James C. Bednarz

SKEWED BROOD SEX RATIO AND SEX-BIASED HATCHING SEQUENCE IN HARRIS'S HAWKS

Nestling sex ratios in Harriss hawks show a significant bias toward males at the age of sexing, as predicted by Fishers equilibrium hypothesis. Our data, however, do not support current hypotheses of parental manipulation of offspring sex ratios in response to varying environmental or social cues. The overall bias in offspring sex ratios was due to a highly significant skew toward males in first-hatched nestlings. We propose that sex-dependent hatching sequence may represent a proximal mechanism that upholds an adaptive sex-ratio skew toward males, the less expensive sex. If this mechanism were not employed by Harriss hawks, we submit that the dimorphic-size advantage of female nestlings added to the developmental advantage of hatching first might result in maladaptive brood reduction. Our observations show that nests in which males hatch first fledged significantly more young than nests in which females hatch first. Therefore, we suggest that female breeders may enhance their fitness by skewing the sex ratio of the first-hatched offspring toward males

Identifying predators clarifies predictors of nest success in a temperate passerine

1. Nest predation negatively affects most avian populations. Studies of nest predation usually group all nest failures when attempting to determine temporal and parental activities, habitat or landscape predictors of success. Often these studies find few significant predictors and interpret patterns as essentially random. 2. Relatively little is known about the importance of individual predator species or groups on observed patterns of nest success, and how the ecology of these predators may influence patterns of success and failure. 3. In 2006 and 2007, time-lapse, infrared video systems were deployed at nests of Swainsons warblers (Limnothlypis swainsonii Audubon) in east-central Arkansas to identify dominant nest predators and determine whether factors predicting predation differed among these predators. 4. Analysis of pooled data yielded few predictors of predation risk, whereas separate analyses for the three major predator groups revealed clear, but often conflicting, patterns. 5. Predation by ratsnakes (Elaphe obsoleta) and raptors was more common during the nestling period, whereas predation by brown-headed cowbirds (Molothrus ater) occurred more during incubation. Additionally, the risk of predation by raptors and cowbirds decreased throughout the breeding season, whereas ratsnake predation risk increased. 6. Contrary to expectations, predation by ratsnakes and cowbirds was more common far from edges, whereas raptor predation was more common close to agricultural edges. 7. Collectively, our results suggest that associating specific predators with the nests they prey on is necessary to understand underlying mechanisms.

Estimating Territory and Home-Range Sizes: Do Singing Locations Alone Provide an Accurate Estimate of Space Use?

ABSTRACT. Documenting space-use patterns of animals is important for understanding their ecology and providing for their conservation. The concept of territory refers to a defended area, whereas the concept of home range refers to any area used by an animal during its normal activities. Few studies have explored quantitatively how these areas differ, and the use of these terms in the literature has, at times, been inconsistent. Many studies attempt to estimate territory size of a bird by territory mapping, which involves using locations obtained by following a singing bird. This is often assumed to be an effective measure of territory size, but few studies have examined the efficacy of this technique. Here, we used radiotelemetry to estimate breeding home-range and territory sizes of Swainsons Warbler (Limnothlypis swainsonii), a Neotropical migrant songbird. We found that territories were much larger than had been estimated by territory-mapping methods and suggest that researchers use caution when assuming that territory mapping accounts for all areas used by a bird. We also found that home ranges were significantly larger than territories, and these relatively large home ranges necessitate protection of large areas of habitat for Swainsons Warblers. We emphasize the importance of precise terminology when referring to areas of space use and suggest that radiotracking may uncover previously unknown use patterns and important ecological information in species that have not been radiotracked.

POPULATION GENETICS OF THE GALÁPAGOS HAWK (BUTEO GALAPAGOENSIS): GENETIC MONOMORPHISM WITHIN ISOLATED POPULATIONS

Abstract Because of their smaller size and isolation, island populations tend to be more divergent and less genetically variable than mainland populations. We collected DNA samples from nine Galápagos Hawk (Buteo galapagoensis) island populations, covering the species’ entire range. Neutral minisatellite DNA markers were used to calculate within-island genetic diversity and between-island genetic differentiation (FST). Typically, these markers mutate too quickly to be informative in such studies. However, in very small, isolated populations, concerns about high mutational rate are obviated by the relative force of genetic drift. Individuals within islands had the highest levels of reported genetic uniformity of any natural bird population, with mean within-population band-sharing similarity values ranging from 0.693 to 0.956, increasing with decreasing island size. Galápagos Hawks exhibit cooperative polyandry to varying degrees across islands; however, we did not find an association between degree of polyandry and genetic variability. Between-island FST values ranged from 0.017 to 0.896, with an overall archipelago value of 0.538; thus, most populations were genetically distinct. Also, we documented higher levels of genetic similarity between nearby populations. Our results indicated negligible gene flow among most Galápagos Hawk populations, and genetic drift has played a strong role in determining structure at these minisatellite loci. Genética de Poblaciones de Buteo galapagoensis: Monomorfismo Genético dentro de Poblaciones Aisladas

EMERGING CONCEPTS AND RESEARCH DIRECTIONS IN THE STUDY OF CAVITY-NESTING BIRDS: KEYSTONE ECOLOGICAL PROCESSES

The papers in this special section of The Condor derive from a symposium entitled ‘‘The Ecology of Cavity Nesters: Keystone Processes,’’ which we organized for the Third North American Conference, held in September 2002 in New Orleans, Louisiana. The purpose of this symposium was to examine the current state of knowledge about cavity-nesting birds, ubiquitous in the world’s forests, and discuss directions for future research and conservation. The unifying theme of the symposium, and what we believe is a key paradigm in understanding ecological systems, is the concept of keystone interactions and processes. Keystone species are those that exhibit disproportionate influence over the structure and function of their community due to some lifehistory trait or interactions with other species (Paine 1969). Keystone processes, then, may be defined as the interactions between one or more keystone species with both biotic and abiotic factors, that influence resource creation, use, and exchange for many species. Our symposium covered a broad range of species and processes involving cavity-nesting birds. The papers that follow were developed from the presentations included in the symposium. Although it has been recognized for decades that primary cavity nesters (i.e., excavators) provided resources that facilitate the survival and viability of many organisms in forest communities, investigations into how these connections and interactions work have been rare. Notable exceptions include groundbreaking explorations

The influence of habitat, prey abundance, sex, and breeding success on the ranging behavior of Prairie Falcons

We studied the ranging behavior and habitat selection of radio-tagged Prairie Falcons (Falco mexicanus) during the breeding season in southwestern Idaho. The distribution and numbers of Townsends ground squirrels (Spermophilus townsendii), the primary prey of Prairie Falcons in our study area, varied in response to drought during the study period. Prairie Falcons ranged over large areas (ca. 300 km 2 ) and increased their foraging ranges in response to declining ground squirrels. Reptiles and birds were preyed upon most frequently when squirrels were rare. Males and females differed little in their use of space. Successful pairs ranged over smaller areas than non-nesters and unsuccessful pairs. Falcons nesting near habitat most suitable for ground squirrels ranged over smaller areas than those nesting farther from such habitat. Home ranges contained significantly more winterfat (Ceratoides lanata) and native perennial grasses (especially Poa secunda), and significantly less salt desert shrubs and exotic annual grasses than expected based on availability. Salt desert shrubs were found less than expected, based on availability in core areas within home ranges. Selection for winterfat and bluegrass in core areas was contingent upon selection at the larger scale of the home range; falcons with-home ranges containing more winterfat and bluegrass than expected based on availability were less selective in their placement of core areas with respect to these habitats. We believe salient features of Prairie Falcon home ranges result largely from patchy distribution of landscape features associated with different densities and availabilities of Townsends ground squirrels.

Status, Habitat Use, and Management of Red-Shouldered Hawks in Iowa

In recent years the red-shouldered hawk (Buteo lineatus) has declined in numbers in Iowa (Brown 1964, Koenig 1975) and nationwide (Cohen 1970:14, 36; Brown 1971; Hackman and Henny 1971). In the Midwest, it is listed as rare or endangered in Illinois, Iowa, Michigan, Missouri, and Wisconsin (Roosa 1977, Merz 1978). The decline probably is due directly or indirectly to habitat alteration (Todd 1940, Cohen 1970, Henny et al. 1973, Oberholser 1974, Portnoy 1974, Campbell 1975, Bock and Lepthien 1976). The objectives of this study were to systematically survey potential redshouldered hawk nesting habitat in Iowa using a land-use data base, and to provide information on habitat use and management recommendations.

Habitat and Landscape Effects on Brood Parasitism, Nest Survival, and Fledgling Production in Swainson's Warblers

Abstract Numerous factors, including nest predation and brood parasitism, may limit populations of neotropical migratory birds. However, nest predation and brood parasitism are not constant, and temporal, biological, habitat, and landscape factors can affect the likelihood of these events. Understanding these patterns is important for species of conservation concern for which managers seek to provide quality habitat. One such species, the Swainsons warbler (Limnothlypis swainsonii), is a neotropical migrant that breeds primarily in bottomland hardwood forests of the southeastern United States. Little is known of factors influencing reproductive success of this rare, yet locally abundant, species. From 2004 through 2007, we examined factors influencing reproductive success of Swainsons warblers at 2 sites in eastern Arkansas, USA, St. Francis National Forest and White River National Wildlife Refuge. We used 2-stage modeling to assess the relationship between 1) temporal and biological, and 2) habitat and landscape factors and brood parasitism, nest survival, and fledgling production. Brood parasitism was greater in this population (36%) than reported elsewhere (≤10%), but decreased throughout the breeding season. Nest survival was comparable to or lower than in other populations of this species and increased throughout the breeding season. The probability of brood parasitism was greater near forest edges. Although nests of Swainsons warblers were often associated with giant cane (Arundinaria gigantea), nest survival had a weak negative association with cane density. For nests that were successful, the best predictor of number of Swainsons warblers fledged was brood-parasitism status: nonparasitized nests fledged 2.75 young, whereas parasitized nests fledged 0.60 Swainsons warblers. Our findings suggest that managing and restoring relatively high-elevation bottomland forests that are located far from agricultural edges should increase Swainsons warbler productivity.

Factors influencing home-range size of Swainson's Warblers in eastern Arkansas.

Abstract. Understanding factors affecting space-use patterns can lead to improved knowledge of the ecology and habitat needs of a species. We radiotracked 37 male Swainsons Warblers (Limnothlypis swainsonii) at two sites in eastern Arkansas, White River National Wildlife Refuge, a bottomland site, and St. Francis National Forest, a site with birds in both bottomlands and uplands. We found substantial variation in home-range size and evaluated habitat and nonhabitat predictors of home-range size by information-theoretic techniques. We also evaluated how breeding phase affected home-range size. Several habitat variables were good predictors of home-range size, including understory density, vine-stem density, and other stem-density variables. Overall, vine-stem density and understory density were the best predictors of home-range size, and nonhabitat factors were poor predictors. A three-variable model including density of giant cane (Arundinaria gigantea), vine, and shrub stems as separate variables was a much better predictor of home-range size than a model using total stem density, suggesting that Swainsons Warblers respond differently to different stem types. Birds using areas with fewer vines and more cane exploited smaller home ranges, suggesting better habitat quality. Birds that were tracked a greater proportion of time during the incubation and nestling periods used larger home ranges. Though our study found that birds held smaller home ranges in areas of more cane and fewer vines, a combination of stem factors and understory density appears to explain home-range size best. We recommend that management for Swainsons Warbler focus on maintenance of uniformly dense understory vegetation within forests.

VARIATION IN MORPHOLOGY AND MATING SYSTEM AMONG ISLAND POPULATIONS OF GALÁPAGOS HAWKS

Abstract Interspecific variation in sexual size dimorphism has commonly been attributed to variation in social mating system, with dimorphism increasing as intrasexual competition for mates increases. In birds, overall body size has also been found to correlate positively with size dimorphism. In this study, we describe variation in morphology and mating system across six populations of the endemic Galápagos Hawk (Buteo galapagoensis). Galápagos Hawks exhibit cooperative polyandry, a mating system in which long-term social groups contain a single female and multiple males. Comparisons among islands revealed significant differences in overall body size for both adults and immatures. Populations ranged from completely monogamous to completely polyandrous, with varying mean group sizes. Data did not support our prediction that sexual size dimorphism would increase with the degree of polyandry (number of males per group) or with body size; there was no correlation between mating system and sexual dimorphism. We did find a significant negative relationship between degree of polyandry and body size among islands, opposite of the pattern predicted. Variación en Morfología y Sistema de Apareamiento entre Poblaciones de Buteo galapagoensis Resumen. Variación interespecífica en dimorfismo sexual ha sido atribuída comúnmente a variaciones del sistema social de apareamiento, de tal manera que el dimorfismo aumenta conforme aumenta la competencia intrasexual por parejas reproductivas. También se ha encontrado que el tamaño corporal se correlaciona positivamente con el dimorfismo. En este estudio describimos la variación morfológica y el grado de poliandría de seis poblaciones de Buteo galapagoensis, una especie que exhibe un sistema de apareamiento denominado poliandría cooperativa. En este sistema los grupos de individuos reproductivos incluyen una sola hembra y múltiples machos. Se comprobó que existen diferencias significativas en el tamaño del cuerpo de adultos y juveniles entre islas. Las poblaciones muestreadas variaron entre monógamas y completamente poliándricas, y el tamaño promedio de los grupos fue variable. Los datos no apoyaron las predicciones establecidas inicialmente pues el grado de dimorfismo sexual no aumentó con el nivel de poliandría (número de machos por grupo) ni con el tamaño corporal, ni hubo una correlación entre el sistema de apareamiento y el dimorfismo sexual. La relación entre el tamaño corporal y el sistema de apareamiento fue contraria a la que se predijo: hubo una correlación negativa significativa entre el grado de poliandría y el tamaño corporal entre islas.