Influence of Fragmentation on Shrubsteppe-Obligate Passerines

Publication Date

3-2003

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Biology

Department

Biology

Supervisory Committee Chair

Steven T. Knick

Abstract

I examined reproductive success and post-fledging dispersal of shrubsteppe-obligate passerines to determine if either mechanism might contribute to the reduced abundance previously reported for these species in fragmented habitat. During 1999 and 2000, I monitored nests of sage thrashers (Oreoscoptes montanus), sage sparrows (Amphispiza belli), and Brewer's sparrows (Spizella breweri) in fragmented, edge, and contiguous shrub steppe habitat in southwestern Idaho. The combination of nest success and relative abundance among sage thrashers and Brewer's sparrows was consistent with trends I would expect if contiguous habitat functioned as a source and fragmented habitat as a sink (only fragmented and contiguous treatments were consistently compared among all species). Nest success of these two species was significantly less in fragmented habitat, whereas relative abundance between treatments was similar. Conversely, nest success and relative abundance of sage sparrows were similar between treatments. I fit logistic regression models of nest success for sage thrashers and Brewer's sparrows using both local and landscape scale predictor variables. Heterogeneity of shrub habitat (Angular Second Moment) was included in models for both species (but at different scales) and was used to create maps predicting the relative probability of nest success for each species in my study area.

I examined post-fledging dispersal of sage thrashers from contiguous and edge habitats only, as no birds fledged in fragmented habitat. Twenty individuals from 10 nests (1999, n = 7; 2000, n = 3) were monitored up to 44 days. To evaluate habitat associations of juveniles as they dispersed, principal components analysis (PCA) was applied to environmental measures around nests and dispersal locations. No correlation was found between habitat occupied by juveniles and time during the first 30 days away from the nest and habitat variability increased with time. Furthermore, dispersal distances varied greatly among juveniles. Lack of habitat affinity suggests that post-fledging dispersal might not be responsible for lower numbers of shrubsteppe-obligate birds in fragmented habitat.

My results suggest that dissimilar reproductive success between fragmented and contiguous habitat could contribute to reduced numbers of shrubsteppe-obligate birds in fragmented habitat. I also found no evidence that post-fledging dispersal might function to decrease bird densities in fragmented habitat and it may actually facilitate an exchange of individuals between habitats when they are adjacent. This potential source - sink scenario could lower the population equilibrium. Therefore, I recommend that managers should concentrate on preserving habitat conducive to successful reproduction. This might include identifying larger landscape configurations, via Geographic Information System (GIS) applications, that are correlated with high nest success (e.g. in my study, low shrub heterogeneity and high shrub composition was positively related to the nest success of sage thrashers and Brewer's sparrows). Subsequently, variables at smaller scales should be evaluated. Until we determine the relationship between natal and breeding habitats of shrubsteppe-obligate passerines, protecting habitats that correlate with high nest success likely is our most effective tool to minimize population declines.

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