that in some fragments the influence of fragment size and isolation can be more important than this positive environmental factor. Fragment size and isolation affect both communities but the last one seems to be a stronger threatening factor for the epiphylls. The least isolated and the largest fragments housed the richest floras - especially in relation to the epiphylls - and had the greatest proportion of shade species. Habitat fragmentation negatively affected epiphytic and epiphyllous bryophytes and increased the representation of species with larger niches (generalists) while decreasing the representation of species with smaller niches (typically found in shady or in sunny areas). The results suggests that the critical fragment size necessary for bryophyte preservation must be correlated with insularity levels; for epiphytes, however, it is likely that low levels of isolation cannot compensate borderline effects as with the epiphyllous bryophytes.     

Nestedness of bird assemblages in fragmented Afromontane forest: the effect of plantation forestry in the matrix

Bird species composition (61 species) of Afromontane forest fragments (n = 19) embedded in a natural grassland matrix or a plantation forestry matrix were compared. Forest bird assemblages in a natural grassland matrix displayed a nested non-random species distribution. Species loss was significantly area-dependent and predictable. Assemblages in the plantation-dominated matrix were also nested but were neither area- nor isolation-dependent and these potential processes had equal influence over bird species composition. Forests patches <50 ha in plantation were more species rich than those in a grassland matrix and visa versa for patches >50 ha. The effect of the matrix is revealed here with arboreal plantation cover promoting dispersal across the landscape, favouring abundant generalist bird species in smaller patches than expected in the plantation matrix, and the loss of some specialists and species living at low densities from large (>50 ha) patches. Because of a significant species-area relationship and possible area-dependent extinction, the largest forest patch in the nested series in a grassland matrix protects most bird species (89%). However, in a plantation matrix the absence of a species-area relationship requires the combination of many larger forest patches to protect most bird species. Plantation forests in the matrix significantly alter forest bird assemblage structure and composition. To avoid the effects of insularisation and to maintain the viability of bird communities in Afromontane forests requires conservation of the larger forests in a natural matrix. However, for forests in a plantation matrix, the management of dispersal processes by manipulation of the matrix may be as important to conserving species and communities as is minimising extinctions. Avoiding placing plantations near large forests, increasing the size of small patches, and where afforestation is unavoidable, placing plantations in the vicinity of small rather than large forest patches, are preferable management practices.     

What is the appropriate paradigm for riparian forest conservation?

Because forest width is thought to be the most relevant metric to ecological communities, it has persisted as the dominant paradigm and focus of management recommendations for riparian forest conservation. Consequently, managers may overlook important effects of the surrounding landscape matrix. We determined if characteristics of the landscape matrix, particularly the amount of urban development surrounding a forest, were better predictors of bird communities than forest width. We sampled breeding-bird communities 3 times each June 2001-2004 in 33 riparian forest sites (69-565 m wide) in Ohio, USA. We examined if bird community structure and composition were more closely associated with forest width or the amount of urban development within 1 km of each forest using canonical correlation analysis. Results indicate that the landscape matrix surrounding these relatively large forest tracts explained >94% of the variation in bird communities compared to <6% explained by forest width. Numbers of Neotropical migrants were negatively associated with urbanization in the landscapes (r = −0.43), whereas residents (r = 0.57) and short-distance migrants (r = 0.41) were positively associated with urbanization. Similar patterns persisted at the individual species level, particularly for Neotropical migrants as 76% of species in this guild were negatively related to urban development. Our findings suggest that the traditional approach to conserving riparian forests is not sufficient and that explicit consideration of the surrounding landscape matrix also should be a key component in conservation efforts.     

Effects of ground fires on understorey bird assemblages in Amazonian forests

Avifaunal responses to understorey fire disturbance and subsequent changes in habitat structure were examined within 20 burnt and unburnt forest plots of 0.25 ha (10×250 m), 10-15 months after an unprecedented understorey fire swept through the Tapajós-Arapiuns Extractive Reserve of central Brazilian Amazonia following the severe 1997-1998 El-Niño dry season. Although these surface fires in the previously undisturbed primary forest were relatively mild, they resulted in dramatic changes in forest structure consistent with those found elsewhere in Amazonia. Bird species negatively affected by these changes tended to be the least common, the most disturbance-sensitive, and habitat specialists. Considering different guilds, ant followers, dead-leaf gleaners, terrestrial gleaners, and arboreal sallying insectivores were the most negatively affected, whereas nectivores and arboreal granivores became more abundant in the burnt forest. The results highlight the severe consequences of even relatively mild surface fires in neotropical forests, and the importance of controlling haphazard frontier expansion for the conservation of susceptible species that are endemic to fire-prone regions.     

Bird conservation in fragmented Mediterranean forests of Spain: effects of geographical location, habitat and landscape degradation

The effects of habitat fragmentation on forest bird assemblages were analysed in 214 holm oak (Quercus ilex) remnants spread across the northern and southern plateaux of central Spain. Bird richness was highly dependent on fragment area for all species regardless of isolation, and barely affected by habitat traits. Geographical location was associated with high differences in richness of bird assemblages, which included 17 species exclusive to northern remnants and one exclusive to southern remnants. This supports the hypothesis that habitat suitability deteriorates sharply from north to south for forest birds in Spain. The species-area relationships of bird assemblages sampled in fragmented forests along a broad continental gradient (from Norway to southern Spain) showed that true forest birds only nest in woodlands >100 ha in southern Spain, whereas the full complement of forest species occurs in much smaller fragments in central-western Europe. Loss of species that are particularly sensitive to habitat fragmentation accounts for these differences between dry Spanish and mesic European woodlands. These results are explained by the low habitat suitability of Spanish woodlands, associated with the restrictive conditions for plant regeneration in the Mediterranean climate and long-standing human usage. There is, therefore, a particular need to develop management strategies that conserve birds, and probably other forest organisms, in Mediterranean regions by preventing habitat deterioration and decreases in fragment size, and by conserving all woods >100 ha.     

Simulating anthropogenic impacts to bird communities in tropical rain forests

We used an aggregated modelling approach to simulate the impacts of anthropogenic disturbances on the long-term dynamics of faunal diversity in tropical rain forests. We restricted our study to bird communities even though the approach is more general. We developed a model called BioDiv which simulated the establishment of hypothetical bird species in a forest. Our model was based on the results of a simple matrix model which calculated the spatio-temporal dynamics of a tropical rain forest in Malaysia. We analysed the establishment of bird species in a secondary forest succession and the impacts of 60 different logging scenarios on the diversity of the bird community. Of the three logging parameters (cycle length, method, intensity), logging intensity had the most servere impact on the bird community. In the worst case the number of bird species was reduced to 23% of the species richness found in a primary forest.     

Effect of forest fragmentation on fruit and seed predation of the tropical dry forest tree Ceiba aesculifolia

Forests fragmentation reduces the density of natural plant populations forming patches of the remaining individuals. One of the biotic interactions that can be affected by forest fragmentation and is poorly studied is seed predation. We determined the effects of forest fragmentation on seed and fruit predation in Ceiba aesculifolia by comparing trees in continuous forest with trees in fragmented forest. We compared the following variables: (a) frequency of fruit predation by Collie’s squirrel (Sciurus colliaei) in each habitat; (b) frequency of the cotton-staining bug seed predator (Dysdercus, Orden Hemiptera) in each habitat; (c) the effect of seed predation on germination frequency and time; and (d) the effect of different life stages of Dysdercus on seed viability. In continuous habitat, 100% of the trees presented fruits with squirrel predation while only 34% of trees in fragmented habitats presented fruit predation. In continuous forest 27% of the trees contained fruits with the seed predator Dysdercus, while only 2% of the trees in fragmented forest presented Dysdercus. The initial weight of damaged seeds was greater than seeds that were not damaged indicating that seed predators select heavier seeds to feed upon. Frequency of seed germination was affected by different life stages; pre-adults decreased germination significantly more than nymphs and adults. Seed predation significantly increased the time it took for germination to occur. Our study shows that forest fragmentation significantly affects predation patterns of squirrels and cotton-staining bugs. Reduction of natural seed predators in forest fragments may have long-term consequences on forest structure and diversity.     

Movements of neotropical understory passerines affected by anthropogenic forest edges in the Brazilian Atlantic rainforest

Edge effects are suggested to have great impact on the persistence of species in fragmented landscapes. We tested edge avoidance by forest understory passerines in the Brazilian Atlantic Rainforest and also compared their mobility and movement patterns in contiguous and fragmented landscapes to assess whether movements would increase in the fragmented landscape. Between 2003 and 2005, 96 Chiroxiphia caudata, 38 Pyriglena leucoptera and 27 Sclerurus scansor were radio-tracked. The most strictly forest species C. caudata and S. scansor avoided forest edges, while P. leucoptera showed affinities for the edge. Both sensitive species showed larger mean step length and maximal observed daily distance in the fragmented forest versus the unfragmented forest. P. leucoptera did not show any significant difference. There were no significant differences in proportional daily home range use for any of the three species. Our results suggested that fragmentation and the consequent increase in edge areas do influence movement behavior of sensitive forest understory birds that avoided the use of edges and increased the speed and distance they covered daily. For the most restricted forest species, it would be advisable to protect larger patches of forest instead of many small or medium fragments connected by narrow corridors. However, by comparing our data with that obtained earlier, we concluded that movement behavior of resident birds differs from that of dispersing birds and might not allow to infer functional connectivity or landscape-scale sensitivity to fragmentation; a fact that should be taken into consideration when suggesting conservation strategies.     

Predation of artificial nests in a fragmented landscape in the tropical region of Los Tuxtlas, Mexico

Predation rates of artificial nests were investigated in a fragmented landscape in the lowlands of Los Tuxtlas in southern Mexico. Hen and plasticine eggs were used to assess predation pressure in four habitats: the interior of forest fragments, the forest-pasture edge, corridors of residual forest vegetation and linear strips of live fences across pastures. Three sites per habitat were used in three experimental trials. Hen and plasticine ground nests with three eggs each were alternated every 50 m along transects at each site. Predation rates on each type of nest were monitored for 9 days. Survey of potential avian and mammalian potential nest predators were conducted at each site prior to the experimental trails. Readings of amount of light illuminating the ground were taken by each nest at each site to assess exposure of nests. In general, average predation rates were significantly higher for both hen and plasticine nests in the forest-pasture edge and in the corridors than in the interior of the forest fragments. While birds and mammals were the principal predators on hen eggs in the forests, mammals were responsible for the majority (?70%) of eggs damaged at the other habitats. Surveys of potential nest predators showed that avian and mammalian potential nest predators were significantly more common at the forest-pasture edges and at the other habitats than in the forests. Readings of light reaching the ground suggest that concealment of nests by the vegetation may play an important role in predation risk. Our results are consistent with reports from other Neotropical rainforests indicating an increase of artificial nest predation pressures from forest interior to open habitats. Restoration of forest fragments, allowing the vegetation to grow along the forest-pasture edge and the planting of arboreal crops at the forest-pasture edges may be measures that could increase cover and nest protection.     

Bats in a fragmented landscape: Species composition, diversity and habitat interactions in savannas of Santarém, Central Amazonia, Brazil

Species response to forest fragmentation may strongly vary according to ecological requirements, shape, spatial configuration and connectivity of fragments, and the structure of the surrounding matrix. Bats are natural candidates for studies on forest fragmentation. However, the conclusions of such studies are often contradictory. We present the results of a study on the effects of forest fragmentation on bat communities in Alter do Chão, Santarém area, in Central Amazonia, Brazil, an area composed by forests and fragments surrounded by savanna-like vegetation. We compared the species composition between habitats, tested the effect of size, shape and density of trees of forested areas on the presence species and their relative frequency of captures, and investigated the savannas as an ecological barrier for the dispersion of bats. With an effort of 5678 mistnet-hours, we captured 3740 bats (64 species). Multidimensional Scaling indicated no strong separation between the habitats sampled, however, savanna sites were grouped distinctly of fragments and forest sites. Multiple linear regressions indicated no significant correlation between the number of bat species recorded and the size, shape or tree density in forest sites and fragments. There was a significant correlation between the number of captures and the variables tested, explained by the shape of the sites, but not by size or tree density. In general, sites with bigger shape indices had fewer captures. Ten of the 20 most captured species showed no significant difference between the three habitats. The bat communities in Alter do Chão were not strongly affected by forest fragmentation, and the savannas did not appear to act as an ecological barrier to bats.     

Do regional gradients in land-use influence richness, composition and turnover of bird assemblages in small woods?

Small patches of natural or semi-natural habitat have an important role in the conservation of biodiversity in human-dominated environments. The values of such areas are determined by attributes of the patch as well as its context in the surrounding land mosaic. There is a need for better understanding of the ways in which assemblages are influenced by patch context and the scale over which this occurs. Here we examine the influence of regional environmental gradients on the richness, annual turnover and composition of breeding bird species in small woods in south-eastern England. Regional gradients were defined independently of woods by an ordination of attributes for 5 km × 5 km landscape units across a 2100 km² region. Patch-level attributes, particularly area, were the most important predictors for most bird variables. For woodland migrants and woodland-dependent species, variables representing the context of each wood, either at a local or regional scale, explained significant additional variance in species richness after accounting for wood area, but did not do so for species turnover. Significant context effects for woodland-dependent species related to the extent of hedges and woodland cover in the local vicinity (<1 km radius), whereas for woodland seasonal migrants the best predictors of richness after patch area were two regional environmental gradients. The initial cue to settlement for migrants may be at a coarse regional scale, with selection for suitable landscapes that have a greater extent of woodland cover. Edge species showed different responses: they were influenced by the diversity of structural features in woods, and were a more-dominant component of the avifauna in isolated woods in open fenland environments of the region. Significant relationships between coarse regional gradients (25 km² units) and bird assemblages in small woods (0.5-30 ha) suggest that population and community processes in the avifauna operate across a broader scale than local patch neighbourhoods. They also highlight the importance of adopting a landscape or regional perspective on potential changes to land-use in rural environments, and on the conservation management of small reserves.     

Bats in continuous forest, forest fragments and in an agricultural mosaic habitat-island at Los Tuxtlas, Mexico

In order to document the responses of bats to destruction and fragmentation of their natural habitat and the value of different types of man-made vegetation for bat conservation in the Neotropics, bats were sampled with mist nets to compare species richness and species composition in a tract of continuous forest, forest fragments and a habitat-island consisting of a mosaic of forest and arboreal crops in Los Tuxtlas, southern Mexico. We captured 3835 bats representing 39 species: 76% were captured in continuous forest, 74% in the mosaic habitat and 87% in forest fragments. In the mosaic habitat we captured 43% of the total number of bats, 33% in the forest fragments and 24% in continuous forest. On average the habitats studied had 64% species in common. Evidence of continuous breeding activity was determined for a high number of species at the three habitats (> 70% lactating and 65% with embryos). A few bat species (Carollia brevicauda, Pteronotus parnelli, Sturnira lilium, Artibeus jamaicensis, Dermanura phaeotis, Vampyrodes caraccioli, Glossophaga soricina, Dermanuta toltecus, Cheoroniscus godmani, Platyrrhinus helleri) dominated the sample, but their relative dominance varied among habitats. Recapture of bats provided evidence for inter-habitat movement. The co-occurrence of the three habitats helps conserve a diverse assemblage of bat species in the local landscape.     

The effects of forest fragmentation on euglossine bee communities (Hymenoptera: Apidae: Euglossini)

Despite scientific and media attention on pollinator declines, there is still only a rudimentary understanding of the response of bees—the most important group of pollinators worldwide—to ongoing land use changes. Euglossine bees are an ecologically important Neotropical clade of forest-dependent pollinators. Despite the fact that euglossines are well studied relative to other groups of tropical bees, only three previous studies, all from Brazil, address the response of euglossines to forest fragmentation. In this study, I tripled the maximum sample size of previous efforts by sampling male euglossines in 22 forest fragments ranging in area from 0.25 ha to 230 ha in southern Costa Rica, using chemically baited Van Someren traps. Abundance of euglossine bees was significantly positively related to forest fragment size, negatively related to shape (edge:area ratio), and marginally related to fragment isolation. Euglossine species richness showed similar, but weaker trends: richness was significantly positively related to the quantity of forest edge, marginally negatively related to fragment area, and not related to fragment isolation. The positive relationship between euglossine richness and abundance and forest fragment edge is consistent with other studies that have found high euglossine density in secondary or disturbed forest. The data suggest that individual euglossines move between forest fragments, as has been shown in other systems. Still, forest fragmentation appears to affect euglossine bees more strongly than other bee groups in the study region. Their large flight range and positive relationship with forest edges may help to buffer the negative effects of fragmentation, allowing euglossines to utilize even the very smallest forest fragments in the study area.     

Avian assemblages along a gradient of urbanization in a highly fragmented landscape

Our goal was to evaluate how avian assemblages varied along a gradient of urbanization in the highly fragmented landscape of coastal southern California. We measured species richness and abundance of birds within continuous blocks of habitat, within urban habitat fragments that varied in landscape and local habitat variables, and within the urban matrix at different distances from the wildland interface. These comparisons allowed us to characterize patterns of avifaunal response to a gradient of urban fragmentation. At the fragment scale, we found that fragment area was a strong, positive predictor of the total number of breeding species detected per fragment; total bird abundance per point count also increased with fragment size. Tree cover was higher in small fragments, as was the abundance of birds that typically occupy wooded habitats. Comparisons between core, fragment, and urban transects revealed differing patterns of response of individual bird species to urbanization. In unfragmented habitat, we recorded a relatively high diversity of urbanization-sensitive birds. In urban transects, these species were rare, and a relatively few species of non-native and anthropophilic birds were common. These urbanization-enhanced birds were also recorded in previous urban gradient studies in northern California and Ohio. Bird communities along the urban gradient reached their highest richness and abundance in fragments. The marked difference in vegetation structure between urban and natural landscapes in this arid shrubland system likely contributed to this pattern; the presence of native shrubs and exotic trees in fragments enabled both shrub and arboreal nesters to co-occur. As is characteristic of biotic homogenization, urban fragmentation in coastal southern California may increase local diversity but decrease overall regional avifaunal diversity.