Avian Resource Partitioning Along a Montane Sere
Avian vegetation-use and foraging behavior were studied along a spruce-fir sere in the Bear River Mountains of northern Utah and southern Idaho from 1976 to 1978 on four 10 ha plots--one each in meadow, aspen, fir and spruce. A total of 71 species were encountered, of which 43 bred in at least one of three breeding seasons.
Avian vegetation-use was quantified on the 3 forested plots by sampling a total of 197 (0.05 ha) plots around singing male birds of 15 species. Also, 75 random plots (25 in each seral stage) were sampled to characterize the available vegetation. Principal component analysis (PCA) on all avian samples showed that, based on vegetationuse, avian communities in the seral stages were distinct, although some bird species showed affinities for vegetation within one seral stage that was characteristic of another seral stage. White-crowned Sparrow and Yellow-bellied Sapsucker showed an affinity for fir in aspen stands; Oregon Junco and Ruby-crowned Kinglet used aspen and spruce, respectively, in fir stands; Chipping Sparrow and Western criminant function analysis (DFA) of the avian vegetation-use data confirmed the PCA relationships.
PCA of avian vegetation-use data within each seral stage showed that in aspen, differences between broad-leaf and coniferous vegetation explained the distribution of several species. Relatively little variation existed in the vegetation physiognomy of the aspen seral stage. In fir and spruce, subtle differences in coniferous vegetation explained the distribution of most species.
PCA and DFA of the random vegetation samples were similar to the analyses of the avian vegetation-use data when comparing aspen to fir and spruce, suggesting that gross morphological differences in vegetation are sufficient to separate avian communities when vegetational characteristics of a particular seral stage are distinct. However, the vegetation-use pattern of the bird communities in fir and spruce stands overlapped to a greater degree than did the random vegetation samples in fir and spruce, suggesting that bird communities in these two seral stages are partitioning the use of vegetation on a coarser scale than that defined merely by changes in vegetation due to succession.
Foraging behavior data were collected on 3 species that occurred in aspen, fir, and spruce; 3 species that occurred in fir and spruce; and 5 species that occur in fir and spruce, but for which a large sample of foraging behavior was obtained only in fir. PCA of the foraging behavior data determined that; 1) each species studied tended to forage in a similar manner in different seral stages, 2) different foraging types, e.g., flycatchers, foliage gleaners, exhibited distinct foraging patterns, and 3) most species did not change vegetation types in different seral stages.
Colwell and Futuyma niche breadth and overlap measures were calculated for the foraging behavior data, and generally confirmed that foraging technique was the most conservative of the foraging classes studied. Foraging niche breadths and overlaps were relatively high, suggesting a reduction of competition between species. Ricklefs and Cox's (1977) index of morphological similarity confirmed that most of the foliage-gleaning species were quite similar morphologically. This occurrence of many similar species may be due to the structure of western conifers, or possibly species are keying on temporally superabundant food resources.
It is concluded that it is the distinctiveness of the vegetation within a seral stage that will determine the population response of avian species along a sere. Possible ways in which vegetation physiognomy of each seral stage may affect avian community structure are discussed. Foraging behavior is more conservative than vegetationuse and tends to be independent of seral stage for most species.
Smith, Kimberly G. (1981). Avian resource partitioning along a montane sere. PhD. 124p.