Elf owl home range and habitat study, 2015 annual report
The elf owl (Micrathene whitneyi), a covered species under the Lower Colorado River (LCR) Multi-Species Conservation Program (MSCP), occurs within the LCR MSCP planning area during spring and summer, but its current distribution is much more restricted than in the past. In 2015, Great Basin Bird Observatory and University of Arizona completed the first season of field work in a three-year project designed to provide the Bureau of Reclamation (Reclamation) with data characterizing elf owl responsiveness and habitat use in riparian areas. The goal of the first field season was to characterize patterns of elf owl occurrence across a broad study area in Arizona. Of specific interest was to assess the frequency with which elf owls occupy areas dominated by riparian vegetation, the significance of riparian vegetation to elf owls, and environmental factors that are good predictors of elf owl occurrence. Data were collected across several important environmental gradients to provide maximal interpretational and biological context. These included geographic (i.e. latitude and longitude), elevation, and vegetation gradients, and included three riparian (mesic, xeric, exotic) and three upland vegetation types (desert woodland, mixed or arborescent desertscrub, shrubland). A stratified sampling design was developed to optimally allocate field effort across these gradients and vegetation types. Data were collected using standardized discovery surveys for elf owls which incorporate call-play back techniques, and by characterizing environmental attributes focused on vegetation structure and composition at each station at which owl surveys were performed regardless of whether or not an owl detection occurred.
Discovery surveys were performed along 112 transects that included 1,397 stations across southern and western Arizona, totaling 193 kilometers (km) of transect effort. A total of 855 elf owl detections were recorded during discovery surveys, which we estimate represented 553 unique individuals. Probability of elf owl occurrence increased markedly with presence of mature (>3 m tall and >20 cm dbh) saguaro cacti (Carnegiea gigantea). Occurrence probabilities at stations dominated by mesic and xeric riparian vegetation were much higher in areas with saguaros (mean ± SE = 0.43 ± 0.03 mesic; 0.42 ± 0.03 xeric) than without saguaros (0.13 ± 0.01 mesic; 0.11 ± 0.02 xeric), and lowest at stations dominated by exotic riparian vegetation without saguaros (0.00-0.04 ± 0.00-0.02). In the absence of any riparian vegetation, probability of elf owl occurrence was low regardless of whether saguaros were present (0.12-0.20 ± 0.04-0.18) or absent (0.00-0.09 ± 0.00-0.06), although sample sizes were low (n = 55). With regard to plant species composition in riparian areas, probability of elf owl occurrence increased with cover of broadleaf deciduous trees other than willow (Salix sp.) and with cover of mesquite (Prosopis spp.). By surveying a large number of riparian areas across southern and western Arizona, we identified potential study sites for more intensive efforts in future years of this project. Coupled with data to be gathered during the upcoming 2016-17 field seasons, this project will greatly augment the understanding of how elf owls use riparian vegetation along the sampled gradients.