Phillip M. Stepanian

Climate and weather impact timing of emergence of bats.

Interest in forecasting impacts of climate change have heightened attention in recent decades to how animals respond to variation in climate and weather patterns. One difficulty in determining animal response to climate variation is lack of long-term datasets that record animal behaviors over decadal scales. We used radar observations from the national NEXRAD network of Doppler weather radars to measure how group behavior in a colonially-roosting bat species responded to annual variation in climate and daily variation in weather over the past 11 years. Brazilian free-tailed bats (Tadarida brasiliensis) form dense aggregations in cave roosts in Texas. These bats emerge from caves daily to forage at high altitudes, which makes them detectable with Doppler weather radars. Timing of emergence in bats is often viewed as an adaptive trade-off between emerging early and risking predation or increased competition and emerging late which restricts foraging opportunities. We used timing of emergence from five maternity colonies of Brazilian free-tailed bats in south-central Texas during the peak lactation period (15 June–15 July) to determine whether emergence behavior was associated with summer drought conditions and daily temperatures. Bats emerged significantly earlier during years with extreme drought conditions than during moist years. Bats emerged later on days with high surface temperatures in both dry and moist years, but there was no relationship between surface temperatures and timing of emergence in summers with normal moisture levels. We conclude that emergence behavior is a flexible animal response to climate and weather conditions and may be a useful indicator for monitoring animal response to long-term shifts in climate.

Estimating animal densities in the aerosphere using weather radar: To Z or not to Z?

Weather radars provide near-continuous recording and extensive spatial coverage, which is a valuable resource for biologists, who wish to observe and study animal movements in the aerosphere over a wide range of temporal and spatial scales. Powerful biological inferences can be garnered from radar data that have been processed primarily with the intention of understanding meteorology. However, when seeking to answer certain quantitative biological questions, e.g., those related to density of animals, assumptions made in processing radar data for meteorological purposes interfere with biological inference. In particular, values of the radar reflectivity factor (Z) reported by weather radars are not well suited for biological interpretation. The mathematical framework we present here allows researchers to interpret weather radar data originating from biological scatterers (bioscatterers) without relying on assumptions developed specifically for meteorological phenomena. The mathematical principles discussed are used to interpret received echo power as it relates to bioscatterers. We examine the relationships among measurement error and these bioscatter signals using a radar simulator. Our simulation results demonstrate that within 30-90 km from a radar, distances typical for observing aerial vertebrates such as birds and bats, measurement error associated with number densities of animals within the radar sampling volume are low enough to allow reasonable estimates of aerial densities for population monitoring. The framework presented for using radar echoes for quantifying biological populations observed by radar in their aerosphere habitats enhances use of radar remote-sensing for long-term population monitoring as well as a host of other ecological applications, such as studies on phenology, movement, and aerial behaviors.

Nocturnally migrating songbirds drift when they can and compensate when they must

The shortest possible migratory route for birds is not always the best route to travel. Substantial research effort has established that birds in captivity are capable of orienting toward the direction of an intended goal, but efforts to examine how free-living birds use navigational information under conditions that potentially make direct flight toward that goal inefficient have been limited in spatiotemporal scales and in the number of individuals observed because of logistical and technological limitations. Using novel and recently developed techniques for analysis of Doppler polarimetric weather surveillance radar data, we examined two impediments for nocturnally migrating songbirds in eastern North America following shortest-distance routes: crosswinds and oceans. We found that migrants in flight often drifted sideways on crosswinds, but most strongly compensated for drift when near the Atlantic coast. Coastal migrants’ tendency to compensate for wind drift also increased through the night, while no strong temporal differences were observed at inland sites. Such behaviors suggest that birds migrate in an adaptive way to conserve energy by assessing while airborne the degree to which they must compensate for wind drift.

Extracting Migrant Flight Orientation Profiles Using Polarimetric Radar

Seasonal animal migration is characterized by aligned flight of airborne organisms across large spatial expanses. This large-scale alignment results in azimuthal patterns in polarimetric radar products. The following overviews some such patterns and introduces a technique for obtaining vertical profiles of migrant flight orientation by exploiting azimuthal symmetries in the polarimetric radar product of copolar correlation coefficient ρHV . This method is compared with several Doppler-velocitybased techniques for measuring flight direction, and a sensitivity analysis is performed. Finally, the method is applied to a case of nocturnal migration over the Southern Great Plains, demonstrating the utility of the technique in the study of animal migratory behavior within the airspace.

Seasonal differences in landbird migration strategies

ABSTRACT Migrating birds make strategic decisions at multiple temporal and spatial scales. They must select flight altitudes, speeds, and orientations in order to maintain preferred directions of movement and to minimize energy expenditure and risk. Spring flights follow a rapid phenology, but how this rapid transit translates to in-flight decisions is not clear. We described flight strategies of nocturnally migrating landbirds using 6 weather surveillance radars during spring (2013–2015) and fall (2013–2014) migratory periods in the eastern United States to investigate seasonal decision-making patterns and how climate change may influence these trends. During spring, we found groundspeed and airspeed of migrants to be significantly higher than those of fall migrants; compensation for wind drift was also significantly greater during spring. Our results indicate that birds make more rapid and precise flights in spring that are only partially explained by meteorological phenomena. Future applications at greater spatial scales will allow direct comparisons of in-flight behaviors with predictions from migration theory.

The role of the US Great Plains low-level jet in nocturnal migrant behavior

The movements of aerial animals are under the constant influence of atmospheric flows spanning a range of spatiotemporal scales. The Great Plains nocturnal low-level jet is a large-scale atmospheric phenomenon that provides frequent strong southerly winds through a shallow layer of the airspace. The jet can provide substantial tailwind assistance to spring migrants moving northward, while hindering southward migration during autumn. This atmospheric feature has been suspected to play a prominent role in defining migratory routes, but the flight strategies used with respect to these winds are yet to be examined. Using collocated vertically pointing radar and lidar, we investigate the altitudinal selection behavior of migrants over Oklahoma during two spring and two autumn migration seasons. In general, migrants choose to fly within the jet in spring, often concentrating in the favorable wind speed maximum. Autumn migrants typically fly below the jet, although some will rapidly climb to reach altitudes above the inhibiting winds. The intensity of migration was relatively constant throughout the spring due to the predominantly favorable southerly jet winds. Conversely, autumn migrants were more apt to delay departure to wait for the relatively infrequent northerly winds.

Electromagnetic Model Reliably Predicts Radar Scattering Characteristics of Airborne Organisms

The radar scattering characteristics of aerial animals are typically obtained from controlled laboratory measurements of a freshly harvested specimen. These measurements are tedious to perform, difficult to replicate, and typically yield only a small subset of the full azimuthal, elevational, and polarimetric radio scattering data. As an alternative, biological applications of radar often assume that the radar cross sections of flying animals are isotropic, since sophisticated computer models are required to estimate the 3D scattering properties of objects having complex shapes. Using the method of moments implemented in the WIPL-D software package, we show for the first time that such electromagnetic modeling techniques (typically applied to man-made objects) can accurately predict organismal radio scattering characteristics from an anatomical model: here the Brazilian free-tailed bat (Tadarida brasiliensis). The simulated scattering properties of the bat agree with controlled measurements and radar observations made during a field study of bats in flight. This numerical technique can produce the full angular set of quantitative polarimetric scattering characteristics, while eliminating many practical difficulties associated with physical measurements. Such a modeling framework can be applied for bird, bat, and insect species, and will help drive a shift in radar biology from a largely qualitative and phenomenological science toward quantitative estimation of animal densities and taxonomic identification.

An introduction to radar image processing in ecology

Summary Use of radar in ornithology, chiropterology and entomology continues to increase, driven in part by widespread online data availability. In addition to research applications, rapid growth in areas such as wind energy and aviation has prompted the use of radar for conservation. While a variety of research applications motivate ecologists to gain basic radar literacy, the ability to process and analyse radar data sets can be a daunting task that may dissuade inexperienced ecological radar users. This effect is exacerbated by vague radar methodologies in the ecology literature, as well as the combination of complex techniques and unfamiliar terminology in other radar-focused disciplines. While radar data come in many formats and levels of detail, a common type is the two-dimensional radar image. As rasters of data with associated spatial coordinates, radar images are relatively easy to manipulate, especially for those familiar with basic raster computations. Furthermore, because radar image data require relatively small storage space, they can be readily downloaded from a number of online sources. With this in mind, radar images provide a convenient foundation for ecological applications. A primer on radar image interpretation and processing is presented, with a focus on image composition for typical atmospheric surveillance radar scans. Additionally, a selection of existing ecological radar image processing methods are overviewed. As a starting point, a basic algorithm for automated image processing is outlined that may be modified to create specialized workflows. Three examples of the application of this algorithm are included, illustrating its modification and use for automated feature extraction. By outlining a basic algorithm, we hope to provide a clear starting point for the beginning radar user. When combined with additional existing methods, this algorithm provides a wide range of refinements and modifications that can pave a path towards sophisticated radar processing workflows. In the long term, the ability of ecologists to independently analyse radar data will lead to better ecological interpretation of radar data and a more informed application to conservation policy.

Influence of atmospheric properties on detection of wood-warbler nocturnal flight calls

Avian migration monitoring can take on many forms; however, monitoring active nocturnal migration of land birds is limited to a few techniques. Avian nocturnal flight calls are currently the only method for describing migrant composition at the species level. However, as this method develops, more information is needed to understand the sources of variation in call detection. Additionally, few studies examine how detection probabilities differ under varying atmospheric conditions. We use nocturnal flight call recordings from captive individuals to explore the dependence of flight call detection on atmospheric temperature and humidity. Height or distance from origin had the largest influence on call detection, while temperature and humidity also influenced detectability at higher altitudes. Because flight call detection varies with both atmospheric conditions and flight height, improved monitoring across time and space will require correction for these factors to generate standardized metrics of songbird migration.

Precipitation Spectra Analysis Over China With High-Resolution Measurements From Optimally-Merged Satellite/Gauge Observations—Part II: Diurnal Variability Analysis

Timing and diurnal variation of summer precipitation is analyzed over China using a new high-resolution (0.1°, hourly) satellite-gauge merged surface rainfall dataset that spans from 2008 through 2013. The results show that: 1) both precipitation amount (PA) and frequency (PF) show strong diurnal cycles with local solar time (LST); 2) peak times of PA (PAPT) primarily occur from 15 LST to 00 LST in most parts of the Tibet Plateau (TP), Xinjiang (XJ), Northwestern China (NW), Northeastern China (NE), and Southern China (SC), and the PAPT occurs from 00 LST to 09 LST in southern TP, Eastern XJ, western NW, southern NE, eastern Northern China (NC), and most parts of Southwestern China (SW); 3) the PAPT transitions eastward with time, occurring at ~15 LST in central TP, at midnight in SW, and at 15-18 LST in the eastern coastal regions that are in the lower reach of Yangtze River and in the north side of Wuyi Mountains; 4) peak times of PF (PFPT) show a similar spatial pattern with PAPT, but with a small temporal (1-2 h) lead; 5) peak times of precipitation intensity (PIPT) occur during the 18-00 LST time frame in the southeastern TP and central SW regions. The PIPT along the upper Yangtze River valley occurs around 00-06 LST. The PIPT occurs in the morning at around 06-09 LST in the mid-lower Yangtze River valley and most parts of SC. This study on the diurnal cycle of precipitation over China can be used as a reference to validate atmospheric and hydrologic models, and also to guide hydrometerological research and applications.