Effects of Regional Cold Fronts and Localized Weather Phenomena on Autumn Migration of Raptors and Landbirds in Southwest Idaho

Weather has a significant effect on avian migration, but whether the influence is similar across diverse geographic regions and across all species remains to be determined. We evaluated the effect of regional cold fronts and localized weather phenomena on the timing of autumn migration of multiple species of landbirds and raptors in southwest Idaho. The focus of the analysis was on total landbirds and the ten most common landbird species, along with total raptors and the eight most common raptor species. Using 13 years of data from the Idaho Bird Observatory in southwest Idaho (1997–2009), including standardized mist-net captures of landbirds and counts of raptors during autumn migration, we determined significant patterns that advance our understanding of the variables influencing avian migration in the West. Our data show a depression of numbers of most migratory species on the days immediately before, during, and after the passage of a cold front, with peak flights of most species occurring several days prior to or after cold fronts. This pattern was further substantiated by a detailed analysis of many weather variables illustrating that the majority of species choose to migrate during calmer winds, high pressure, and between cold fronts when the opportunity presents itself. In the Intermountain West, cold fronts are fewer in fall than in much of the rest of North America, so migrants may have greater choice of conditions under which to migrate and this behavior may be more common

Stopover Ecology of Autumn Landbird Migrants in the Boise Foothills of Southwestern Idaho

The topography of western North America provides a complex landscape for landbird migrants, and stopover patterns in this region are poorly understood. We examined seven years of stopover data (1997–2003) from a montane area in southwestern Idaho to determine whether this area provides suitable stopover habitat. We compared the proportion of birds recaptured, stopover duration, and changes in energetic condition within and among species and between two mist-netting sites located in different habitats. The proportion of birds recaptured ranged from zero to over 20%, and fewer than 5% of individuals were recaptured in most species. Mean minimum stopover durations from recapture data ranged from 1 to 10 days; most species averaged less than 6 days. Stopover duration estimates from open-population models were comparable but generally greater than estimates from recapture data. As found in stopover studies from other regions, stopover metrics varied within and among species in Idaho. However, most migrants in this study exhibited an ability to gain mass, evidenced both by recapture data and by regression of energetic condition against time since sunrise. These data imply that montane habitats in Idaho are suitable stopover sites. It follows that these habitats might serve an important role for many landbird migrants during the period of late summer molt and autumn migration, a time when many lowland areas of the West, including some riparian systems, are especially arid. We suggest that including montane nonriparian habitats in future stopover ecology studies will allow for a more complete understanding of migrant habitat needs in the West

Molt Strategies and Age Differences in Migration Timing Among Autumn Landbird Migrants in Southwestern Idaho

Intraspecific patterns of autumn migration timing are not well known, particularly in the western United States. Here, we (1) describe autumn migration timing and age ratios of landbird migrants in southwestern Idaho, (2) examine differences in timing among age and sex classes, and (3) demonstrate how prebasic molt strategies affect migration timing differences between age classes. As a group, Neotropical migrants were most common from late July through early September, whereas temperate migrants were most common from mid-September into early October. Proportion of hatch-year birds was 74.5% for all migrants combined and ranged from 33.3% to 100% for individual species. Timing differences between sex classes were detected in only a few species and no general patterns emerged. In 22 of 31 Neotropical and temperate migrants examined, there were significant differences in timing between adults and hatch-year birds. In species in which adults begin fall migration before replacing flight feathers, adults migrated earlier than hatch-year birds. Conversely, in species in which adults molt flight feathers on or near the breeding grounds before departing on fall migration, hatch-year birds migrated earlier than adults in all but one case. Therefore, it appears that molt strategy is a powerful determinant of intraspecific migration timing differences and, to our knowledge, this is the first study to document this pattern among migrant passerines of North America

Variable Shifts in the Autumn Migration Phenology of Soaring Birds in Southern Spain

While alteration of the migratory habits of birds is widely regarded as one of the most evident ecological effects of climate change, studies reporting shifts in migration phenology for long-lived, long-distance migrants have been few. Using time series of count data collected in southern Spain during autumn migration, we examined the magnitude and direction of phenological shifts for six common species of soaring birds. Many current methods for investigating phenological change rely on continuous data sets; however, these data may be unavailable for a variety of reasons. We used a cross-correlation analysis, which allowed us to compare recent data on the timing of migration from 1999–2011 to a historic data set collected during 1976–1977. The direction of phenological shifts for autumn migration was species-specific. White Storks Ciconia ciconia and Black Kites Milvus migrans appeared to have delayed passage, Black Storks Ciconia nigra and European Honey Buzzards Pernis apivorus have advanced their migratory timing, and we found no clear phenological change for Short-toed Eagles Circaetus gallicus or Booted Eagles Hieraaetus pennatus

Morphological Changes in American Kestrels (\u3cem\u3eFalco sparverius\u3c/em\u3e) at Continental Migration Sites

Many American kestrel (Falco sparverius) populations are declining across North America. Potential causes include mortality from reduction in food availability, a changing climate, habitat degradation, an increase in avian predators, disease, and toxins. We analyzed American kestrel count and banding data from seven raptor migration sites throughout North America with at least 20 years of migration data. We used count data to determine the year at which the kestrel population began a significant decline and then used banding records to determine whether body mass and wing chord declined after this point. We found reductions in kestrel body mass at three sites and reductions in kestrel wing chord at five sites. Our results indicate declines in body size at the majority of sites are consistent with the hypotheses that food availability, impacts of a changing climate, or predation risk may be contributing to population declines

Morphological changes in American kestrels (Falco sparverius) at continental migration sites

Many American kestrel (Falco sparverius) populations are declining across North America. Potential causes include mortality from reduction in food availability, a changing climate, habitat degradation, an increase in avian predators, disease, and toxins. We analyzed American kestrel count and banding data from seven raptor migration sites throughout North America with at least 20 years of migration data. We used count data to determine the year at which the kestrel population began a significant decline and then used banding records to determine whether body mass and wing chord declined after this point. We found reductions in kestrel body mass at three sites and reductions in kestrel wing chord at five sites. Our results indicate declines in body size at the majority of sites are consistent with the hypotheses that food availability, impacts of a changing climate, or predation risk may be contributing to population declines

Metamaterial fibres for subdiffraction imaging and focusing at terahertz frequencies over optically long distances

Using conventional materials, the resolution of focusing and imaging devices is limited by diffraction to about half the wavelength of light, as high spatial frequencies do not propagate in isotropic materials. Wire array metamaterials, because of their extreme anisotropy, can beat this limit; however, focusing with these has only been demonstrated up to microwave frequencies and using propagation over a few wavelengths only. Here we show that the principle can be scaled to frequencies orders of magnitudes higher and to considerably longer propagation lengths. We demonstrate imaging through straight and tapered wire arrays operating in the terahertz spectrum, with unprecedented propagation of near field information over hundreds of wavelengths and focusing down to 1/28 of the wavelength with a net increase in power density. Applications could include in vivo terahertz-endoscopes with resolution compatible with imaging individual cells.Alessandro Tuniz, Korbinian J. Kaltenecker, Bernd M. Fischer, Markus Walther, Simon C. Fleming, Alexander Argyros and Boris T. Kuhlme