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ABSTRACT. We tested the hypothesis that the spatial distributions of foraging least, crested and parakeet auklets (Aethia pusilla, A. cristatella and A. psittacula, respectively) in the shallow passes of the Aleutian Islands would be determined by physical mechanisms that control near-surface prey concentrations. We recorded currents using an Acoustic Doppler Current Profiler, volume scattering using 200 and 420 kHz scientific echosounders, and the numbers of foraging birds. Zooplankton were sampled using a multiple opening/closing net and environmental sampling system (MOCNESS). Prey choice of birds was ascertained by collecting foraging birds and examining their stomach contents. Most sampling occurred between 8 July and 6 August 1993, when we conducted 50 passages along a transect that crossed a sill between Unalga and Kavalga Islands, western Aleutian Islands, thereby samplinq the loraglng activity ot auklets at a variety of times of day and tidal phases. We found that the abundance of foraging individuals of each of the 3 auklet species was a function of tidal speed. Auklet species were selective about the species of prey taken. Regardless of tidal direct~on, crested auklets foraged on euphausiids upwclled on the upstream side of the pass, whereas least auklets consumed copepods concentrated in near-surface convergences on the downstream side. Parakeet auklets foraged over the top of the pass and took fish and invertebrates. Tidal speed and direction influenced the distance between the peak numbers of some, but not all, species of auklets. Auklet prey preferences dictated where they foraged in the pass and the physical mechanisms exploited for successful foraging. Thus, in this instance, resource partitioning by these closely related planktivores was enhanced by a spatial segregation forced by the physical processes that enhanced the availability of prey. Our findings emphasize the important role of physical processes in the structuring of marine communities.

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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 [Aethiapusilla]) and its zooplanktonic food resources suggest that fractal approaches can yield new ecological insights into complex spatial patterns deriving from animal movements.

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 ofnonlipid 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 a potentially important constraint on migrating hummingbirds, because if they deplete their fat stores and allow their body mass to fall below 3.5 g, they incur a substantial cost in terms of greatly increased time spent on the subsequent stopover.

Three 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 do~ninance, 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.

We studied hydrographic structure, zooplankton distributions, and foraging by planktivorous seabirds in the Anadyr Strait, northern Bering Sea, during 4 summer cruises (1984)(1985)(1986) 1993). The western portion of the strait was occupied by cold, dense Anadyr water that was mixed from top to bottom. This mixed water was separated from the stratified Bering Shelf water on the eastern side of the strait by a sharp surface front (the 'Anadyr Front'). Net sampling indicated that calanoid copepods were the numerically dominant component of the zooplankton, and that densities of several species were elevated in the frontal zone, apparently due to mechanical accumulation resulting from surface convergence. Hydroacoustic surveys showed that overall zooplankton biomass was concentrated along the thermocline and at the front. Although the location of the Anadyr Front was highly variable over time scales as short as 1 d, large numbers of least auklets Aethia pusilla often flew 25 to 50 km from their breeding colonies to feed at the front. Diet samples indicated that the copepod Neocalanusplumchrus was the principal prey taken by least auklets both at the front and away from it, indicating that heavy use of the distant frontal habitat was due to the higher densities of their preferred prey (i.e. rather than absence of suitable prey species closer to shore). Whenever aggregations of least auklets were found away from the front, there was evidence that they were exploiting near-surface hgh-density patches of zooplankton, though the exact mechanisms responsible for the formation of such patches are unclear. In contrast to least auklets, crested auklets Aethia cristatella were usually found away from the front. In several cases, compact aggregations of crested auklets were located over acoustically observed epibenthic layers of zooplankton. Hydrographic data suggested that intense subsurface jets and/or upwelling along the eastern side of the strait might have increased the availability of the crested auklets' preferred euphausiid prey. Thus, spatial segregation of the 2 principal planktivores in Anadyr Strait likely arises because different physical mechanisms cause concentrations of preferred prey originating at different depths.

The 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.