Academic journal article The American Midland Naturalist

Territory Characteristics of Cassin's Sparrows in Northwestern Oklahoma

Academic journal article The American Midland Naturalist

Territory Characteristics of Cassin's Sparrows in Northwestern Oklahoma

Article excerpt

Introduction

Cassin's sparrows (Aimophila cassinii) have decreased at rates of 1.35% per y nationally and 2.30% per y in Oklahoma from 1966-2007 (Sauer et al., 2008). Dunning et al. (1999) suggested that Cassin's sparrows appear in high densities in suitable habitat, yet there is limited information regarding the breeding habitat of Cassin's sparrows. This is partially due to their extreme abundance fluctuations, lack of breeding site fidelity, and secretive migration patterns (Ruth, 2000). The lack of year to year site fidelity may be based on variable precipitation patterns in regions in which the species occurs (Andrews and Righter, 1992; Baumgartner and Baumgartner, 1992; and Gordon, 2000).

Cassin's sparrow breeding habitat consists of a shrub component which may include Artemesia sp., mesquite (Prosopis sp.), hackberry (Celtis spp.), rabbitbrush (Chrysothamnus sp.), and oak (Quercus spp.) (Ruth, 2000). Their numbers have generally been shown to be positively correlated to shrub cover and negatively correlated to bare ground (McLachlan, 2007; Rogers and Sexon, 1990; Doxon, 2005). Territory characteristics within shrublands are poorly documented for this species. Schnase (1984) found their mean territory size to be 2.6 ha in southern Texas (n = 21) and Ruth (2000) found ten territories per 40 ha plot in Arizona for an average of a territory every 4 ha.

Based on the need for more detailed data on the natural history of Cassin's sparrows, we evaluated physical and biotic properties of their territories in northwestern Oklahoma during 2008. We compared habitat characteristics within territories with characteristics at nonterritorial sites and at the larger landscape to investigate territory size and habitat use during the breeding season.

Study Area

We conducted our study on a private ranch located at latitude 36.392793° and longitude -99.674557° in Ellis County, Oklahoma. The ranch covers 4856 ha. This site receives 55.9 cm of precipitation annually with a mean annual temperature of 15.3 C (Tyrl et al., 2008). It consists of sand sagebrush (Artemisia filifolia) mixed-prairie with sand sagebrush, and sand plum (Prunus angustifolia) as dominant shrubs. Shrub composition is patchy and grasses such as little bluestem (Schizachyrium scoparium), side-oats grama (Bouteloua curtipendula), blue grama (B. gracilis), and buffalograss (Buchloe dactyloides) occur throughout. Soils consist primarily of the Lincoln, Eda and Eda-Tivoli complexes. Topography varies from flat to rolling (Tyrl et al., 2008). The area is moderately grazed annually by cattle.

Methods

From 5 Jun.-22Jun. 2008, we used territory mapping with song playback to estimate area of Cassin's sparrow breeding territories (Bibby et al, 2000; Budnik et al, 2000). This time period was chosen to avoid migrating birds, to ensure that birds detected were breeding within the study area. We used the Oklahoma Breeding Bird Atlas (Reinking, 2004) as a general reference of breeding chronology. We chose a 214 ha patch of sand sagebrushdominated rangeland because preliminary surveys revealed this area to contain dozens of breeding pairs. The survey period stopped once the entire area had been completely mapped and all Cassin's territories determined (i.e., 22Jun. 2008). Within the study area, we conducted a survey by walking parallel transects moving west to east 100 m apart such that the entire area was covered. We surveyed 50 m on both sides of the transect using a compass. We recorded the location of each Cassin's sparrow detected that exhibited territorial behavior such as song or display flights. We also used song playback (Stokes Field Guide to Bird Songs, Boston, MA) to elicit a response along the transect lines, using an iPOD Shuffle portable MP3 player (MP3; Apple, Cupertino, CA) connected to a Radio Shack 40-1441 portable folding amplified speaker system (speaker; Radio Shack, Fort Worth, TX) to play Cassin's sparrow songs. Once a bird was detected, we moved the speaker backwards and around the perimeter of a possible territory until the male would not move any further towards the playback song. …

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