. The Canadian field-naturalist. 1996 BuFORD, Capen, and Williams: Fledgling Brood Density of Forest Birds 645. 0 5 10 15 20 25 30 35 40 45 50 55 Distance (m) Figure 1. Frequency distribution of perpendicular distances measured from 29 line transects to fledgling broods on Group I sites on the Green Mountain National Forest, Vermont, 1993 and 1994. (4 values > 60 m not shown.) assist selection of the best-fitting model, and com- putes density and variance estimates. Histograms of distance data were inspected for evidence of violat- ing statistical assumptions and for appropriate cut- points
. The Canadian field-naturalist. 1996 BuFORD, Capen, and Williams: Fledgling Brood Density of Forest Birds 645. 0 5 10 15 20 25 30 35 40 45 50 55 Distance (m) Figure 1. Frequency distribution of perpendicular distances measured from 29 line transects to fledgling broods on Group I sites on the Green Mountain National Forest, Vermont, 1993 and 1994. (4 values > 60 m not shown.) assist selection of the best-fitting model, and com- putes density and variance estimates. Histograms of distance data were inspected for evidence of violat- ing statistical assumptions and for appropriate cut- points for grouping data. Grouping of distances was an effective method for improving estimator robust- ness when assumptions may have been violated (Buckland et al. 1993). We also truncated outliers (-5% of the distances), which were not helpful in modeling the detection function and could have led to an overparameterized model. We then used pro- gram DISTANCE to fit and select the best of four models for the detection function. Distances were partitioned into groups, with the partition points adjusted as necessary to improve model fit. Data partitioning and truncation steps used were all stan- dard distance sampling procedures described in Buckland etal. (1993). The "half-normal model with hermite polynomi- als" was selected as the best fit to the detection func- tion for both Group I and Group II sites. Model selection was based on Akaike's Information Criterion (AIC), although chi-squared goodness-of- fit statistics were also considered. Buckland et al. (1993) preferred AIC over the chi-squared test for several reasons. Following model selection we used DISTANCE to compute brood density and variance estimates with a bootstrap technique. This analysis procedure was conducted separately for Group I and Group II sites, to test for differences in fledgling brood den- sity between the two groups. Results We detected 508 broods representing 38 species (Table 1). This total gr
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