Territorial Dynamics and Spatial Utilization of a Fragmented Habitat by Coastal Cactus Wrens (Campylorhynchus brunneicapillus)

http://www.ucgis.org/oregon/papers/solek.htm

ABSTRACT

This study focuses on a population of coastal Cactus Wrens (Campylorhynchus brunneicapillus) in eastern Los Angeles County, California and investigates the role of habitat in shaping territorial dynamics and behavior. I am investigating how Cactus Wren behavior is correlated with territory morphology (i.e. size, shape, and placement) and certain vegetation and landscape characteristics. I am also interested in describing interterritory variation in regards to territory morphology and the vegetation characteristics unique to each territory, determining if territorial configurations change over the course a year, and determining if some territory placements are preferable to others due to unique combinations of vegetation and landscape features. I am utilizing ArcView GIS as a tool to address my specific ecological questions about the species. ArcView has been used to create a spatial database from aerial orthophotographs of the study site. The ArcView Spatial Analyst extension will be applied in the data analyses as a means to interpret and quantify the effects of habitat edges and small-scale fragmentation on the spatial utilization and territorial behavior of this population of birds. Data analyses will include multivariate statistical approaches to quantify interterritory variation and correlation /regression techniques to examine relationships between territory morphology, vegetation structure, and behavior. Preliminary data indicate that Cactus Wren territories are highly variable in regards to morphology and vegetation composition and structure, but further research is necessary before any conclusions regarding the specific research objectives can be formulated.

BACKGROUND AND INTRODUCTION

The Cactus Wren, Campylorhynchus brunneicapillus, is the largest species of wren in North America. It is non-migratory and resident throughout its range, which includes most of the U.S. southwest and extends south to Baja California and central portions of mainland Mexico. Both a coastal and desert (interior) population of these birds exist in California (Rea and Weaver 1990, Harper and Salata 1991). Coastal populations, limited to the Pacific slope region of southern California and northern Baja California, are unique in that they are obligate inhabitants of Coastal Sage Scrub. This shrubland plant community is confined to the mediterranean-climate zone in North America and characterized by low, semi-woody vegetation (Desimone and Burk 1992). Coastal Cactus Wrens build their conspicuous, oblong nests exclusively in prickly pear and cholla cacti (Opuntia spp.), common components of this vegetative community in southern California.

Life history information on coastal populations of the Cactus Wren is limited. Long-term ecological studies of the species have been conducted on the more widely distributed desert populations, particularly in Arizona and New Mexico (Anderson and Anderson 1973, Marr and Raitt 1983), with little attention given to the coastal California populations. Coastal populations of the Cactus Wren have been severely impacted by development throughout southern California (Mock 1993, K. Garret, W. Wehtje, pers. comm.). Habitat loss, degradation, and fragmentation are the major issues affecting the viability of these populations. This decline is indicative of the substantial loss of the Coastal Sage Scrub plant community in southern California due to displacement from urbanization and grazing pressures (O'Leary 1995). While the species is currently not on any Federal or state lists, certain coastal populations have been proposed for federal threatened status in the past (Rea 1986). The coastal Cactus Wren is a California State Species of Special Concern and, in 1993, was selected as one of three target species in California's Natural Communities Conservation Program (NCCP) and a surrogate for conservation of the Coastal Sage Scrub plant community in southern California (Atwood 1998). Further investigations are merited due to the paucity of information on these unique and threatened populations.

The concept of territory and its role in a bird's life has been of interest to ecologists for some time (Howard 1920). In passerines, it has been most commonly described as a defended area with the primary function of insuring access to resources or mates (Nice 1941). Variation in territory utilization patterns and morphology, such as size, configuration, and placement, exists on both the interspecies and intraspecies level (Wiens 1972). The types of variation that exist among individual Cactus Wren territories and the influence of this variation on behavior have yet to be examined in the species. Through my research, I intend to quantitatively describe interterritory variation relative to territory morphology and the vegetation and landscape features using a Geographic Information System (GIS). I am also interested in determining if specific territory morphology and habitat characteristics are correlated with activity patterns, if the vegetation composition and structure, landscape features, and activity patterns are indicators of overall territory quality, and if territory size, configurations, and placements are dynamic throughout the course of a year.

STUDY SITE AND METHODS

The study site is located in the San Jose Hills at the eastern edge of the San Gabriel Valley in Los Angeles County, California. This project focuses on a discrete population of coastal Cactus Wrens inhabiting a 20 ha. parcel of undeveloped Coastal Sage Scrub habitat located on the campus of California State Polytechnic University, Pomona. The vegetation is composed mainly of low shrubs, including California sagebrush (Artemisia californica), California buckwheat (Eriogonum fasiculatum), and several species of sages (Salvia mellifera, S. leucophylla, and S. apiana). The common tree species are California Black Walnut (Juglans californica) and Elderberry (Sambucus mexicana). The habitat has been internally fragmented by a network of unpaved access roads and is bordered on most sides by agricultural fields, an interstate highway, and suburban development. While degraded, it still contains relatively contiguous areas of Coastal Sage Scrub vegetation and large stands of prickly pear cactus (Opuntia littoralis) that support a population of approximately 10-15 pairs of coastal Cactus Wrens.

A 1:3600 aerial photograph of the study area was electronically scanned at 600 dpi. This digital image was orthorectified using a Trimble TSC1 GPS and "heads-up" digitized in ArcView 3.2 GIS to create the spatial database This database will be the primary tool for describing and analyzing the major landscape and vegetation characteristics within and adjacent to each Cactus Wren territory. The vegetation and substrate types of interest include the tree and shrub species and the surface areas covered by of cactus, scrub, annual plants and grasses, and bare ground, respectively. From each territory, data will be collected on the tree/shrub species encountered, canopy cover of trees and shrubs, percent cover of each of the major vegetation and substrate types, vegetation height, mean slope and aspect, and locations of fence lines, snags, and posts. The spatial arrangement of territories in relation to the various anthropogenic features such roads, fence lines, agricultural fields, and structures will also be considered. This information will compose the major spatial layers and associated attribute data necessary for the statistical analyses. Classification of the vegetation and substrate types will be performed through the ArcView Image Analysis extension. Application of the ArcView Spatial Analyst extension, buffer operations, and proximity analysis will provide a means of interpreting and quantifying the effects of habitat edges and small-scale fragmentation on the territorial behavior of this population of birds. Habitat information unavailable from the aerial photo (e.g. vegetation height) will be gathered on site.

Efforts to color band this population of Cactus Wrens were initiated in 1994. To date, 65 birds have been marked with a unique combination of colored, split plastic leg bands. Adult and juvenile Cactus Wrens are captured using nylon Avinet mistnets and morphometric data gathered on each individual. Birds are banded and released in situ. Nestlings are banded from the nest if it is accessible with minimal disturbance to the integrity of the nest and the surrounding vegetation. As males and females are monomorphic, sex is determined by the presence of physical breeding characteristics (brood patch or cloacal protrubrance) or behavior based on subsequent field observations of the individual (Pyle 1987).

Ten focal Cactus Wren territories were delineated in March 2000. Territorial boundaries were determined through the use of recorded playbacks of Cactus Wren calls in order to elicit territorial responses (Falls 1986). The locations of any territorial response (vocalizations and behavioral displays) were plotted on field maps. By connecting the outermost points of the locations of these behaviors, a polygon of maximum size was formed to denote a territory. (Bibby et al 1992). Territorial delineations will be performed several times over the course of the study. Variation among the individual territories will be compared and analyzed using the appropriate multivariate statistical tests (e.g. MANOVA). Correlation analyses will be performed between territory size and the area covered by the major vegetation types of interest (trees, shrubs, cacti, and scrub), the frequency of occurrence of the major plant species, and the frequency of occurrence of the taller vegetation within each territory (e.g. trees/shrubs > 2m). Activity budgets of individuals on their territories will allow me to distinguish any spatial variation between territory boundaries and foraging areas. This data is being entered into the ArcView GIS spatial database and analyzed. The interface tools available in ArcView will assist in determining the size of individual territories, documenting changes in their conformation over the course of the study, and describing the vegetation structure and composition within each territory.

Color-banded territorial males are currently being observed once a week within their respective territories for 40-minute blocks using 10x40 binoculars and a 60x spotting scope. Activity budgets on each territory will be conducted throughout the course of the study during the morning (0600-1000 hrs.) to include the pre-nesting, nesting, and post-nesting periods. Activity budgets are conducted only on the territorial males. The type, duration, and location of the major activities performed such as foraging, perching, singing, display, defense, and behaviors related to nesting are recorded. This information will be entered into the GIS spatial database and be used to determine if activity patterns are affected by variables such as territory size, configuration, and placement and be analyzed using correlation analyses. To determine if foraging patterns are dependent on the coverage of the vegetation found within a territory, observed territory use, expressed as a percent of total foraging within an entire territory, will be regressed against the percent area covered by each of the major vegetation types of interest.

PRELIMINARY RESULTS AND DISCUSSION

As this is study ongoing, with the field portion of the project scheduled to be complete by October 2000, statistical results are not available. Preliminary data on the ten focal Cactus Wren territories indicate substantial variation among territories in regards to size (Figure 1) and vegetation composition (Figure 2). Territory sizes in this population range from 0.13 ha. to 0.8 ha (Figure 1), which is consistent with findings from other studies of the species (Anderson and Anderson 1973, Rea and Weaver 1990, Steintz 1997). The number of taller trees and shrubs found within each territory varies considerably, and may be a structural component of the territory affecting behavioral patterns. Territorial disputes and other interactions between non-affiliated individuals are rare in this population, and boundaries between adjacent territories appear to be rarely crossed. I expect territory configurations, activity patterns, and areas of utilization within territories to be dynamic throughout the course of the study, but continued fieldwork is necessary to support this.

I have found an inverse relationship between the amount of cactus cover and territory size, with smaller territories containing larger areas of cactus cover (Figure 3). One possible explanation for this is that increased cactus cover provides more protection for a foraging Cactus Wren. A bird foraging under the safety of cactus cover would be less susceptible to predators and able to forage more efficiently within a smaller area. Cactus Wrens holding territories containing fewer and/or smaller patches of cactus would be more exposed and need to be more vigilant while foraging. This factor may reduce the effectiveness of their foraging strategies and require that they forage over a larger area, thereby necessitating a larger territory (Morrison et al 1998). Alternatively, cactus cover may provide more favorable microhabitat conditions, such as cooler temperatures, that support a more abundant insect fauna. Cactus Wrens spend a major portion of their time foraging on the ground and within cactus for insects. If more cactus cover is available, a Cactus Wren could therefore afford to have a smaller territory since the foraging opportunities are more favorable in these types of areas. Time budget data is in the process of being analyzed to determine if there is a correlation between behavior and territory size, placement, and vegetation/landscape structure, which will lend further insight into these types of relationships.

Based on direct observation and banding records, male Cactus Wrens in this population exhibit a high degree of site fidelity, with some individuals holding and defending the same territory for more than three years. There appears to be a high turnover rate of females in this population, with male-female bonds fluid from one season to the next. While some juveniles have established territories in close proximity to their natal nests, most disappear from the area by the end of their first summer. In addition, unbanded individuals consistently appear in the population at the beginning of each breeding season, indicating that some dispersal and exchange with other populations is occurring.

ACKNOWLEDGEMENTS

This study is made possible through the support of the http://www.csupomona.edu/~cgisr/ Center for Geographic Information System Research(CGISR) at California State Polytechnic University, Pomona, with special thanks to Director Cheryl Hickam and Coordinator Keith Mann. I would also like to thank Biological Sciences Graduate Services for providing a graduate research grant, Dr. Sara Garver of the Department of Geography and Anthropology, and Dr. Lazlo Szijj and Dr. Gary Carlton of the Biological Sciences Department.

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