The foraging behavior of six species of insectivorous foliage-gleaning birds was studied in three different communities in Arizona to examine the role of resource partitioning in coexistence. I recorded data during winter and spring, the harshest seasons. Two to four species coexisted in each community. I recorded plant species, plant life-form, foraging height, plant portion, perch size, capture size, capture technique, and foraging rate. Foraging behavior was compared to the plant distribution profile, interspecifically among sympatric guild members within a season, intraspecifically between seasons, and intraspecifically between communities within a season. Species differed most in plant species selected, foraging height, and capture technique, and varied the least in plant portion, perch site, and capture site. All species overlapped in most behaviors, but differed from other guild members in at least two foraging variables. Permanent residents showed the greatest differences. These findings are consistent with niche complementarity and the idea that competition has led to partitioning of the resources. However, other hypotheses cannot be ruled out.

I used age structure to examine the role of fire disturbance and climate on the population dynamics of the subalpine forest in the southern Sierra Nevada. I cored trees on ten 0.1 ha plots (3300-3400 m elevation) that varied in species composition, from single-species foxtail pine (Pinus balfouriana) or lodgepole pine (Pinus contorta, var. murrayana), to mixed-species stands of both pines. Crossdating was used to produce accurate dates of tree recruitment and fire events. Age structure varied by plot species composition: lodgepole pine recruitment pattern is pulsed, sometimes forming single-cohort patches in response to fire; foxtail pine plots have a more steady pattern of recruitment; mixed-species plots show an intermediate recruitment pattern. Fire may maintain a species composition mosaic in the subalpine forest. Foxtail pine regeneration may increase in areas opened by fire, although not immediately following fire. Low-intensity fire may spread over areas larger than previously reported under certain conditions in the subalpine zone. In addition, unusually frequent, extreme, and/or extended periods of drought may severely limit subalpine tree regeneration. Growing season frost events and grazing before 1900 may also have affected trees establishing in the subalpine zone.

This study compared the effects of sampling metrics (basal area, density) on community structure analyses. Additionally, an oak woodland classification system was developed from the basal area data for the southwestern United States. Minimum-variance clustering was used to define communities within the oak woodlands. The communities defined by basal area data were more evenly distributed, showed more distinct communities, and were more homogeneous than the density-based communities. Thirteen communities were identified in the classification of oak woodlands, with mean basal area varying by an order of magnitude. Elevation had a major influence on the distribution of the communities.

I predicted the likely effects of fire on the abundance of grassland birds by determining which vegetal features influenced the distribution and abundance of grassland birds at the Buenos Aires National Wildlife Refuge, Arizona. Abundance of pyrrhuloxia (Cardinalis sinuatus) (r² = 0.363), Lucy's warbler (Vermivora luciae) (r² = 0.348), and total abundance of birds (r² = 0.358) was positively correlated with increasing density of mesquite (Prosopis velutina), whereas abundance of cactus wren (Campylorhynchus brunneicappillus) (r² = 0.452) was negatively correlated with increasing mesquite density. Abundance of loggerhead shrikes (Lanius ludovicianus) (r² = 0.693) was positively correlated with increasing environmental patchiness. Fire is likely to reduce the density of mesquite, thus I predict that those species positively correlated with mesquite density will decline following fire whereas species negatively correlated with mesquite density will benefit.

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