Sexual signal loss: The link between behaviour and rapid evolutionary dynamics in a field cricket.

Sexual signals may be acquired or lost over evolutionary time, and are tempered in their exaggeration by natural selection. In the Pacific field cricket, Teleogryllus oceanicus, a mutation ("flatwing") causing loss of the sexual signal, the song, spread in <20 generations in two of three Hawaiian islands where the crickets have been introduced. Flatwing (as well as some normal-wing) males behave as satellites, moving towards and settling near calling males to intercept phonotactic females. From 2005 to 2012, we surveyed crickets and their responses to conspecific song, noting the morph and number of males and females before and after experimental playbacks. The three Hawaiian islands consistently contained different proportions of flatwing crickets, ranging from about 90% of males on Kauai to 50% on Oahu to rare on the Big Island of Hawaii. Flatwing and normal-wing males do not appear to differ in responsiveness to playback, a behaviour that should influence the likelihood of a male encountering a phonotactic female. Instead, male and female crickets from populations in which little to no calling song is perceptible during development tended to seek out callers more readily than crickets that developed in noisier environments. Such increased phonotaxis makes females more likely to find either the caller to which they are responding or to encounter a flatwing (or normal male satellite) that has also been attracted to the song. Our evidence suggests that pre-existing behavioural plasticity (manifest as flexible responses to social-particularly acoustic-information in the environment) is associated with the rapid spread of the flatwing trait. Different social environments select for differential success of flatwing or normal-wing males, which in turn alters the social environment itself.

Alternative Reproductive Tactics in Context: How Demography, Ecology, and Behavior Affect Male Mating Success.

Exploitation of sexual signals by predators or parasites increases costs to signalers, creating opportunities for establishment of alternative reproductive tactics (ARTs). In field crickets, males calling may attract acoustically orienting parasitoid flies. Alternatively, males behaving as satellites forgo calling and attempt to intercept females attracted to callers. We modeled the contribution of calling versus satellite behavior to male reproductive success in the larger context of variation in ecology (parasitism rate, background mortality), demography (density, sex ratio), and female behavior (phonotaxis, mating choosiness). Male mating success was most influenced by number of females (standardized effect size 0.42), followed by female choosiness (0.33), background mortality (-0.31), number of males (-0.28), and parasitism rate (-0.21). The smallest effects were phonotaxis (0.10) and satellite behavior (-0.09). Although satellite behavior ameliorated negative effects of parasitism, its comparative effect was slight. ARTs seem most likely to evolve and persist when a single selection pressure on signaling is particularly strong.

Elevation and latitude interact to drive life-history variation in precocial birds: a comparative analysis using galliformes.

Elevational gradients provide a powerful laboratory for understanding the environmental and ecological drivers of geographic variation in avian life-history strategies. Environmental variation across elevational gradients is hypothesized to select for a trade-off of reduced fecundity (lower clutch size and/or fewer broods) for higher offspring quality (larger eggs and/or increased parental care) in higher elevation species and populations. In birds, a focus on altricial species from north temperate latitudes has prevented an evaluation of the generality of this trade-off, and how it is affected by latitude and intrinsic factors (development mode). We performed a comparative analysis controlling for body size and phylogenetic relationships on a global data set of 135 galliform species to test (i) whether higher elevation precocial species have lower fecundity (smaller clutch and/or fewer broods) and invest more in offspring quality (greater egg mass) and (ii) whether latitude influences the traits involved and/or the trade-off, and (iii) to identify ecological and environmental drivers of life-history variation along elevational gradients. Life-history traits showed significant interaction effects across elevation and latitude: temperate higher elevation species had smaller clutches and clutch mass, larger eggs and shorter incubation periods, whereas more tropical species had larger clutches, eggs and clutch mass, and longer incubation periods as elevation increased. Number of broods and body mass did not vary with elevation or latitude. Latitudinal gradient in clutch size was observed only for low-elevation species. Significantly, an overlooked latitude-by-elevation interaction confounds our traditional view of clutch size variation across a tropical-to-temperate gradient. Across all latitudes, higher elevation species invested in offspring quality via larger eggs but support for reduced fecundity resulting from smaller clutches was found only along temperate elevational gradients; contrary to expectations, tropical high-elevation species showed increased fecundity. Variation in nest predation risk could explain differences between temperate and tropical elevational gradients, but we lack a consistent mechanism to explain why predation risk should vary in this manner. Alternatively, a resource availability hypothesis based on physical attributes that globally differ between elevation and latitude (seasonality in day length and temperature) seems more plausible.

Alternative Reproductive Tactics Arising from a Continuous Behavioral Trait: Callers versus Satellites in Field Crickets.

Alternative reproductive tactics may arise when natural enemies use sexual signals to locate the signaler. In field crickets, elevated costs to male calling due to acoustically orienting parasitoid flies create opportunity for an alternative tactic, satellite behavior, where noncalling males intercept females attracted to callers. Although the caller-satellite system in crickets that risk detection by parasitoids resembles distinct behavioral phenotypes, a male's propensity to behave as caller or satellite can be a continuously variable trait over several temporal scales, and an individual may pursue alternate tactics at different times. We modeled a caller-satellite-parasitoid system as a spatially explicit interaction among male and female crickets using individual-based simulation. Males varied in their propensity to call versus behave as a satellite from one night to the next. We varied mortality, density, sex ratio, and female mating behavior, and recorded lifetime number of mates as a function of a male's probability of calling (vs. acting as a satellite) along a gradient in parasitism risk. Frequently, the optimal behavior switched abruptly from being pure caller (call every night) to pure satellite (never call) as parasitism rate increased. However, mixed strategies prevailed even with high parasitism risk under conditions of higher background mortality rate, decreasing density, increasing female-biased sex ratio, and increasing female choosiness. In natural populations, high parasitoid pressure alone would be unlikely to yield fixation of pure satellite behavior.

Diffuse migratory connectivity in two species of shrubland birds: evidence from stable isotopes.

Connecting seasonal ranges of migratory birds is important for understanding the annual template of stressors that influence their populations. Brewer's sparrows (Spizella breweri) and sagebrush sparrows (Artemisiospiza nevadensis) share similar sagebrush (Artemisia spp.) habitats for breeding but have different population trends that might be related to winter location. To link breeding and winter ranges, we created isoscapes of deuterium [stable isotope ratio (δ) of deuterium; δ2H] and nitrogen (δ15N) for each species modeled from isotope ratios measured in feathers of 264 Brewer's and 82 sagebrush sparrows and environmental characteristics at capture locations across their breeding range. We then used feather 2Hf and 15Nf measured in 1,029 Brewer's and 527 sagebrush sparrows captured on winter locations in southwestern United States to assign probable breeding ranges. Intraspecies population mixing from across the breeding range was strong for both Brewer's and sagebrush sparrows on winter ranges. Brewer's sparrows but not sagebrush sparrows were linked to more northerly breeding locations in the eastern part of their winter range. Winter location was not related to breeding population trends estimated from US Geological Survey Breeding Bird Survey routes for either Brewer's or sagebrush sparrows. Primary drivers of population dynamics are likely independent for each species; Brewer's and sagebrush sparrows captured at the same winter location did not share predicted breeding locations or population trends. The diffuse migratory connectivity displayed by Brewer's and sagebrush sparrows measured at the coarse spatial resolution in our analysis also suggests that local environments rather than broad regional characteristics are primary drivers of annual population trends.

Reproductive biology of Hawaiian lava crickets.

Insects have spread across diverse ecological niches, including extreme environments requiring specialized traits for survival. However, little is understood about the reproductive traits required to facilitate persistence in such environments. Here, we report on the reproductive biology of two species of endemic Hawaiian lava crickets (Caconemobius fori and Caconemobius anahulu) that inhabit barren lava flows on the Big Island. We examine traits that reflect investment into reproduction for both male and female lava crickets and compare them to the non-extremophile Allard's ground cricket (Allonemobius allardi) in the same sub-family. Lava cricket females possessed fewer, but much larger eggs than ground crickets, while males do not provide the costly nuptial gifts that are characteristic of the Nemobiinae subfamily. Lava crickets also have longer ovipositors relative to their body length than related Caconemobius species that occupy cave habitats on the Hawaiian islands. The differences in reproduction we report reveal how these little-known cricket species may increase survival of their offspring in the resource-deprived conditions of their hot, dry environments.

How avian nest site selection responds to predation risk: testing an 'adaptive peak hypothesis'.

1. Nest predation limits avian fitness, so birds should favour nest sites that minimize predation risk. Nevertheless, preferred nest microhabitat features are often uncorrelated with apparent variation in predation rates. 2. This lack of congruence between theory-based expectation and empirical data may arise when birds already occupy 'adaptive peaks'. If birds nest exclusively in low-predation microhabitats, microhabitat and nest predation may no longer be correlated even though predation ultimately shaped microhabitat selection. 3. This 'adaptive peak hypothesis' was tested for a population of Yellow Warblers (Dendroica petechia) focusing on two nest microhabitat features: concealment and height. Experimental nests measured relative predation risk both within and outside the microhabitat range typically occupied by natural nests to examine whether nest site choices made by birds restricted our ability to detect microhabitat effects on predation. 4. Within the natural range (30-80% concealment, >75 cm height), microhabitat-predation relationships were weak and inconsistent, and similar for experimental and natural nests. Over an extended range, however, experimental predation rates were elevated in exposed sites (<30% concealed), indicating a concealment-related 'adaptive plateau'. 5. Clay egg bite data revealed a concealment effect on avian predators, and the abundance of one avian predator group correlated with nest concealment among years, suggesting these predators may cue birds to modulate nest concealment choices. 6. This study demonstrates how avian responses to predation pressure can obscure the adaptive significance of nest site selection, so predation influences may be more important than apparent from published data.

Combined effects of landscape composition and heterogeneity on farmland avian diversity.

Conserving biodiversity on farmland is an essential element of worldwide efforts for reversing the global biodiversity decline. Common approaches involve improving the natural component of the landscape by increasing the amount of natural and seminatural habitats (e.g., hedgerows, woodlots, and ponds) or improving the production component of the landscape by increasing the amount of biodiversity-friendly crops. Because these approaches may negatively impact on economic output, it was suggested that an alternative might be to enhance the diversity (compositional heterogeneity) or the spatial complexity (configurational heterogeneity) of land cover types, without necessarily changing composition. Here, we develop a case study to evaluate these ideas, examining whether managing landscape composition or heterogeneity, or both, would be required to achieve conservation benefits on avian diversity in open Mediterranean farmland. We surveyed birds in farmland landscapes of southern Portugal, before (1995-1997) and after (2010-2012) the European Union's Common Agricultural Policy (CAP) reform of 2003, and related spatial and temporal variation in bird species richness to variables describing the composition, and the compositional and configurational heterogeneity, of the natural and production components of the landscape. We found that the composition of the production component had the strongest effects on avian diversity, with a particularly marked effect on the richness of farmland and steppe bird species. Composition of the natural component was also influential, mainly affecting the richness of woodland/shrubland species. Although there were some effects of compositional and configurational heterogeneity, these were much weaker and inconsistent than those of landscape composition. Overall, we suggest that conservation efforts in our area should focus primarily on the composition of the production component, by striving to maximize the prevalence of biodiversity-friendly crops. This recommendation probably applies to other areas such as ours, where a range of species of conservation concern is strongly associated with crop habitats.

Independent effects of food and predator-mediated processes on annual fecundity in a songbird.

We investigated the relative importance and interaction of ecological processes affecting annual fecundity in birds by simultaneously manipulating food availability and nest predation risk in a small songbird, the Wrentit (Chamaea fasciata). From 2000 to 2002 we provided supplemental food to individual Wrentit territories, and during 2002 we altered nest predation risk by providing supplemental food to their principal predators, Western Scrub-Jays (Aphelocoma californica). These experiments were conducted during a period of high interannual variation in rainfall, with 2002 being one of the driest years on record. Food-supplemented Wrentits in a normal predation environment produced an average of 0.54 more fledglings per year than control pairs over the three breeding seasons. During the feeding plus predation manipulation experiment, Wrentit food supplementation and lowered nest predation risk each independently increased the probability that a Wrentit pair would fledge young; however, the interaction between food supplementation and altered nest predation risk was not significant. Thus, even in an extreme drought year, both food and nest predation had equal but independent effects on reproductive success and annual fecundity. Combining supplemental food with reduced nest predation did not result in a synergistic increase in annual fecundity, primarily because Wrentits did not produce multiple broods. Our results suggest that whether food and predation have additive or synergistic effects on reproductive success depends on the life history of the species and the environment in which they live.

GIS-based niche modeling for mapping species' habitat.

Ecological "niche modeling" using presence-only locality data and large-scale environmental variables provides a powerful tool for identifying and mapping suitable habitat for species over large spatial extents. We describe a niche modeling approach that identifies a minimum (rather than an optimum) set of basic habitat requirements for a species, based on the assumption that constant environmental relationships in a species' distribution (i.e., variables that maintain a consistent value where the species occurs) are most likely to be associated with limiting factors. Environmental variables that take on a wide range of values where a species occurs are less informative because they do not limit a species' distribution, at least over the range of variation sampled. This approach is operationalized by partitioning Mahalanobis D2 (standardized difference between values of a set of environmental variables for any point and mean values for those same variables calculated from all points at which a species was detected) into independent components. The smallest of these components represents the linear combination of variables with minimum variance; increasingly larger components represent larger variances and are increasingly less limiting. We illustrate this approach using the California Gnatcatcher (Polioptila californica Brewster) and provide SAS code to implement it.

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.

Landscape Characteristics of Fragmented Shrubsteppe Habitats and Breeding Passerine Birds.

We examined the influence of local and landscape-level attributes of fragmented habitats in shrubsteppe habitats on the breeding distributions of Sage (Amphispiza belli) and Brewer's (Spizella breweri) Sparrows, Sage Thrashers (Oreoscoptes montanus), Horned Larks (Eremophila alpestris), and Western Meadowlarks (Sturnella neglecta) in the Snake River Plains of southwestern Idaho. We developed habitat (resource) selection models for each species by combining bird counts conducted from 1991 through 1933 with local vegetation characteristics and landscape attributes derived from satellite imagery. Site selection by shrubsteppe species (Sage and Brewer's Sparrows, and Sage Thrashers) depended on local vegetation cover and landscape features, such as the patch size of shrub habitats or the spatial similarity of sites. Marginal sites for these species (with species present in one of three years) were intermediate between unoccupied (never present) and occupied sites along environmental gradients characterized by increasing size of shrub habitat patches and total shrub cover and by decreasing disturbance. Horned Larks and Western Meadowlarks, typical grassland species, were not sensitive to landscape features, and their occupancy depended on the amount of grassland or shrub cover. In contrast to shrubsteppe species, sites that varied by occupancy rates of Western Meadowlarks did not significantly differ in vegetation or landscape components. Our results demonstrate that fragmentation of shrubsteppe significantly influenced the presence of shrub-obligate species. Because of restoration difficulties, the disturbance of semiarid shrubsteppe may cause irreversible loss of habitat and significant long-term consequences for the conservation of shrub-obligate birds. Características del paisaje en ambientes fragmentados de estepas arbustivas y pájaros paserínidos en estado reproductivo.

Resumen: Examinamos la influencia de los atributos a nivel local y del paisaje de los hábitats fragmentados dentro de ambientes de estepas arbustivas, sobre las distribucion reproductiva de los paserinidos Amphispiza belli y Spizella breweri, Oreoscoptes montanus, Eremophila alpestris y Sturnella neglecta en las planicies del Río Snake del Sudoeste de Idaho. Desarrollamos modelos de selección de hábitats (recursos) para cada especie, combinando conteos de pájaros llevados a cabo entre 1991 y 1993 con características de la vegetación local y los atributos del hábitat derivados de imágenes de satelite. La selección de los sitios por parte de las especies de arbustos de las escarpas o pendientes (Amphispiza belli y Spizella breweri) dependió de la cobertura de la vegetación local y las características del paisaje, tales como el tamaño de los parches de los ambientes arbustivos o similitud espacial de los sitios. Los sitios marginales para esas especies (aquellos con las especies presentes en solo 1 de los 3 años) fueron intermedios entre los desocupados (nunca presentes) y los sitios ocupados, a lo largo de gradientes ambientales caracterizados por un incremento en el tamaño de los parches del ambiente arbustivo y la cobertura total de arbustos y un decrecimiento en la perturbación. Eremophila alpestris y Sturnella neglecta, especies típicas de pastizales, no fueron sensibles a las características del paisaje y dependieron de la cantidad de cobertura de pastizal o de arbustos. En forma opuesta a las especies de las estepas arbustivas, los sitos que variaron en cuanto al grado de ocupación de Sturnella neglecta, no presentaron variaciones significativas en los components de la vegetación o del paisaje. Nuestros resultados demuestran que la fragmentacion del paisaje de estepas arbustivas influenció significativamente la presencia de especies asociadas a los arbustos. Debido a las dificultades en la restauración, la perturbación de ambientes semi-áridos de estepas arbustivas puede causar la pérdida irreversible del hábitat y tener consecuencias significativas a largo plazo para la conservación de los pájaros asociados a los arbustos.

The role of habitat in avian community composition: physiognomy or floristics?

It has been proposed that within rather broad habitat types the distribution and abundance of bird species may be more closely associated with plant taxonomic composition than with the structure and configuration of the vegetation. Birds from a sample of eight representative grassland habitats in middle and western North America are consistent with this hypothesis. Over half (55%) of the variation in bird community composition was associated with floristic variation, but only a third (35%) was associated with physiognomy. Separating the interacting effects of floristics and physiognomy from each other served to accentuate the difference between them with respect to the avifauna. It is postulated that bird species/plant taxa associations, especially within similar habitat types, are mediated by the specific food resources that different plant taxa provide. Summary indices such as diversity measures obscure the taxonomic information content of plant or animal assemblages, and the use of such indices has likely impeded detection of the relationships described here.

Bioenergetics and diet in a simple community of shrubsteppe birds.

Simulation model estimates of bioenergetics are coupled with observations of diet selection and arthropod prey abundances to assess (1) the role of bird populations in trophic energy fluxes in a temporally heterogeneous shrubsteppe ecosystem, and (2) the degree to which those populations may be limited by food.The model estimates a total annual energy demand of 2.91 kcal m-2 yr-1 by the entire passerine avifauna during 1974, with daily demands varying from 0.0025 to 0.0260 kcal m-2. Coupling energy requirements with estimates of arthropod availability implies that bird demands on the insect standing crop never exceeded 0.7% per day of that standing crop during the breeding season or summer.Overall, the bioenergetic estimates imply that these birds are unlikely to be important in ecosystem processes and, reciprocally, are unlikely to be limited by food resources even during peak energy demands. As a consequence, I suggest that biological interactions such as competition play a relatively minor role in structuring the bird community in this variable environment.

Temporal variation in habitat structure and shrubsteppe bird dynamics.

We investigate the role of temporal variation in habitat physiognomy in influencing the dynamics of shrubsteppe bird populations and communities. During a 3-y (1977-1979) study of 14 sites in the northern Great Basin of North America, annual precipitation varied substantially, with one of the driest years on record followed by one of the wettest. This resulted in significant physiognomic variation (increasing height and coverage of vegetation, decreasing horizontal patchiness), mediated largely by changes in the annual elements of the flora, particularly forbs and grasses. Shrub species coverage values, on the other hand, demonstrated no statistically detectable year-to-year changes, nor were they correlated with any physiognomic variation. Despite large scale physiognomic changes, no bird species' abundance varied in a statistically significant fashion; neither could variation in bird abundances be correlated with variation in either physiognomy or shrub species coverages.Multivariate analyses revealed essentially the same patterns as the univariate analyses: substantial changes in physiognomy, few changes in shrub species coverages or bird species abundances, and little correlation of temporal variation among the three data sets. Calculation of the Euclidean distances that sites "moved" in multivariate physiognomic, bird species, or shrub species hyperspaces yields synthetic gradients of annual "turnover" of sites with respect to those data sets. Sites identified as demonstrating high physiognomic turnover were characterized by high coverage of grass and forbs, while low turnover sites had greater coverage of shrubs and higher shrub species diversity. Relatively high bird turnover sites had greater numbers of Western Meadowlarks and Black-throated Sparrows, while more stable sites had high numbers of Brewer's Sparrows. Physiognomically, high bird turnover sites were grassier and had greater total vegetation coverage, while low bird turnover sites had more bare ground and higher horizontal patchiness. A site's position on the avifauna turnover axis, however, was uncorrelated with its position on the physiognomic turnover axis. Shrub species showed virtually no annual turnover.Reanalysis of a previous Principal Components Analysis (PCA) of these same data sets that was applied without regard to year of sampling revealed that the first physiognomic component (41% of the total physiognomic variability) did in fact have a strong temporal element, and that this element was consistent with the changes in univariate characters noted above. No other physiognomic component could be associated with annual variation, nor could any components of parallel bird abundance or shrub species coverage PCA's.Regional patterns indicate that sites tended to be very consistent from year to year in their relationships to one another as defined by their relative locations in either physiognomic or shrub species hyperspaces, but varied independently of one another with respect to their bird species abundances and composition. Examination of the temporal consistency of site relationships between "bird space" and "vegetation space" reveals that bird communities are to a large degree independent of a site's physiognomic position, but instead are strongly associated with its position in shrub species hyperspace.The overall patterns that emerge from these analyses are consistent with the so-called "checkerboard effect" that results from the apparently random annual redistribution of individual birds, and leads to the conclusion that populations of shrubsteppe birds are not existing at maximum density or "carrying capacity." Such observations are consistent with contentions that these populations lack close biological coupling with coexisting species and that interactions among these species (e.g. competition) are likely to play little if any role in the organization of their communities.

Diet niche relationships among North American grassland and shrubsteppe birds.

We consider the dietary relationships of the numerically dominant breeding bird species in four North American grassland/shrubsteppe habitats, sampled over 2-3 consecutive years. Overall, the diets of these species contained primarily insects: orthopterans comprised 29% of the diet biomass, coleopterans 24%, and lepidopteran larvae 23%, while seeds contributed 15% of the average diet. These diets varied substantially, however, and we evaluated several aspects of this variation. Intersexual differences in diets within a species were few, despite the occurrence of significant sexual size dimorphism in several species. For many species, however, there were substantial shifts in dietary composition between years at a given location; overall, the average between-year similarity of species' dietary composition was 70%. Different species exhibited rather different diet patterns. Horned Larks were relatively omnivorous, had broad diet composition niches, and varied considerably in diets between different locations. Meadowlarks were also broad-niched and geographically variable in their diets, but were the most highly carnivorous of the species we considered. Dietary niche breadths of Grasshopper Sparrows were intermediate, but diet composition was rather stable, both between years and between locations. Chestnut-collared Longspurs exhibited narrow diet niches, but substantial annual variation: each year this species apparently exploited a different but limited set of prey types rather heavily. Larger avian predators generally consumed a broader array of functional groups of prey, but did not differ in the taxonomic variety of their diets from small birds. Variation in diet composition between individuals within local populations was considerable; in most species, an individual contained on the average 30-40% of the prey taxa represented in entire population smaples.Patterns of dietary overlap among species were quite inconsistent from year to year at most locations, although at the shrubsteppe site overlap among all species present was consistently quite high. Relatively few cooccurring species pairs exhibited low diet overlap. The degree of diet niche overlap was unrelated to body size differences of the birds, despite as much as six-fold differences in weight among some coexisting species. Relationships of the bird species on another dimension of the trophic niche, prey size, also differed substantially between sites and years. The ranking of co-occurring species by the mean sizes of the prey they consumed generally did not parallel their rankings by body sizes, and in some cases the smallest and the largest species present ate prey of similar sizes. At the shrubsteppe site, all the breeding species exhibited quite similar frequency distributions of prey sizes in their diets.As species number and diversity increased in the breeding avifaunas, diet niche breadths generally decreased, species packing by prey size decreased, and diet composition niche overlap remained relatively unchanged. These trends are in at least partial agreement with predictions of diffuse competition theory, but the patterns were derived from broad inter-site comparisons of overall site averages, and the relationships generally did not hold within local assemblages of species. In general, our attempts to match values of dietary niche features with site characteristics failed to demonstrate close agreement with the predictions of prevailing ecological theory based upon assumptions of resource limitation and competition. Instead, our findings seem generally most consistent with the suggestion that food is not normally limiting to bird populations in these systems, and individuals and populations are exploiting the food resources in an opportunistic fashion, which leads to considerable individual, between-year, and between-location variation in diet compositions and interspecific overlaps.Our attempts to discern clear relationships that accord with theoretical expectations in these avian assemblages are thwarted by our lack of detailed information on the resource base and by the lack of clear tests that will separate alternative hypotheses of community organization and structuring. We suggest that these complications may compromise the findings of many community studies.

Foraging ecology of the California gnatcatcher deduced from fecal samples.

The California gnatcatcher is a threatened species essentially restricted to coastal sage scrub habitat in southern California. Its distribution and population dynamics have been studied intensely, but little is known about its diet. We identified arthropod fragments in 33 fecal samples of the California gnatcatcher to gain insight into its foraging ecology and diet. Fecal samples were collected from adult males, adult females, fledglings, and nestlings. Leaf- and planthoppers (Homoptera) and spiders (Araneae) predominated numerically in samples. Spider prey was most diverse, with eight families represented. True bugs (Hemiptera) and wasps, bees, and ants (Hymenoptera) were only minor components of the gnatcatcher diet. Gnatcatcher adults selected prey to feed their young that was larger than expected given the distribution of arthropod size available in their environment, and chicks were provisioned with larger prey items and significantly more grasshoppers and crickets (Orthoptera) and spiders than adults consumed themselves. Both adults and young consumed more sessile than active prey. Further studies are needed to determine whether arthropods sampled in coastal sage scrub that are common in fecal samples are good indicators of California gnatcatcher habitat.

Effects of parents and Brown-headed Cowbirds (Molothrus ater) on nest predation risk for a songbird.

Nest predation limits avian fitness, so ornithologists study nest predation, but they often only document patterns of predation rates without substantively investigating underlying mechanisms. Parental behavior and predator ecology are two fundamental drivers of predation rates and patterns, but the role of parents is less certain, particularly for songbirds. Previous work reproduced microhabitat-predation patterns experienced by Yellow Warblers (Setophaga petechia) in the Mono Lake basin at experimental nests without parents, suggesting that these patterns were driven by predator ecology rather than predator interactions with parents. In this study, we further explored effects of post-initiation parental behavior (nest defense and attendance) on predation risk by comparing natural versus experimental patterns related to territory density, seasonal timing of nest initiation, and nest age. Rates of parasitism by Brown-headed Cowbirds (Molothrus ater) were high in this system (49% nests parasitized), so we also examined parasitism-predation relationships. Natural nest predation rates (NPR) correlated negatively with breeding territory density and nonlinearly (U-shaped relationship) with nest-initiation timing, but experimental nests recorded no such patterns. After adjusting natural-nest data to control for these differences from experimental nests other than the presence of parents (e.g., defining nest failure similarly and excluding nestling-period data), we obtained similar results. Thus, parents were necessary to produce observed patterns. Lower natural NPR compared with experimental NPR suggested that parents reduced predation rates via nest defense, so this parental behavior or its consequences were likely correlated with density or seasonal timing. In contrast, daily predation rates decreased with nest age for both nest types, indicating this pattern did not involve parents. Parasitized nests suffered higher rates of partial predation but lower rates of complete predation, suggesting direct predation by cowbirds. Explicit behavioral research on parents, predators (including cowbirds), and their interactions would further illuminate mechanisms underlying the density, seasonal, and nest age patterns we observed.

Silent night: adaptive disappearance of a sexual signal in a parasitized population of field crickets.

Sexual signals are often critical for mate attraction and reproduction, although their conspicuousness exposes them to parasites and predators. We document the near-disappearance of song, the sexual signal of crickets, and its replacement with a novel silent morph, in a population subject to strong natural selection by a deadly acoustically orienting parasitoid fly. On the Hawaiian Island of Kauai, more than 90% of male field crickets (Teleogryllus oceanicus) shifted in less than 20 generations from a normal-wing morphology to a mutated wing that renders males unable to call (flatwing). Flatwing morphology protects male crickets from the parasitoid, which uses song to find hosts, but poses obstacles for mate attraction, since females also use the males' song to locate mates. Field experiments support the hypothesis that flatwings overcome the difficulty of attracting females without song by acting as 'satellites' to the few remaining callers, showing enhanced phonotaxis to the calling song that increases female encounter rate. Thus, variation in behaviour facilitated establishment of an otherwise maladaptive morphological mutation.

CALLING SONGS OF FIELD CRICKETS (TELEOGRYLLUS OCEANICUS) WITH AND WITHOUT PHONOTACTIC PARASITOID INFECTION.

The field cricket Teleogryllus oceanicus has been introduced to Hawaii, where it is parasitized by an acoustically orienting parasitoid fly, Ormia ochracea. Previous work showed that call parameters from parasitized populations differ from those in unparasitized populations in a direction expected if selection by flies is occurring. Here we examined songs of males collected in the field and compare calling song characters of crickets later found to harbor parasitoid larvae with those of males free of parasitoids. The two groups differ significantly in several song characteristics, particularly the trill-like long chirp given at the beginning of each song. Males with longer long chirps containing shorter interpulse intervals are more likely to be parasitized, suggesting that the flies find such males more attractive. Depending on the traits females prefer, sexual selection may oppose natural selection in altering T. oceanicus song in parasitized populations.