Urban flight: understanding individual and population-level responses of Nearctic-Neotropical migratory birds to urbanization

Breakwaters as habitats for synanthropes: Spatial associations of vertebrates and vegetation with anthropogenic litter

Urban infrastructures can provide 'novel' habitats for marine and terrestrial animals and plants, enhancing their ability to adapt to urban environments. In particular, coastal infrastructures characterized by a complex three-dimensional morphology, such as breakwaters, could provide species refuges and food. We investigated the role of breakwaters in providing habitat for vertebrates and plants, and the influence of anthropogenic litter in regulating the value of these structures as habitat. We sampled vertebrate and plant species and quantified the amount of anthropogenic litter on breakwaters and adjacent rocky habitats at several sites in three different countries (Italy, Spain and Chile). We found breakwaters to accumulate more litter items (e.g. especially plastics) than adjacent rocky habitats by means of their large-scale (i.e., 1 m) structural complexity. Birds, which used the artificial infrastructure as transitory habitat, reached similar abundances in breakwaters compared with adjacent rocky platforms. In contrast, synanthropic mammal species, such as Rattus norvegicus and feral cats, were slightly more frequent on breakwaters and appeared to use them as permanent habitat. Plants were frequent in the upper zone of breakwaters and, even though many macrophyte species can trap litter, their cover correlated negatively with anthropogenic litter density. Therefore, breakwaters provide either transitory or permanent habitats for different species, despite functioning as a sink for anthropogenic litter. Thus, new infrastructure should be designed with lower structural complexity in their supralittoral zone limiting the proliferation of synanthropic species. In addition, restricting public access to sensitive areas and enforcing littering fines could enhance the ecological value of these novel habitats by reducing the benefits to pest species.

Surviving in cities: the case of a year-round territorial bird in the Neotropics

Urban expansion has been identified as one of the main threats to biodiversity because it can negatively affect wildlife populations. However, wildlife population dynamics have not been studied in one of the most rapidly urbanizing regions in the world—the Neotropics. To examine the effect of urbanization on the population dynamics of Neotropical wildlife, we used recapture data from a marked population (2011–2017) of White-eared Ground-Sparrow (Melozone leucotis) across an urban–rural gradient in the Costa Rican Central Valley. Additionally, we tested if this effect differed between males and females. Contrary to our prediction, ground-sparrow survival rates were higher in urban and suburban sites than in the rural site, and we found that survival was positively correlated to the proportion of urban surface inside each territory (β = 0.90, 95% CI: 0.71–0.97). We did not find differences in survival rates between the sexes and the mean survival rate was high overall (0.79 ± 0.06). Surprisingly, our results suggest that the survival rate of this urban avoider is positively influenced by urbanization, and therefore, suggest that the potential cost to some urban avoiders may not be driven by reduced survival in more urbanized environments, and could be driven by reductions in other vital rates. Therefore, we encourage research to evaluate multiple vital rates of urban avoiders and urban adapters to achieve more comprehensive knowledge on how urbanization is affecting avian populations in the Neotropics.

Double-brooding and annual breeding success of great tits in urban and forest habitats

Urban areas differ from natural habitats in several environmental features that influence the characteristics of animals living there. For example, birds often start breeding seasonally earlier and fledge fewer offspring per brood in cities than in natural habitats. However, longer breeding seasons in cities may increase the frequency of double-brooding in urban compared with nonurban populations, thus potentially increasing urban birds’ annual reproductive output and resulting in lower habitat difference in reproductive success than estimated by studies focusing on first clutches only. In this study, we investigated 2 urban and 2 forests great tit Parus major populations from 2013 to 2019. We compared the probability of double-brooding and the total number of annually fledged chicks per female between urban and forest habitats, while controlling for the effects of potentially confounding variables. There was a trend for a higher probability of double-brooding in urban (44% of females) than in forest populations (36%), although this was not consistent between the 2 urban sites. Females produced significantly fewer fledglings annually in the cities than in the forest sites, and this difference was present both within single- and double-brooded females. Furthermore, double-brooded urban females produced a similar number of fledglings per season as single-brooded forest females. These results indicate that double-brooding increases the reproductive success of female great tits in both habitats, but urban females cannot effectively compensate in this way for their lower reproductive output per brood. However, other mechanisms like increased post-fledging survival can mitigate habitat differences in reproductive success.

Which Traits Influence Bird Survival in the City? A Review

Urbanization poses a major threat to biodiversity worldwide. We focused on birds as a well-studied taxon of interest, in order to review literature on traits that influence responses to urbanization. We review 226 papers that were published between 1979 and 2020, and aggregate information on five major groups of traits that have been widely studied: ecological traits, life history, physiology, behavior and genetic traits. Some robust findings on trait changes in individual species as well as bird communities emerge. A lack of specific food and shelter resources has led to the urban bird community being dominated by generalist species, while specialist species show decline. Urbanized birds differ in the behavioral traits, showing an increase in song frequency and amplitude, and bolder behavior, as compared to rural populations of the same species. Differential food resources and predatory pressure results in changes in life history traits, including prolonged breeding duration, and increases in clutch and brood size to compensate for lower survival. Other species-specific changes include changes in hormonal state, body state, and genetic differences from rural populations. We identify gaps in research, with a paucity of studies in tropical cities and a need for greater examination of traits that influence persistence and success in native vs. introduced populations.

Consistent choice of landscape urbanization level across the annual cycle in a migratory waterbird species

Rapid urbanization has a great impact on avian distribution, ecology, habitat selection, and behavior. Recent avian studies indicated that individuals remain consistent in their behavioral responses to human disturbance across short periods of time. However, there is still little information about keeping consistent behaviors in distinct locations across different stages of the annual cycle. In this study, we aimed to test for long-term consistency in habitat selection with respect to urbanization in a migratory waterbird species, the Eurasian coot Fulica atra. For this purpose, we individually marked ca. 300 coots from four populations that varied in urbanization level and tracked their habitat preferences during the non-breeding season. We found that individuals from urban breeding populations selected habitats with a higher share of artificial areas during the non-breeding season, when compared to non-urban individuals. Also, a comparison of non-breeding sites selected by birds from our study populations with random sites showed that urban birds selected sites with higher urbanization level than resulting from random availability. Finally, we found a seasonal variation in habitat preferences in coots—individuals from all study populations selected more urbanized areas as the non-breeding season progressed. The results indicate that birds are able to remain consistent in habitat preferences not only at a large geographical scale, but also across different seasons. Marked between-population variation in habitat selection across the annual cycle may reflect personality differences of coots from urban and non-urban populations, and it stays in line with the personality-matching habitat choice hypothesis.

Humans Dominate the Social Interaction Networks of Urban Free-Ranging Dogs in India

Research on human-animal interaction has skyrocketed in the last decade. Rapid urbanization has led scientists to investigate its impact on several species living in the vicinity of humans. Domesticated dogs (Canis lupus familiaris) are one such species that interact with humans and are also called man's best friend. However, when it comes to the free-ranging population of dogs, interactions become quite complicated. Unfortunately, studies regarding free-ranging dog-human interactions are limited even though the majority of the world's dog population is free-ranging. In this study, we observed twelve groups of free-ranging dogs in their natural habitat, the streets. We quantified their interactions at the intra (dog-dog) and interspecific (dog-human) levels. The study areas were divided into two zones, namely - intermediate and high flux, based on human activity or movement. Social network analysis revealed higher instances of interspecific than intraspecific interactions, irrespective of the human flux zones. Humans, in significantly higher occasions, initiated both positive and negative behaviors in comparison to dogs. Our findings conclude that humans are a crucial part of the interaction network of Indian free-ranging dogs.

Personality-dependent breeding dispersal in rural but not urban burrowing owls

Dispersal propensity has been correlated with personality traits, conspecific density and predation risk in a variety of species. Thus, changes in the relative frequency of behavioural phenotypes or in the ecological pressures faced by individuals in contrasting habitats can have unexpected effects on their dispersal strategies. Here, using the burrowing owl Athene cunicularia as a study model, we test whether changes in the behavioural profile of individuals and changes in conspecific density and predation pressure associated with urban life influence their breeding dispersal decisions compared to rural conspecifics. Our results show that breeding dispersal behaviour differs between rural and urban individuals. Site fidelity was lower among rural than among urban birds, and primarily related to an individual's behaviours (fear of humans), which has been reported to reflect individual personality. In contrast, the main determinant of site fidelity among urban owls was conspecific density. After taking the decision of dispersing, urban owls moved shorter distances than rural ones, with females dispersing farther than males. Our results support a personality-dependent dispersal pattern that might vary with predation risk. However, as multiple individuals of two populations (one urban, one rural) were used for this research, differences can thus also be caused by other factors differing between the two populations. Further research is needed to properly understand the ecological and evolutionary consequences of changes in dispersal behaviours, especially in terms of population structuring and gene flow between urban and rural populations.

Urbanization, environmental stabilization and temporal persistence of bird species: a view from Latin America

Background

A scarcely studied consequence of urbanization is the effect of temporal stabilization of the environment on bird communities. This alteration is thought to dampen environmental variations between day and night, seasons and years, promoting a temporal persistence of bird composition in urban areas. The aim of this study was to review current evidence of temporal stabilization of biotic and abiotic factors in urban environments and the potential effects of such stabilization on temporal variation of bird species presence at different temporal scales.

Methods

I selected the literature by searching published articles and book chapters using Scopus and Google scholar. I only included articles that compared the temporal variation of bird composition or resources between different levels of urbanization.

Results

In general, there is evidence of temporal stabilization of abiotic and biotic factors at the three time scales considered. At the diurnal scale, the main factor considered was artificial light in the context of light pollution. At the seasonal and interannual scales, several case studies found a smaller temporal variation of primary productivity in urban than in natural and rural areas. Bird species composition showed more stabilization in urban environments at the three temporal scales: (1) several case studies reported bird activity at night, associated with artificial light; (2) studies in urban parks and along urbanization gradients showed smaller seasonal variation of bird composition in the more urbanized areas; and (3) in general, case studies along urbanization gradients showed smaller interannual variation of bird composition in the more urbanized areas, although some studies showed no relationships or opposite trends than expected.

Discussion

The published evidence suggests that urban areas dampen the natural cycles at several temporal scales. The stabilization of biotic and abiotic factors, such as light, temperature, food and habitat structure, is desynchronized from natural diurnal, seasonal and interannual cycles. However, there is a dearth of long-term comparisons of bird composition and studies that simultaneously analyze the relationship between resources and bird composition stabilization at the seasonal and interannual scales. More research is needed in the Southern hemisphere, where there is a lack of studies dealing with the seasonal and interannual variations of primary productivity along urbanization gradients and nocturnal activity of bird species. A future research agenda should include differentiation of spatial and temporal homogenization of avifaunas.

A review of urban impacts on avian life-history evolution: Does city living lead to slower pace of life?

The concept of a pace-of-life syndrome describes inter- and intraspecific variation in several life-history traits along a slow-to-fast pace-of-life continuum, with long lifespans, low reproductive and metabolic rates, and elevated somatic defences at the slow end of the continuum and the opposite traits at the fast end. Pace-of-life can vary in relation to local environmental conditions (e.g. latitude, altitude), and here we propose that this variation may also occur along an anthropogenically modified environmental gradient. Based on a body of literature supporting the idea that city birds have longer lifespans, we predict that urban birds have a slower pace-of-life compared to rural birds and thus invest more in self maintenance and less in annual reproduction. Our statistical meta-analysis of two key traits related to pace-of-life, survival and breeding investment (clutch size), indicated that urban birds generally have higher survival, but smaller clutch sizes. The latter finding (smaller clutches in urban habitats) seemed to be mainly a characteristic of smaller passerines. We also reviewed urbanization studies on other traits that can be associated with pace-of-life and are related to either reproductive investment or self-maintenance. Though sample sizes were generally too small to conduct formal meta-analyses, published literature suggests that urban birds tend to produce lower-quality sexual signals and invest more in offspring care. The latter finding is in agreement with the adult survival hypothesis, proposing that higher adult survival prospects favour investment in fewer offspring per year. According to our hypothesis, differences in age structure should arise between urban and rural populations, providing a novel alternative explanation for physiological differences and earlier breeding. We encourage more research investigating how telomere dynamics, immune defences, antioxidants and oxidative damage in different tissues vary along the urbanization gradient, and suggest that applying pace-of-life framework to studies of variation in physiological traits along the urbanization gradient might be the next direction to improve our understanding of urbanization as an evolutionary process.

Ground beetles in city forests: does urbanization predict a personality trait?

BACKGROUND

Urbanization leads to substantial changes in natural habitats with profound effects on wildlife. Understanding behavioural responses to such environmental change is essential for identifying which organisms may adapt, as behaviour is often the first response to altered conditions. Individuals in more urbanized habitats may be expected to be more exploratory and bolder than their conspecifics in less urbanized habitats as they may be better able to cope with novel challenges.

METHODS

In a two-year field study we tested ground beetles from differently urbanized forests for their exploratory behaviour (in a novel environment) and their risk-taking (death-feigning). In total, we tested ca. 3,000 individuals of four forest-dwelling ground beetle species from eight within-city forest patches. In the second year, we also transferred ca. 800 tested individuals of two species to the laboratory to test for consistent behavioural differences (i.e. personality differences) under standardised conditions.

RESULTS

Individuals were generally more exploratory in more urbanized than in less urbanized areas but only in one year of the study. Exploratory behaviour was not predicted by population density but increased with temperature or showed a temperature optimum. Exploration was consistent over time and individuals that were more exploratory also took higher risks.

DISCUSSION

We demonstrated that species which are generally less directly exposed to human activities (e.g., most invertebrates) show behavioural responses to urbanization. Effects of urbanization were year-dependent, suggesting that other environmental conditions interacted with effects of urbanization on beetle behaviour. Furthermore, our results indicate that different personality compositions might cause behavioural differences among populations living in differently urbanized habitats.

Free-ranging dogs prefer petting over food in repeated interactions with unfamiliar humans

Dogs (Canis lupus familiaris) are the first species to have been domesticated and, unlike other domesticated species, they have developed a special bond with their owners. The ability to respond to human gestures and language, and the hypersocial behaviours of dogs are considered key factors that have led them to become man's best friend. Free-ranging dogs provide an excellent model system for understanding the dog-human relationship in various social contexts. In India, free-ranging dogs occur in all possible human habitations. They scavenge among garbage, beg for food from humans, give birth in dens close to human habitations, and establish social bonds with people. However, there is ample dog-human conflict on the streets, leading to morbidity and mortality of dogs. Hence, the ability to assess an unfamiliar human before establishing physical contact could be adaptive for dogs, especially in the urban environment. We tested a total of 103 adult dogs to investigate their response to immediate social and long-term food and social rewards. The dogs were provided a choice of obtaining food either from an experimenter's hand or the ground. The dogs avoided making physical contact with the unfamiliar human. While immediate social reward was not effective in changing this response, the long-term test showed a strong effect of social contact. Our results revealed that these dogs tend to build trust based on affection, not food. This study provides significant insights into the dynamics of dog-human interactions on the streets and subsequent changes in behaviour of dogs through the process of learning.

Conventional oil and natural gas infrastructure increases brown-headed cowbird (Molothrus ater) relative abundance and parasitism in mixed-grass prairie

The rapid expansion of oil and natural gas development across the Northern Great Plains has contributed to habitat fragmentation, which may facilitate brood parasitism of ground-nesting grassland songbird nests by brown-headed cowbirds (Molothrus ater), an obligate brood parasite, through the introduction of perches and anthropogenic edges. We tested this hypothesis by measuring brown-headed cowbird relative abundance and brood parasitism rates of Savannah sparrow (Passerculus sandwichensis) nests in relation to the presence of infrastructure features and proximity to potential perches and edge habitat. The presence of oil and natural gas infrastructure increased brown-headed cowbird relative abundance by a magnitude of four times, which resulted in four times greater brood parasitism rates at infrastructure sites. While the presence of infrastructure and the proximity to roads were influential in predicting brood parasitism rates, the proximity of perch sites was not. This suggests that brood parasitism associated with oil and natural gas infrastructure may result in additional pressures that reduce productivity of this declining grassland songbird.

Conventional oil and natural gas infrastructure increases brown-headed cowbird (Molothrus ater) relative abundance and parasitism in mixed-grass prairie

The rapid expansion of oil and natural gas development across the Northern Great Plains has contributed to habitat fragmentation, which may facilitate brood parasitism of ground-nesting grassland songbird nests by brown-headed cowbirds (Molothrus ater), an obligate brood parasite, through the introduction of perches and anthropogenic edges. We tested this hypothesis by measuring brown-headed cowbird relative abundance and brood parasitism rates of Savannah sparrow (Passerculus sandwichensis) nests in relation to the presence of infrastructure features and proximity to potential perches and edge habitat. The presence of oil and natural gas infrastructure increased brown-headed cowbird relative abundance by a magnitude of four times, which resulted in four times greater brood parasitism rates at infrastructure sites. While the presence of infrastructure and the proximity to roads were influential in predicting brood parasitism rates, the proximity of perch sites was not. This suggests that brood parasitism associated with oil and natural gas infrastructure may result in additional pressures that reduce productivity of this declining grassland songbird.

Breeding Dispersal by Birds in a Dynamic Urban Ecosystem

Changes in land cover during urbanization profoundly affect the diversity of bird communities, but the demographic mechanisms affecting diversity are poorly known. We advance such understanding by documenting how urbanization influences breeding dispersal-the annual movement of territorial adults-of six songbird species in the Seattle, WA, USA metropolitan area. We color-banded adults and mapped the centers of their annual breeding activities from 2000-2010 to obtain 504 consecutive movements by 337 adults. By comparing movements, annual reproduction, and mate fidelity among 10 developed, 5 reserved, and 11 changing (areas cleared and developed during our study) landscapes, we determined that adaptive breeding dispersal of sensitive forest species (Swainson's Thrush and Pacific wren), which involves shifting territory and mate after reproductive failure, was constrained by development. In changing lands, sensitive forest specialists dispersed from active development to nearby forested areas, but in so doing suffered low annual reproduction. Species tolerant of suburban lands (song sparrow, spotted towhee, dark-eyed junco, and Bewick's wren) dispersed adaptively in changing landscapes. Site fidelity ranged from 0% (Pacific wren in changing landscape) to 83% (Bewick's wren in forest reserve). Mate fidelity ranged from 25% (dark-eyed junco) to 100% (Bewick's wren). Variation in fidelity to mate and territory was consistent with theories positing an influence of territory quality, asynchronous return from migration, prior productivity, and reproductive benefits of retaining a familiar territory. Costly breeding dispersal, as well as reduced reproductive success and lowered survival cause some birds to decline in the face of urbanization. In contrast, the ability of species that utilize edges and early successional habitats to breed successfully, disperse to improve reproductive success after failure, and survive throughout the urban ecosystem enables them to maintain or increase population size.

Community-level demographic consequences of urbanization: an ecological network approach

Ecological networks are known to influence ecosystem attributes, but we poorly understand how interspecific network structure affect population demography of multiple species, particularly for vertebrates. Establishing the link between network structure and demography is at the crux of being able to use networks to understand population dynamics and to inform conservation. We addressed the critical but unanswered question, does network structure explain demographic consequences of urbanization? We studied 141 ecological networks representing interactions between plants and nesting birds in forests across an urbanization gradient in Ohio, USA, from 2001 to 2011. Nest predators were identified by video-recording nests and surveyed from 2004 to 2011. As landscapes urbanized, bird-plant networks were more nested, less compartmentalized and dominated by strong interactions between a few species (i.e. low evenness). Evenness of interaction strengths promoted avian nest survival, and evenness explained demography better than urbanization, level of invasion, numbers of predators or other qualitative network metrics. Highly uneven networks had approximately half the nesting success as the most even networks. Thus, nest survival reflected how urbanization altered species interactions, particularly with respect to how nest placement affected search efficiency of predators. The demographic effects of urbanization were not direct, but were filtered through bird-plant networks. This study illustrates how network structure can influence demography at the community level and further, that knowledge of species interactions and a network approach may be requisite to understanding demographic responses to environmental change.

High urban breeding densities do not disrupt genetic monogamy in a bird species

Urbanization causes widespread endangerment of biodiversity worldwide. However, some species successfully colonize cities reaching higher densities than in their rural habitats. In these cases, although urban city dwellers may apparently be taking advantage of these new environments, they also face new ecological conditions that may induce behavioural changes. For example, the frequency of alternative reproductive behaviours such as extra-pair paternity and intraspecific brood parasitism might increase with breeding densities. Here, using a panel of 17 microsatellites, we tested whether increments in breeding densities such as those associated with urban invasion processes alter genetic monogamy in the burrowing owl Athene cunicularia. Our results show low rates of extra-pair paternity (1.47%), but relatively high levels of intraspecific brood parasitism (8.82%). However, we were not able to detect differences in the frequency at which either alternative reproductive behaviour occurs along a strong breeding density gradient. Further research is needed to properly ascertain the role of other social and ecological factors in the frequency at which this species presents alternative reproductive strategies. Meanwhile, our results suggest that genetic monogamy is maintained despite the increment in conspecific density associated with a recent urban invasion process.

Emulating natural disturbances for declining late-successional species: a case study of the consequences for cerulean warblers (Setophaga cerulea)

Forest cover in the eastern United States has increased over the past century and while some late-successional species have benefited from this process as expected, others have experienced population declines. These declines may be in part related to contemporary reductions in small-scale forest interior disturbances such as fire, windthrow, and treefalls. To mitigate the negative impacts of disturbance alteration and suppression on some late-successional species, strategies that emulate natural disturbance regimes are often advocated, but large-scale evaluations of these practices are rare. Here, we assessed the consequences of experimental disturbance (using partial timber harvest) on a severely declining late-successional species, the cerulean warbler (Setophaga cerulea), across the core of its breeding range in the Appalachian Mountains. We measured numerical (density), physiological (body condition), and demographic (age structure and reproduction) responses to three levels of disturbance and explored the potential impacts of disturbance on source-sink dynamics. Breeding densities of warblers increased one to four years after all canopy disturbances (vs. controls) and males occupying territories on treatment plots were in better condition than those on control plots. However, these beneficial effects of disturbance did not correspond to improvements in reproduction; nest success was lower on all treatment plots than on control plots in the southern region and marginally lower on light disturbance plots in the northern region. Our data suggest that only habitats in the southern region acted as sources, and interior disturbances in this region have the potential to create ecological traps at a local scale, but sources when viewed at broader scales. Thus, cerulean warblers would likely benefit from management that strikes a landscape-level balance between emulating natural disturbances in order to attract individuals into areas where current structure is inappropriate, and limiting anthropogenic disturbance in forests that already possess appropriate structural attributes in order to maintain maximum productivity.

Hormones in the city: endocrine ecology of urban birds

Urbanization dramatically changes the landscape, presenting organisms with novel challenges and often leading to reduced species diversity. Urban ecologists have documented numerous biotic and abiotic consequences of urbanization, such as altered climate, species interactions, and community composition, but we lack an understanding of the mechanisms underlying organisms' responses to urbanization. Here, I review findings from the nascent field of study of the endocrine ecology of urban birds. Thus far, no clear or consistent patterns have been revealed, but we do have evidence that urban habitat can shape endocrine traits, and that those traits might contribute to adaptation to the urban environment. I suggest strong approaches for future work addressing exciting questions about the role of endocrine traits in mediating responses to urbanization within species across the globe.

Anthropogenic resource subsidies decouple predator-prey relationships

The extent to which resource subsidies affect food web dynamics is poorly understood in anthropogenic landscapes. To better understand how species interactions are influenced by subsidies, we studied breeding birds and nest predators along a rural-to-urban landscape gradient that varied in subsidies provided to generalist predators. We hypothesized that resource subsidies in urban landscapes would decouple predator-prey relationships, as predators switch from natural to anthropogenic foods. From 2004 to 2009, we surveyed nest predators and monitored 2942 nests of five songbird species breeding in 19 mature forest stands in Ohio, USA. Eighteen species were video-recorded depredating nests. Numbers of avian and mammalian nest predators were positively associated with the amount of urban development surrounding forests, with the exception of Brown-headed Cowbirds (Molothrus ater). Although nest survival strongly declined with detections of nest predators in rural landscapes, nest survival and predator numbers were unrelated in urban landscapes. Thus, the strength of interaction between breeding birds and nest predators diminished as landscapes surrounding forested parks became more urbanized. Our work suggests that decoupling of predator-prey relationships can arise when synanthropic predators are heavily subsidized by anthropogenic resources. In this way, human drivers can alter, and completely disarticulate, relationships among species that are well established in more natural systems.

Urban ecological systems: scientific foundations and a decade of progress

Urban ecological studies, including focus on cities, suburbs, and exurbs, while having deep roots in the early to mid 20th century, have burgeoned in the last several decades. We use the state factor approach to highlight the role of important aspects of climate, substrate, organisms, relief, and time in differentiating urban from non-urban areas, and for determining heterogeneity within spatially extensive metropolitan areas. In addition to reviewing key findings relevant to each state factor, we note the emergence of tentative "urban syndromes" concerning soils, streams, wildlife and plants, and homogenization of certain ecosystem functions, such as soil organic carbon dynamics. We note the utility of the ecosystem approach, the human ecosystem framework, and watersheds as integrative tools to tie information about multiple state factors together. The organismal component of urban complexes includes the social organization of the human population, and we review key modes by which human populations within urban areas are differentiated, and how such differentiation affects environmentally relevant actions. Emerging syntheses in land change science and ecological urban design are also summarized. The multifaceted frameworks and the growing urban knowledge base do however identify some pressing research needs.

A method for detecting undervalued resources with application to breeding birds

Anthropogenic changes to ecosystems can decouple habitat selection and quality, a phenomenon well illustrated by ecological traps in which individuals mistakenly prefer low-quality habitats. Less recognized is the possibility that individuals might fail to select high-quality habitat because of the absence of some appropriate cue. This incorrect assessment of resource quality can lead to relatively high-quality resources being undervalued, whereby they support fewer individuals than optimal. We developed a habitat selection model to predict the expected patterns in patch-level density, fitness, and individual quality derived from either accurate assessment of habitat quality or from undervaluing of habitat patches (i.e., quality is not correctly assessed). Unlike previous habitat selection models, we explicitly and simultaneously incorporated variation in both individual and habitat quality into our estimates of realized fitness. Although multiple mechanisms can reduce patch-average density, fitness, and individual quality in less preferred patches, only undervaluation results in the occupation of higher-quality territories by similar-quality individuals in less preferred vs. preferred patches. We then looked for evidence of undervaluation in our seven-year data set of Acadian Flycatchers (Empidonax virescens) occupying forests in urbanizing landscapes in Ohio, USA. We suspected that forests within more urban landscapes may be undervalued in our study system because (1) urban forests typically support lower densities of Neotropical migratory birds than rural forests and (2) anthropogenic disturbance and habitat alterations are likely to result in mismatches between cues typically used in habitat selection and actual habitat quality. In contrast to our predictions, field data suggest that urban forests are not undervalued. Our work not only expands upon previous habitat selection models by considering undervaluation, but also demonstrates how predictions derived from our model can be tested using a long-term empirical data set.

Quantifying avian nest survival along an urbanization gradient using citizen- and scientist-generated data

Despite the increasing pace of urbanization little is known about the factors that limit bird populations (i.e., population-level processes) within the urban/suburban land-use matrix. Here, we report rates of nest survival within the matrix of an urban land-use gradient in the greater Washington, D.C., USA, area for five common songbirds using data collected by scientists and citizens as part of a project called Neighborhood Nestwatch. Using program MARK, we modeled the effects of species, urbanization at multiple spatial scales (canopy cover and impervious surface), and observer (citizen vs. scientist) on nest survival of four open-cup and one cavity-nesting species. In addition, artificial nests were used to determine the relative impacts of specific predators along the land-use gradient. Our results suggest that predation on nests within the land-use matrix declines with urbanization but that there are species-specific differences. Moreover, variation in nest survival among species was best explained by urbanization metrics measured at larger "neighborhood" spatial scales (e.g., 1000 m). Trends were supported by data from artificial nests and suggest that variable predator communities (avian vs. mammalian) are one possible mechanism to explain differential nest survival. In addition, we assessed the quality of citizen science data and show that citizens had no negative effect on nest survival and provided estimates of nest survival comparable to Smithsonian biologists. Although birds nesting within the urban matrix experienced higher nest survival, individuals also faced a multitude of other challenges such as contaminants and invasive species, all of which could reduce adult survival.