Bird species occupancy trends in Southeast Mexico over 1900-2020: Accounting for sighting record absences.

Long-term land-use change impacts tropical bird communities through population-level and functional diversity effects from habitat loss, degradation and fragmentation, leading to land management and conservation challenges. Assessing the temporal impacts of land-use change on occupancy patterns, population change and functional traits of bird species in tropical areas is limited by the treatment of nondetections as true absences or artefacts of low sampling effort during and throughout years. With this in mind, we developed a novel Bayesian species occupancy framework to account for species absences to evaluate bird community changes in Palenque, Chiapas, Mexico, where there is opportunity for study given exceptional records of change across habitats from rainforest to urban centres. We created a novel dataset of population trends for 244 bird species over the years 1900 to 2020 from published short-term field studies, expert field notes and community science pages. Our results show that open area species had higher population increases than forest specialists over time, represented most evidently by the turnover of rainforest specialists for urban species. Modelled influence of functional traits displayed the importance of main habitat types, body mass and habitat and dietary breadth as factors that associated with bird population trends. On average, species with body masses <6.6 and > 948.4 g showed decreasing trends, while all other species showed increasing or stable trends. Our findings illuminate the value of accounting for species absences from several data sources to discover long-term species population trends and affiliated functional traits whose preservation requires conservation and land management action to protect bird ecosystem services. Primary forest conservation is key to maintaining populations of habitat and dietary specialists, such as small understorey insectivorous and large frugivorous species. Protecting rare natural savanna patches from conversion to cattle pasture is vital to prevent further extirpation of native granivores and to slow colonization by exotic and invasive species.

El cambio a largo plazo en el uso de la tierra impacta a las comunidades de aves tropicales a través de los efectos de la diversidad funcional y a nivel de la población debido a la pérdida, degradación y fragmentación del hábitat, lo que lleva a desafíos de gestión y conservación de la tierra. La evaluación de los impactos temporales del cambio de uso de la tierra en los patrones de ocupación, el cambio de población y los rasgos funcionales de las especies de aves en áreas tropicales enfrenta limitaciones al considerar el tratamiento de las no detecciones como ausencias verdaderas o artefactos de bajo esfuerzo de muestreo durante el año y los años. La evaluación de los impactos temporales del cambio de uso de la tierra en los patrones de ocupación, el cambio de población y las características funcionales de las especies de aves en áreas tropicales está limitada por el tratamiento de las no detecciones como verdaderas ausencias o artefactos de bajo esfuerzo de muestreo durante y a lo largo de los años. Con esto en mente, desarrollamos un marco bayesiano novedoso de ocupación de especies para dar cuenta de las ausencias de especies para evaluar los cambios en la comunidad de aves en Palenque, Chiapas, México, donde existe la oportunidad de estudiar dados los registros excepcionales de cambios en los hábitats, desde la selva tropical hasta los centros urbanos. Creamos un nuevo conjunto de datos de tendencias de población para 244 especies de aves durante los años 1900 a 2020 a partir de estudios de campo a corto plazo publicados, notas de campo de expertos y páginas de ciencia comunitaria. Nuestros resultados muestran que las especies de áreas abiertas tuvieron aumentos de población más altos que los especialistas forestales a lo largo del tiempo, representado más evidentemente por la rotación de especialistas de bosques tropicales por especies urbanas. La influencia modelada de los rasgos funcionales mostró la importancia de los principales tipos de hábitat, la masa corporal y el hábitat y la amplitud de la dieta como factores asociados con las tendencias de la población de aves. En promedio, las especies con masas corporales <6,6 g y >948,4 g mostraron tendencias decrecientes, mientras que todas las demás especies mostraron tendencias crecientes o estables. Nuestros hallazgos iluminan el valor de contabilizar las ausencias de especies de varias fuentes de datos para descubrir tendencias de población de especies a largo plazo y rasgos funcionales afiliados cuya preservación requiere acción de conservación y manejo de la tierra para proteger los servicios ecosistémicos de las aves. La conservación de los bosques primarios es clave para mantener las poblaciones de especialistas en hábitat y dieta, como las especies insectívoras pequeñas del sotobosque y las especies frugívoras grandes. Proteger los raros parches naturales de sabana de la conversión en pastos para ganado es vital para evitar una mayor extirpación de los granívoros nativos y para retrasar la colonización por especies invasoras.

Precipitation effects on grassland plant performance are lessened by hay harvest.

Climate and human management, such as hay harvest, shape grasslands. With both disturbances co-occurring, understanding how these ecosystems respond to these combined drivers may aid in projecting future changes in grasslands. We used an experimental precipitation gradient combined with mimicked acute hay harvest (clipping once a year) to examine (1) whether hay harvest influences precipitation effects on plant performance (cover and height) and (2) the role of inter-specific responses in influencing plant performance. We found that hay harvest reduced the strength of precipitation effects on plant performance through changes in bare-ground soil cover. Species performance were mainly influenced by change in abiotic factors, often responding negatively, as hay harvest increased bare-ground amount. Conversely, altered precipitation without hay harvest promoted plant species performance through abiotic factors change first, followed by biotic. Most species, including the dominant grass Schizachyrium scoparium, increased their performance with greater leaf area index (proxy for canopy structure). Our experiment demonstrates that plant performance responds directly to abiotic factors with hay harvest, but indirectly without hay harvest. Positive effects of increasing precipitation were likely due to microhabitat amelioration and resource acquisition, thus inclusion of hay harvest as a disturbance lessens positive impacts of biotic variables on species performance to climate change.

A case for associational resistance: Apparent support for the stress gradient hypothesis varies with study system.

According to the stress gradient hypothesis (SGH), ecological interactions between organisms shift positively as environmental stress increases. In the case of associational resistance, habitat is modified to ameliorate stress, benefitting other organisms. The SGH is contentious due to conflicting evidence and theoretical perspectives, so we adopted a meta-analytic approach to determine if it is widely supported across a variety of contexts, including different kingdoms, ecosystems, habitats, interactions, stressors, and life history stages. We developed an extensive list of Boolean search criteria to search the published ecological literature and successfully detect studies that both directly tested the hypothesis, and those that were relevant but never mentioned it. We found that the SGH is well supported by studies that feature bacteria, plants, terrestrial ecosystems, interspecific negative interactions, adults, survival instead of growth or reproduction, and drought, fire, and nutrient stress. We conclude that the SGH is indeed a broadly relevant ecological hypothesis that is currently held back by cross-disciplinary communication barriers. More SGH research is needed beyond the scope of interspecific plant competition, and more SGH research should feature multifactor stress. There remains a need to account for positive interactions in scientific pursuits, such as associational resistance in tests of the SGH.

Rates of Comorbidities in Very Low Birth Weight Infants Fed an Exclusive Human Milk Diet Versus a Bovine Supplemented Diet.

Background: Our level III neonatal intensive care unit (NICU) implemented the use of an exclusive human milk diet (EHD) and sought to determine its effect on the severe co-morbidities of preterm infants as well as the potential cost-savings due to the anticipated reduction in these co-morbidities. Methods: A retrospective cohort study was completed to determine if an EHD statistically decreased the rate of co-morbidities including length of stay (LOS), days on total parental nutrition (TPN), rates of late onset sepsis, necrotizing enterocolitis (NEC), bronchopulmonary dysplasia (BPD), and severe retinopathy of prematurity (ROP). Results: An EHD significantly decreased the odds of severe ROP (adjusted odds-ratio (aOR)=0.349; 95%CI [0.156, 0.739]; p=0.008) and late onset sepsis (aOR=0.323; 95%CI [0.123, 0.768]; p=0.014). Analysis of cost-effectiveness of an EHD relative to a BSD based on the incremental costs of these co-morbidities determined the net loss in direct hospital costs per patient were estimated to be $420 in 2016 US dollars; however, given the long-term health-care costs and non-pecuniary damages from the co-morbidities of severe ROP and sepsis this net loss appears negligible. Conclusion: This study found that an EHD significantly decreased the odds of severe ROP and late onset sepsis; though not significant, there was a positive trend in decreasing cases of medical NEC; our surgical NEC rates dropped to 0. The benefits of human milk are vital, and the costs are nominal.

Forging a Bayesian link between habitat selection and avoidance behavior in a grassland grouse.

Habitat selection is a basic aspect of the ecology of many species, yet often the term is conflated or confused with both habitat preference and habitat use. We argue that each term fits within a conceptual framework that can be viewed in Bayesian terms and demonstrate, using long-term data on occupancy patterns of a grassland grouse, how prior probability profiles can be estimated. We obtained estimates by specifically focusing on whether and to what extent the Lesser Prairie-Chicken (Tympanuchus pallidicinctus) avoids anthropogenic features such as roads, powerlines, petroleum wells, fences, and buildings, in two study areas, one with denser and one with sparser incidence of features. Grouse strongly avoided large features such as outbuildings and tended to avoid tall features such as powerlines; by contrast, grouse did not or only slightly avoided low, unobtrusive features such as fences. We further examined co-location of pairs of anthropogenic features and found that certain features were avoided so strongly that avoidance distance may be shorter for other features; that is, birds were "pushed toward" some features because they are "pushed away" from others. In each case, our approach points toward a means to incorporate avoidance behavior directly into analytic studies of habitat selection, in that data on use (the posterior, as it were) could be used to infer the selection process provided data on preference (the prior, as it were) could be obtained.

Single Cell Mass Spectrometry Quantification of Anticancer Drugs: Proof of Concept in Cancer Patients.

In clinical cancer medicine, the current inability to quantify intracellular chemotherapy drug concentrations in individual human cells limits the personalization and overall effectiveness of drug administration. New bioanalytical methods capable of real-time measurement of drug levels in live single cancer cells would allow for more adaptive and personalized administration of chemotherapy drugs, potentially leading to better clinical outcomes with fewer side effects. In this study, we report the development of a new quantitative single cell mass spectrometry (qSCMS) method capable of providing absolute drug amounts and concentrations in single cancer cells. Using this qSCMS system, quantitative analysis of the intracellular drug gemcitabine present in individual bladder cancer cells is reported, including in bladder cancer cells isolated from patients undergoing standard-of-care gemcitabine chemotherapy. The development of single cell pharmacology bioanalytical methods can potentially lead to more effective and safely administered drug medications in patients, especially in the treatment of cancer.

The Intersection of Human Disturbance and Diel Activity, with Potential Consequences on Trophic Interactions.

Direct effects of human disturbance on animal populations are well documented across habitats, biomes, and species, but indirect effects of diel have received less attention. An emerging field in applied ecology involves behavioral avoidance of or attraction to humans and their trappings. We posit trophic consequences, in terms of relative risk, for four species of mammals, each of which strongly avoids human activity, in urban reserves of coastal southern California. Two species, one predator and one prey, avoid human activity via a temporal shift to become "more nocturnal"-the species' activity is centered near dawn on days without human activity but nearer to midnight on days with human activity. Diel shifts have brought the species into greater overlap, respectively, with a key prey and a key predator, overlap that may increase encounter rate and thus increase relative risk of predation, with potential consequences for trophic dynamics and cascades: increased risk of predation may depress prey population, either directly (e.g., mortality) or indirectly (e.g., "landscape of fear"). Human use of reserves, especially in high population density regions, needs to be reconsidered either to reduce access or to restrict access entirely to areas that may provide refuge to both predators and prey.

Is extinction forever?

Mistrust of science has seeped into public perception of the most fundamental aspect of conservation-extinction. The term ought to be straightforward, and yet, there is a disconnect between scientific discussion and public views. This is not a mere semantic issue, rather one of communication. Within a population dynamics context, we say that a species went locally extinct, later to document its return. Conveying our findings matters, for when we use local extinction, an essentially nonsensical phrase, rather than extirpation, which is what is meant, then we contribute to, if not create outright, a problem for public understanding of conservation, particularly as local extinction is often shortened to extinction in media sources. The public that receives the message of our research void of context and modifiers comes away with the idea that extinction is not forever or, worse for conservation as a whole, that an extinction crisis has been invented.

Phylogenetic niche conservatism and the evolutionary basis of ecological speciation.

Phylogenetic niche conservatism (PNC) typically refers to the tendency of closely related species to be more similar to each other in terms of niche than they are to more distant relatives. This has been implicated as a potential driving force in speciation and other species-richness patterns, such as latitudinal gradients. However, PNC has not been very well defined in most previous studies. Is it a pattern or a process? What are the underlying endogenous (e.g. genetic) and exogenous (e.g. ecological) factors that cause niches to be conserved? What degree of similarity is necessary to qualify as PNC? Is it possible for the evolutionary processes causing niches to be conserved to also result in niche divergence in different habitats? Here, we revisit these questions, codifying a theoretical and operational definition of PNC as a mechanistic evolutionary process resulting from several factors. We frame this both from a macroevolutionary and population-genetic perspective. We discuss how different axes of physical (e.g. geographic) and environmental (e.g. climatic) heterogeneity interact with the fundamental process of PNC to produce different outcomes of ecological speciation. We also review tests for PNC, and suggest ways that these could be improved or better utilized in future studies. Ultimately, PNC as a process has a well-defined mechanistic basis in organisms, and future studies investigating ecological speciation would be well served to consider this, and frame hypothesis testing in terms of the processes and expected patterns described herein. The process of PNC may lead to patterns where niches are conserved (more similar than expected), constrained (divergent within a limited subset of available niches), or divergent (less similar than expected), based on degree of phylogenetic relatedness between species.

Predictors of occupancy trend across spatial scale.

Many explorations of extinction probability have had a global focus, yet it is unclear whether variables that explain the probability of extinction at large spatial extents are the same as those at small spatial extents. Thus, we used nearly annual presence-absence records for the most recent 40 years of a 110-year data set from Palenque, Mexico, an area with ongoing deforestation, to explore which of >200 species of birds have probabilities of extirpation that are likely to increase. We assessed associations between long-term trends in species presence (i.e., detection in a given year) and body size, geographic range size, diet, dependence on forest cover, taxonomy, and ecological specialization. Our response variable was the estimated slope of a weighted logistic regression for each species. We assessed the relative strength of each predictor by means of a model ranking scheme. Several variables associated with high extinction probability at global extents, such as large body size or small geographic range size, were not associated with occurrence of birds over time at our site. Body size was associated with species loss at Palenque, but occurrence trends of both very large and very small species, particularly the latter, have declined, or the species have been extirpated. We found no association between declining occurrence trend and geographic range size, yet decline correlated with whether a species depends on forest (mean occupancy trend =-0.0380, 0.0263, and 0.0186 for, respectively, species with high, intermediate, or low dependence on forest) and with complex combinations of diet and foraging strata (e.g., occurrence of canopy insectivores and terrestrial omnivores has increased, whereas occurrence of mid-level frugivores and terrestrial granivores has decreased). Our findings emphasize that analyses of local areas are necessary to explicate extirpation risk at various spatial extents.

Resumen: Muchas exploraciones de la probabilidad de extinción han tenido un enfoque global, pero aun no es claro si las variables que explican la probabilidad de extinción en extensiones espaciales grandes son las mismas que en extensiones espaciales pequeñas. Por lo tanto, utilizamos registros de presencia-ausencia casi anuales de los últimos 40 años de una base de datos de 110 años de Palenque, México, un áreas con deforestación continua, para explorar cuales de las >200 especies de aves tienen probabilidades de extirpación que tienen posibilidades de incrementar. Evaluamos asociaciones entre tendencias en la presencia de especies a largo plazo (i.e., detección en un año determinado) y el tamaño corporal, el rango de distribución geográfica, dieta, dependencia en la cobertura forestal, taxonomía y especialización ecológica. Nuestra variable de respuesta fue la pendiente estimada de una regresión logística ponderada para cada especie. Evaluamos la robustez relativa de cada predictor mediante esquema de clasificación de modelos. Varias variables asociadas con una probabilidad de extinción elevada a nivel global, tal como tamaño corporal grande o rango de distribución pequeño, no se relacionaron con la ocurrencia de aves a lo largo del tiempo en nuestro sitio. El tamaño corporal se asoció con la pérdida de especies en Palenque, pero las tendencias en la ocurrencia de especies muy grandes y muy pequeñas, particularmente las últimas, ha declinado, o las especies han sido extirpadas. No encontramos asociación entre la tendencia en declinación de ocurrencia y el tamaño del rango de distribución geográfica, pero la declinación se correlacionó con la dependencia en la cobertura forestal (tendencia de ocupación promedio =−0.0380, 0.0263, y 0.0186 para especies con dependencia alta, intermedia y baja, respectivamente) y con combinaciones complejas de dieta y estrato de forrajeo (e.g., la ocurrencia de insectívoros de dosel y omnívoros terrestres ha aumentado, mientras la ocurrencia de frugívoros de nivel medio y granívoros terrestres ha disminuido). Nuestros resultados enfatizan que los análisis de áreas locales son necesarios para explicar el riesgo de extirpación en varias extensiones espaciales.

Habitat selection and the perceptual trap.

The concept of "ecological traps" was introduced over three decades ago. An ecological trap occurs when, by various mechanisms, low-quality (yielding low fitness) habitat is more attractive than good habitat, thus coaxing individuals to settle there despite a resultant loss of fitness. Empirical work on such traps has increased dramatically in the past decade, but the converse-avoidance of high-quality habitat because it is less attractive, what we term a "perceptual trap" has remained largely unexplored. Even so, depending on conditions (growth rate, strength of habitat preference, and mortality rate), such perceptual traps can be more limiting than ecological traps to population persistence. An example from field experiments with the Lesser Prairie-Chicken (Tympanuchus pallidicinctus) lends empirical support to the concept, and several other potential examples suggest that these traps are perhaps more prevalent than has been appreciated. Because demographic Allee effects are expected to prevent a population from growing sufficiently in a habitat that is avoided, a perceptual trap may persist even though fitness is high. Unlike an ecological trap, which may be negated by increasing habitat quality, biologists will be hard pressed to negate a perceptual trap, which will require determining which cues an animal uses to select high-quality habitat and then devising a means of enhancing those cues so that an animal is lured into the habitat.

Avoidance behavior by prairie grouse: implications for development of wind energy.

New wind-energy facilities and their associated power transmission lines and roads are being constructed at a rapid pace in the Great Plains of North America. Nevertheless, little is known about the possible negative effects these anthropogenic features might have on prairie birds, one of the most threatened groups in North America. We examined radiotelemetry tracking locations of Lesser Prairie-Chickens (Tympanuchus pallidicinctus) and Greater Prairie-Chickens (T. cupido) in two locations in Oklahoma to determine whether these birds avoided or changed movement behavior near power lines and paved highways. We tracked 463 Lesser Prairie-Chickens (15,071 tracking locations) and 216 Greater Prairie-Chickens (5,750 locations) for 7 and 3 years, respectively. Individuals of both species avoided power lines by at least 100 m and Lesser Prairie-Chickens avoided one of the two highways by 100 m. Prairie-chickens crossed power lines less often than expected if birds moved randomly (p < 0.05) but did not appear to perceive highways as a movement barrier (p > 0.05). In addition, home ranges of Lesser Prairie-Chickens overlapped the power line less often than would be expected by chance placement of home ranges; this result was supported by kernel-density estimation of home ranges. It is likely that new power lines (and other tall structures such as wind turbines) will lead to avoidance of previously suitable habitat and will serve as barriers to movement. These two factors will likely increase fragmentation in an already fragmented landscape if wind energy development continues in prairie habitats.

Diversity, seasonality, and context of mammalian roadkills in the southern Great Plains.

Thousands of mammals are killed annually from vehicle collisions, making the issue an important one for conservation biologists and environmental managers. We recorded all readily identifiable kills on or immediately adjacent to roads in the southern Great Plains from March 2004-March 2007. We also recorded distance traveled, whether a road was paved or divided, the number of lanes, and prevailing habitat. Surveys were opportunistic and were conducted by car during conditions of good visibility. Over our 239 surveys and > 16,500 km traveled, we recorded 1412 roadkills from 18 different mammal species (size ranged from Sciurus squirrels to the white-tailed deer, Odocolieus virginianus). The overall kill rate was 8.50/100 km. Four species were prone to collisions: the Virginia opossum (Didelphis virginiana), nine-banded armadillo (Dasypus novemcinctus), striped skunk (Mephitis mephitis), and northern raccoon (Procyon lotor). Together they accounted for approximately 85% (1198) of all roadkills. Mortality rate differed significantly between 2- and 4-lane roads (8.39 versus 7.79/100 km). Kill rates were significantly higher on paved versus unpaved roads (8.60 versus 3.65/100 km), but did not depend on whether a road was divided. Roadkills were higher in spring than in fall (1.5 x), winter (1.4x), or summer (1.3x). The spring peak (in kills/100 km) was driven chiefly by the armadillo (2.76 in spring/summer versus 0.73 in autumn/winter) and opossum (2.65 versus 1.47). By contrast, seasonality was dampened by a late winter/early spring peak in skunk mortalities, for which 41% occurred in the 6-week period of mid-February through March. The raccoon did not exhibit a strong seasonal pattern. Our data are consistent with dispersal patterns of these species. Our results underscore the high rate of highway mortality in the southern plains, as well as differences in seasonality and road type that contribute to mortality. Conservation and management efforts should focus on creating underpasses or using other means to reduce roadkill rates.

Habitat edge, land management, and rates of brood parasitism in tallgrass prairie.

Bird populations in North America's grasslands have declined sharply in recent decades. These declines are traceable, in large part, to habitat loss, but management of tallgrass prairie also has an impact. An indirect source of decline potentially associated with management is brood parasitism by the Brown-headed Cowbird (Molothrus ater), which has had substantial negative impacts on many passerine hosts. Using a novel application of regression trees, we analyzed an extensive five-year set of nest data to test how management of tallgrass prairie affected rates of brood parasitism. We examined seven landscape features that may have been associated with parasitism: presence of edge, burning, or grazing, and distance of the nest from woody vegetation, water, roads, or fences. All five grassland passerines that we included in the analyses exhibited evidence of an edge effect: the Grasshopper Sparrow (Ammodramus savannarum), Henslow's Sparrow (A. henslowii), Dickcissel (Spiza americana), Red-winged Blackbird (Agelaius phoeniceus), and Eastern Meadowlark (Sturnella magna). The edge was represented by narrow strips of woody vegetation occurring along roadsides cut through tallgrass prairie. The sparrows avoided nesting along these woody edges, whereas the other three species experienced significantly higher (1.9-5.3x) rates of parasitism along edges than in prairie. The edge effect could be related directly to increase in parasitism rate with decreased distance from woody vegetation. After accounting for edge effect in these three species, we found evidence for significantly higher (2.5-10.5x) rates of parasitism in grazed plots, particularly those burned in spring to increase forage, than in undisturbed prairie. Regression tree analysis proved to be an important tool for hierarchically parsing various landscape features that affect parasitism rates. We conclude that, on the Great Plains, rates of brood parasitism are strongly associated with relatively recent road cuts, in that edge effects manifest themselves through the presence of trees, a novel habitat component in much of the tallgrass prairie. Grazing is also a key associate of increased parasitism. Areas managed with prescribed fire, used frequently to increase forage for grazing cattle, may experience higher rates of brood parasitism. Regardless, removing trees and shrubs along roadsides and refraining from planting them along new roads may benefit grassland birds.

Comment on "Ivory-billed woodpecker (Campephilus principalis) persists in continental North America".

We reanalyzed video presented as confirmation that an ivory-billed woodpecker (Campephilus principalis) persists in Arkansas (Fitzpatrick et al., Reports, 3 June 2005, p. 1460). None of the features described as diagnostic of the ivory-billed woodpecker eliminate a normal pileated woodpecker (Dryocopus pileatus). Although we support efforts to find and protect ivory-billed woodpeckers, the video evidence does not demonstrate that the species persists in the United States.

Avian reproductive failure in response to an extreme climatic event.

Recently, climate change research has emphasized the potential increase in the frequency and severity of climatic extremes. We compared the reproductive effort and output among four species of passerine birds in coastal southern California, USA, a semi-arid region, during a normal precipitation year (2001) and the driest year in a 150-year climate record (2002). Both reproductive effort and output differed dramatically between years. Mean reproductive output among the four species was 2.37 fledglings/pair in 2001 and 88.4% of all pairs observed attempted at least one nest. The birds attempted a mean of 1.44 nests per pair and were successful in 47.7% of those attempts. In 2002, only 6.7% of the pairs even attempted a nest and only 1.8% were successful, for a total output of 0.07 fledglings per pair. The abundance of suitable arthropod prey items in the environment was also much lower in 2002, suggesting that low food availability was the proximal cause of the reproductive failure. The data for one of these species, the rufous-crowned sparrow (Aimophila ruficeps), were combined with reproductive and rainfall data from a previous 3-year study (1997-1999) in the same sites. The combined data sets suggest that the response of reproduction to rainfall variation is linear, and that the low end of the precipitation range brings the population near reproductive failure. Any change in climate that would increase the frequency of extreme dry conditions would likely endanger populations of these species.

Habitat selection, acoustic adaptation, and the evolution of reproductive isolation.

We examined barriers to gene flow in a hybrid zone of two subspecies of the song sparrow (Melospiza melodia). We focused on how mating signals and mate choice changed along an environmental gradient and gathered data on the morphology, genetics, ecology, and behavior across the zone. Melospiza m. heermanni of the Pacific slope of California and M. m. fallax of the Sonoran Desert, each distinct in plumage, meet across a steep environmental gradient in southeastern California. Although both subspecies occur in riparian habitat, their occupied habitat differs structurally, the former subspecies occurring in areas with denser understory and greater vertical heterogeneity. Song elements varied concomitantly, as predicted by the acoustic adaptation hypothesis, with heermanni having lower-pitched, more widely spaced elements. Females of both subspecies responded more strongly to homotypic than heterotypic song, and addition of subspecific plumage cues increased response if song was homotypic but not if heterotypic. Females thus assess multiple male traits, weighing song more heavily. Males of both subspecies showed significantly greater agonistic response to homotypic song. Microsatellite variation is correlated significantly with plumage variation across the zone and suggests limited gene flow between the taxa. The association of song and plumage with the environment and in turn with assortative mating suggests a means by which reproductive isolation may evolve or be maintained in hybrid zones.