Robert W. Russell

Boundary Layer Clear-Air Radar Echoes: Origin of Echoes and Accuracy of Derived Winds

Abstract Boundary layer clear-air echoes are routinely observed with sensitive, microwave, Doppler radars similar to the WSR-88D. Operational and research meteorologists are using these Doppler velocities to derive winds. The accuracy of the winds derived from clear-air Doppler velocities depends on the nature of the scatterers. This paper uses dual-wavelength and dual-polarization radars to examine the cause of these echoes and the use of Doppler velocities from the clear-air return to estimate winds. The origin of these echoes has been an ongoing controversy in radar meteorology. These echoes have been attributed to refractive-index gradient (Bragg scattering) and insects and birds (particulate scattering). These echoes are most commonly observed over land from spring through autumn. Seldom do they occur over large bodies of water. Widespread clear-air echoes have also been observed in winter when temperatures are above 10°C. Radar reflectivity comparisons of clear-air echoes in Florida and Colorado wer...

Foraging in a fractal environment: Spatial patterns in a marine predator-prey system

Spatial relationships between predators and prey have important implications for landscape processes and patterns. Highly mobile oceanic birds and their patchily distributed prey constitute an accessible model system for studying these relationships. High-frequency echosounders can be used together with simultaneous direct visual observations to quantitatively describe the distributions of seabird consumers and their resources over a wide range of spatial scales, yielding information which is rarely available in terrestrial systems.Recent fine-scale investigations which have used acoustics to study the distribution of foraging marine birds have reported weak or ephemeral spatial associations between the birds and their prey. These results are inconsistent with predictions of optimal foraging, but several considerations suggest that traditional foraging models do not adequately describe resource acquisition in marine environments. Relative to their terrestrial counterparts, oceanic ‘landscapes’ are structurally very simple, but they generally lack visual cues about resource availability.An emerging view assumes that perceptually constrained organisms searching for food in multiscale environments should respond to patterns of resource abundance over a continuum of scales. We explore fractal geometry as a possible tool for quantifying this view and for describing spatial dispersion patterns that result from foraging behavior. Data on an Alaskan seabird (least auklet [Aethia pusilla]) and its zooplanktonic food resources suggest that fractal approaches can yield new ecological insights into complex spatial patterns deriving from animal movements.

Biphasic Mass Gain in Migrant Hummingbirds: Body Composition Changes, Torpor, and Ecological Significance

Body mass of migrant Rufous Hummingbirds (Selasphorus rufus) on refueling stopovers increased on average from 3.2 to 4.6 g over a period ranging from several days to 3 wk. In birds arriving with body masses below °3.5 g, the initial period of mass gain was very slow. This slow gain was not explained by energy costs associated with territory establishment or learning to secure food, since it occurred even in years when nectar resources were superabundant and territoriality was nearly nonexistent. Data on body composition indicate that mass gain up to °3.5 g was due to deposition of nonlipid body components, which we hypothesize to be proteins involved in rebuilding muscle catabolized during the last stage of the recent migratory flight. Following the initial phase of slow mass gain, an accelerating rise in body mass consisted entirely of lipid gain. On average, overnight mass loss decreased prior to migration, suggesting that nocturnal torpor facilitated lipid deposition. The slow phase of mass gain is potentially important constraint on migrating hummingbirds, because if they deplete their fat stores and allow their body mass to fall 3.5 g, they incur a substantial cost in terms of greatly increased time spent on the subsequent stopover.

Interference asymmetries among age-sex classes of rufous hummingbirds during migratory stopovers

SummaryThree age-sex classes of rufous hummingbirds (Selasphorus rufus) overlap temporally and defend feeding territories during migratory stopovers in the Sierra Nevada of California. We demonstrate that these classes differ in their ability to secure and maintain high-quality feeding territories for refueling, and that these differences result in differences in resource use. Data on acquisition of territories, territory characteristics, and responses of territory owners to intruders suggest that several mechanisms are involved in determining dominance, involving sex- and age-related differences in wing disc loading, coloration, and experience. We discuss the implications of these results for understanding intraspecific variation in migration strategies.

Exploitative compensation by subordinate age-sex classes of migrant rufous hummingbirds

SummaryThe three age-sex classes of rufous hummingbirds (Selasphorus rufus) that directly interact on southward migratory stopovers in our California study system differ in territorial ability and resource use. Immature males are behaviorally dominant to adult and immature females and defend the richest territories. Here, we test the hypothesis that the territorially subordinate age-sex classes compensate exploitatively for their exclusion from rich resources. Our results show that females were able to accumulate energy stores at rates comparable to males despite their subordinate territorial status. Territorial females gained body mass at the same rate and in the same pattern as males, and resumed migration at the same body masses. Moreover, during periods when birds were nonterritorial and used dispersed resources, adult and immature females maintained or gained body mass, whereas immature males lost mass. We suggest that females may be energetically compensated by (1) lower costs of flight incurred during foraging and defense, resulting from their lower wing disc loading, and (2) greater success at robbing nectar from rich male territories, resulting from duller coloration (immature females), experience (adult females), and, possibly, hormonal differences. In the future, experiments will be necessary to distinguish the various hypotheses about the mechanisms involved in compensation.

Radar-observed fine lines in the optically clear boundary layer : Reflectivity contributions from aerial plankton and its predators

Sensitive Doppler radars regularly detect fine lines ofenhanced reflectivity in mesoscale boundary-layer convergence zones. Recent studies have concludedthat these ‘fine lines’ are attributable primarily to backscatter fromconcentrations of small, weakly flying insects (‘aerial plankton’)entrained in the convergence zones. Such concentrations are likely tobe attractive to aerial predators that feed on small insects, raising thequestion of whether the presence of the predators themselves maycontribute significantly to the radar-observed fine lines.In this paper, we examine the relative contributions of aerialplankton and its predators to fine-line reflectivity, using field datafrom visual and radar studies together with a compilation of literaturedata on radar cross sections of birds and insects. Visual counts ofbirds and dragonflies in convergence zones, together withsimultaneous remote radar observations during the CaPE project inFlorida, indicated that aerial predators usually contributed little tofine-line reflectivity (median contribution ≈ 2%). Assuming thatthe size distribution of insect targets was spatially invariant, thedensity of insects composing the aerial plankton was inferred to be,on average, about one order of magnitude higher inside convergencezones than in nearby areas.These results suggest that clear-air radar reflectivity may be auseful measure of the quantity of aerial plankton in boundary-layerconvergence zones. This finding is relevant to biology because itindicates that remote sensing techniques can be usefully employed todocument patterns and processes in the distribution of aerial plankton.The results presented here also have relevance for operationalmeteorology, because most of the organisms comprising the planktonprobably serve as passive tracers of horizontal air motions, and aretherefore ideal targets for remotely detecting wind patterns. Incontrast, the aerial predators move actively and rapidly, renderingthem less useful as tracers of wind fields in studies using Dopplerradars. The influence of atmospheric structure on the ecology ofaerial predators and their prey has received little attention, but webelieve that sensitive radars with clear-air observational capabilitiesoffer great potential as research platforms for future studies of aerialplankton and aerial planktivory.

Precipitation scrubbing of aerial plankton : inferences from bird behavior

Abstract I conducted direct visual observations of aerial insect-eating birds concurrently with remote radar observations of aerial plankton before, during, and after the passage of an intense thunderstorm gust front in east-central Florida. Clear skies and convective conditions predominated in the area prior to local passage of the gust front. Shortly after passage, weather conditions shifted to a stratiform regime, with continuous low cloud cover and intermittent drizzly rain. Insectivorous birds appeared in the area briefly at the time of gust front passage, and became common following the onset of the drizzly rain. These birds remained in the area after the cessation of precipitation, feeding actively and very low, until mid-afternoon the following day, when the transition from stratiform back to convective conditions finally occurred. Almost immediately after this transition, the altitude of bird flight began increasing rapidly, and the birds quickly dispersed. I interpreted these observations together with radar data as indicating that (a) large quantities of aerial plankton were entrained by the gust front, “leaked” into the storm outflow, and were subsequently “scrubbed” out of the atmospheric boundary layer by precipitation; (b) after the rain ended, the insect species involved were prevented from reascending by stratiform sky conditions that inhibited thermal convection, and (c) the insects rapidly reascended at the first possible opportunity (i.e., almost immediately following the transition from stratiform back to convective conditions). This detailed case study showed that an individual convergence event in the atmospheric boundary layer caused a spatial redistribution of the aerial plankton which subjected the insects involved to locally intensified predation by avian consumers. However, despite the dramatic short-term effects, the apparently rapid reascent of the aerial plankton suggests that this particular scrubbing event probably had little impact on the local population dynamics of the insect species affected. Data on track directions of a large sample of summer gust fronts in east-central Florida suggest that the potential for net directional displacements of insect populations over seasonal time scales via the cumulative effects of individual convergence events and subsequent scrubbing is probably low.