Abstract
Condors and vultures are distinct from most other terrestrial birds because they use extensive soaring flight for their daily movements. Therefore, assessing resource selection by these avian scavengers requires quantifying the availability of terrestrial-based habitats, as well as meteorological variables that influence atmospheric conditions necessary for soaring. In this study, we undertook the first quantitative assessment of habitat- and meteorological-based resource selection in the endangered California condor (Gymnogyps californianus) within its California range and across the annual cycle. We found that condor use of terrestrial areas did not change markedly within the annual cycle, and that condor use was greatest for habitats where food resources and potential predators could be detected and where terrain was amenable for taking off from the ground in flight (e.g., sparse habitats, coastal areas). Condors originating from different release sites differed in their use of habitat, but this was likely due in part to variation in habitats surrounding release sites. Meteorological conditions were linked to condor use of ecological subregions, with thermal height, thermal velocity, and wind speed having both positive (selection) and negative (avoidance) effects on condor use in different areas. We found little evidence of systematic effects between individual characteristics (i.e., sex, age, breeding status) or components of the species management program (i.e., release site, rearing method) relative to meteorological conditions. Our findings indicate that habitat type and meteorological conditions can interact in complex ways to influence condor resource selection across landscapes, which is noteworthy given the extent of anthropogenic stressors that may impact condor populations (e.g., lead poisoning, wind energy development). Additional studies will be valuable to assess small-scale condor movements in light of these stressors to help minimize their risk to this critically endangered species.
Highlights
All animals require resources critical to their survival, and determining how and why an organism selects among available resources is fundamental to understanding its ecological niche
Resource selection is considered to occur across sequential spatial scales, with the broadest level of selection being that of a geographic range, followed by selection of individual home ranges, selection of coarse-scale habitats within the home range, and selection of microhabitats within coarse-scale habitats.Traditionally, studies of vertebrate resource selection have focused on quantifying use of terrestrial resources because most terrestrial-based species have their needs met by resources that are located on or near ground level
Animals with soaring flight use thermal lift and orographic lift to move across large areas, meteorological conditions vary across temporal and spatial scales [65] and, in turn, influence the degree to which specific areas are used by flying animals [15,51]
Summary
All animals require resources critical to their survival, and determining how and why an organism selects among available resources is fundamental to understanding its ecological niche. Long-term dataset that comprises high-resolution GPS location data to [1] quantify condor selection of terrestrial-based habitats as demarcated by coarse-scale landcover type (e.g., grassland, coniferous forest) [2], assess the relationship between condor use and three meteorological conditions that influence flight conditions for soaring birds (i.e., thermal height, thermal velocity, and wind speed), and [3] evaluate how condor selection of these resources varies across the annual cycle and relative to individual characteristics (i.e., age, sex, breeding status) and factors related to recovery program actions (i.e., rearing method, release location). Our study provides an important framework regarding terrestrial space use by condors that can be used to link habitat use to the risk of lead poisoning across the landscapes in which condors occur
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