Elf Owl home range and habitat study, 2015-2018: Comprehensive project report

The elf owl (Micrathene whitneyi), a covered species under the Lower Colorado River Multi-Species Conservation Program (LCR 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 2015–18, the Great Basin Bird Observatory and the University of Arizona (UA) completed three seasons of field work and associated analyses designed to provide the Bureau of Reclamation (Reclamation) with information characterizing elf owl occurrence, habitat use, and responsiveness to call broadcast surveys. Of specific interest was assessing elf owl occurrence and habitat use in areas dominated by riparian vegetation, determining environmental factors that are good predictors of elf owl occurrence, and developing a well-supported protocol for surveying for elf owl presence/ absence with known detectability that is suitable for the LCR MSCP planning area.

The goal of the first field season in 2015 was to characterize patterns of elf owl occurrence across a broad study area in Arizona, focusing on riparian vegetation. Data were collected using standardized call broadcast (i.e., “discovery”) surveys at survey stations arrayed along transects, and by characterizing environmental attributes at each survey station, regardless of whether or not an owl detection occurred. In contrast, the 2016 and 2017 field seasons focused on confirming and delineating elf owl nests and breeding territories, assessing habitat attributes of these nests and territories, and determining the detection probability of elf owls to call broadcast as a function of distance, survey conditions, and vegetation obstruction. Specific techniques used during the 2016–17 field seasons included passive listening surveys and radio telemetry to obtain a large number of estimated owl locations, nest emergence observations and nest cavity searches, rapid territory assessments, and standardized “response tests” to quantify detection probabilities.

Over 3 years of field work, the following was accomplished:
1) Discovery surveys were conducted on 121 unique transects. Along the unique transects, there were 918 detections of an estimated 590 unique individuals.
2) By obtaining 5,109 location estimates for elf owls via passive listening surveys and radio telemetry, 151 elf owl territories were confirmed and 148 assessed using 1 or more territory delineation methods. Of these, 89 territories were comprised mostly of riparian habitat, and an additional 9 territories were completely riparian.
3) Nest cavity locations were confirmed for 107 of these territories.

4) A total of 200 valid response tests were completed, which were combined with 55 response tests conducted during a previous elf owl research project on the LCR MSCP planning area in 2010–11.
These achievements significantly exceeded the quantitative goals set for this project by Reclamation, the Great Basin Bird Observatory, and the University of Arizona in the original project study plan.

Some key findings from this work include the following:
1) In the absence of riparian vegetation, elf owl occurrence probability was low.
2) Presence of saguaros (Carnegiea gigantea) was one of the best predictors of elf owl occurrence.
3) When saguaros were absent, elf owls were especially uncommon in mesic and exotic riparian woodlands and somewhat more common in xeric riparian woodland.
4) Territory size was relatively consistent across the study area (most territories were 0.5–1.5 hectares as determined by 95% minimum convex polygons), although there was a small number of much larger territories in the western part of the study area.
5) There was suggestive evidence that elf owls in the hotter, more westerly part of the study area that is most similar to the LCR MSCP planning area prefer more northerly facing nest cavities, presumably as a thermoregulatory behavior.
6) Most confirmed nest cavities were in saguaros, though nest cavities in trees were relatively common in the more easterly parts of the study area. However, nest cavity locations were not found for a significant minority of confirmed territories in the western part of the study area, and some of these may have occurred in trees.
7) If suitable cavities are present, and if suitable riparian areas are available, elf owls in the study area appeared to have relatively wide habitat tolerances.
8) Particularly in the western part of the study area, elf owls showed a strong “edge” orientation, with many territories overlapping the riparian/upland ecotone.

9) Detection probabilities to call broadcasts varied with distance, time of night, and season but were less sensitive to vegetation obstruction than expected. Accounting for this variability in defining a standardized survey approach is important.
This project generated a significant volume of information that can assist Reclamation in creating habitat suitable for elf owls. In addition, project results formed the basis for recommending a standardized survey protocol for elf owls in the LCR MSCP planning area along with other recommended protocols for confirming and characterizing elf owl nests, territories, and habitat use.