Lark sparrow (Chondestes grammacus) nest-site selection and success in a mixed-grass prairie

The Auk 120(1):120-129, 2003

ABSTRACT.-Lark Sparrows (Chondestes grammacus) are declining throughout most of their range. Effective management for this species is hampered because relatively little is known about nesting ecology. We studied habitat characteristics affecting Lark Sparrow nest-site selection and nest success at nine study pastures in a southern mixed-grass prairie in Oklahoma. We used a neural-network technique to discriminate between nest and random locations, and bootstrapping with 95% confidence intervals to compare habitat features of successful and unsuccessful nests. We quantified habitat features at the nest and random points during the breeding seasons of 1999 and 2000 among three grazing treatments (control, moderate, and heavy). We located 40 nests during two years of the study, for which crude nest-success was 26.3%. Most nests were located in either moderately grazed pasture (55%) or heavily grazed pasture (40%). The neural model correctly identified nest and random points 91% of the time. Percentage of structural cover, distance to nearest structural element, bare-ground exposure, and percentage of litter cover were the most important nest-site selection criteria according to the model. Simulation analysis indicated points were classified as nest sites if they were 9% structural cover. Successful nests had less bare-ground exposure (... = 6.2 + or - 1.9% [SE]) and more litter cover (... = 18.0 + or - 4.6%) compared to unsuccessful nests (... = 17.5 + or - 3.8% and 10.1 + or - 1.6%, respectively). These results suggest that habitat management for Lark Sparrows in mixed-grass prairie should focus on creating abundant structural cover with moderate levels of litter accumulation and bare ground. Received 4 February 2002, accepted 12 September 2002.

LITTLE IS KNOWN about the breeding ecology of Lark Sparrows (Chondestes grammacus; Martin and Parrish 2000). Available information on Lark Sparrow nest-site selection is limited to descriptive studies conducted in the southern Great Plains (Newman 1970, Walcheck 1970, Renwald 1977, McNair 1984, Suedkamp 2000). Martin and Parrish (2000) reported that most nests were located on bare ground, sometimes in a small depression located at the base of woody plants. However, in some portions of their range, Lark Sparrows also nest in trees and shrubs (Martin and Parrish 2000). Further information regarding vegetation composition and structure of nest sites has not been well documented to date (Suedkamp 2000), nor has the relationship between nest-site location and nest success been evaluated.

Lark Sparrow populations are declining by 3% annually across their range in North America and have declined by 61.2% between 1966 and 1993 (Sauer et al. 1997). Declines are greatest in the eastern and central regions of the Breeding Bird Survey (Sauer et al. 1997, Martin and Parrish 2000). In Oklahoma and Texas, where breeding densities are among the highest in the United States, Lark Sparrows are declining at 3.5% per year and 4.2% per year, respectively (Sauer et al. 1997).

Nest placement will often influence the risk of nest predation (Ricklefs 1969, Martin 1993) and the microclimate experienced during incubation (With and Webb 1993, Gloutney and Clark 1997, Nelson and Martin 1999), thus affecting the ultimate fate of the nesting attempt. Therefore, the selection of a nest site is crucial to the fitness of the breeding pair, and natural selection should strongly influence the nest-site selection process.

Our objectives were twofold: first, we determined nest-site characteristics for Lark Sparrows and evaluated the effect of those characteristics on nesting success. Such knowledge will provide researchers with a better understanding of the breeding ecology of this species and might help elucidate potential causes of the species' decline. We also compared habitat features between successful and unsuccessful nests to determine how those habitat characteristics influenced Lark Sparrow nest success; second, we evaluated various hypotheses regarding nest-site selection behavior proposed to explain patterns of nest-site locations. To those ends, we employed a relatively new method of analysis called "artificial neural networks" (Smith 1996). We used that approach to compare features between nest sites and random points, and then used bootstrapping to investigate variables that were most important to the neural model in distinguishing successful from unsuccessful nests.

METHODS

We collected nest-site selection and success data at the Marvin Klemme Experimental Range Research Station (35[degrees]25'N, 99[degrees]05'W) in western Oklahoma, near Bessie in Washita County. The research station is located in the southern mixed-grass prairie physiographic region (Coupland 1992). Dominant vegetation on the site includes sidcoats grama (Bouteloua curtipendula), purple threeawn (Aristida purpurea), western ragweed (Ambrosia psilostacliya), common broomweed (Amphiachyris dracuncloides), smooth sumac (Rhus glabra), and broom snakeweed (Guiterrezia sarothrae).