James A. Sedgwick

Guild structure of a riparian avifauna relative to seasonal cattle grazing

The avifauna within the willow (Salix spp.) community on the Arapaho National Wildlife Refuge (NWR) was dominated (96% of all observations each year) by 11 species of passerine birds during the summers of 1980-81. Using 28 vegetation variables measured or calculated for randomly selected points and points where birds were sighted, we assigned the species to 3 distinct response guilds relative to historical patterns of seasonal grazing. A eurytopic response guild (habitat generalists) included yellow warblers (Dendroica petechia) (YEWA), savannah sparrows (Passerculus sandwichensis) (SASP), and song sparrows (Melospiza melodia) (SOSP). A stenotopic response guild (habitat specialists) included willow flycatchers (Empidonax traillii) (WIFL). Lincolns sparrows (Melospiza lincolnii) (LISP), and white-crowned sparrows (Zonotrichia leucophrys) (WCSP). The intermediate, mesotopic response guild included American robins (Turdus migratorius) (AMRO), red-winged blackbirds (Agelaius phoeniceus) (RWBL), and brown-headed cowbirds (Molothrus ater) (BHCO). Population densities of the eurytopic response guild differed little between healthy (historically winter-grazed) and decadent (historically summer-grazed) willow communities within a year. Densities of species in the mesotopic response guild differed more dramatically, and stenotopic responseguild species were absent or accidental in decadent willows. Information on habitat use patterns of the individual species between years supported the definition of response guilds; vegetation structure was most variable in habitats of eurytopic species and least variable in habitats of stenotopic species. Comparisons between used and available vegetation features indicated that species in the stenotopic response guild used locations that differed from random on the basis of bush spacing. We hypothesize that the response-guild structure primarily reflects the impact of cattle upon the horizontal patterning of the vegetative community. J. WILDL. MANAGE. 52(2):280-290 Grazing by domestic cattle is the most common historical use of native rangelands in the western United States (Busby 1979). Ecologists have described the impacts of grazing upon breeding bird communities by comparing avian populations on adjacent sites that have been subjected to different levels of grazing (Kantrud 1981, Mosconi and Hutto 1982), on the same site before and after exclusion of grazing (Duff 1979), or both (Bock et al. 1984). Those studies generally have described patterns of species presence or abundance relative to grazing impacts upon vegetation. Birds generally respond to changes in vegetation structure as a consequence of grazing rather than to the presence of cattle per se (Bock and Webb 1984). However, avian responses to vegetative changes are neither easily defined nor consistent among sites (Wiens and Dyer 1975). Ryder (1980) reviewed studies of grazing impacts upon avian communities and categorized species as either increasers or decreasers (in abundance) with the introduction of cattle into their habitats. Studies of grazing impacts, however, generally have not elaborated on the nature of avian behavior or population responses, especially relative to all bird species within a community. Previous studies in northern Colorado have shown that seasonal grazing practices over a 75100-year period may alter the horizontal and vertical structure of a shrub-willow community as potential habitats for birds (Knopf and Cannon 1982). Our objectives were to define avian guilds within the shrub-willow community which respond similarly to structural vegetation parameters, statistically test the validity of the response-guild structure with information on species densities and habitat use, and identify vegetation parameters that cattle alter that appear important to sensitive bird species. We thank E. C. Patten for his interest and local support. Field assistance was provided by B. Bergstrom, K. Bergstrom, J. F. Ellis, E. A.

Habitat relationships and nest site characteristics of cavity-nesting birds in cottonwood floodplains

We examined habitat relationships and nest site characteristics for 6 species of cavity-nesting birds--American kestrel (Falco sparverius), northern flicker (Colaptes auratus), red-headed woodpecker (Melanerpes erythrocephalus), black-capped chickadee (Parus atricapillus), house wren (Troglodytes aedon), and European starling (Sturnus vulgaris)--in a mature plains cottonwood (Populus sargentii) bottomland along the South Platte River in northeastern Colorado in 1985 and 1986. We examined characteristics of cavities, nest trees, and the habitat surrounding nest trees. Density of large trees (>69 cm dbh), total length of dead limbs >10 cm diameter (TDLL), and cavity density were the most important habitat variables; dead limb length (DLL), dbh, and species were the most important tree variables; and cavity height, cavity entrance diameter, and substrate condition at the cavity (live vs. dead) were the most important cavity variables in segregating cavity nesters along habitat, tree, and cavity dimensions, respectively. Random sites differed most from cavity-nesting bird sites on the basis of dbh, DLL, limb tree density (trees with >1 m dead limbs >10 cm diameter), and cavity density. Habitats of red-headed woodpeckers and American kestrels were the most unique, differing most from random sites. Based on current trends in cottonwood demography, densities of cavity-nesting birds will probably decline gradually along the South Platte River, paralleling a decline in DLL, limb tree density, snag density, and the concurrent lack of cottonwood regeneration. J. WILDL. MANAGE. 54(1):112-124 Cavity-nesting birds are a major component of many avian communities. In cottonwood bottomlands they represent 32-43% of the breeding avifauna (Sedgwick and Knopf 1986). In studies of habitat associations of cavity-nesting birds, overstory basal area, understory vegetation, tree dbh and height, and species composition all have been related to densities of cavity nesters (e.g., Anderson and Shugart 1974, McClelland et al. 1979, Raphael and White 1984). The importance of snags (dead trees) to cavity-nesting birds has been especially well documented (Haapanen 1965, Thomas et al. 1979, Scott et al. 1980, Raphael and White 1984). Both the dead and living portions of live trees provide nest substrates for cavity-nesting birds in some forest associations (Carey 1983, Sedgwick and Knopf 1986). Sedgwick and Knopf (1986) concluded that snag densities were not good predictors of breeding habitats for cavitynesting birds in cottonwood bottomlands. Alternatively, limb tree (trees with 21 m dead limbs >10 cm diameter) density and dead limb length (DLL) in live trees, both of which were correlated with cavity density, were better measures of suitable nesting habitat for cavity nest-

Cavity turnover and equilibrium cavity densities in a cottonwood bottomland

A fundamental factor regulating the numbers of secondary cavity nesting (SCN) birds is the number of extant cavities available for nesting. The number of available cavities may be thought of as being in an approximate equilibrium maintained by a very rough balance between recruitment and loss of cavities. Based on estimates of cavity recruitment and loss, we ascertained equilibrium cavity densities in a mature plains cottonwood (Populus sargentii) bottomland along the South Platte River in northeastern Colorado. Annual cavity recruitment, derived from density estimates of primary cavity nesting (PCN) birds and cavity excavation rates, was estimated to be 71-86 new cavities excavated/100 ha

A high incidence of brown-headed cowbird parasitism of willow flycatchers

Brown-headed Cowbirds (Molothrus ater) parasitize both Willow (Empidonax traillii) and Alder (E. alnorum) flycatchers (Friedmann et al. 1977, Friedmann and Kiff 1985). These two flycatchers were considered a single species until 1973 (AOU 1973), which has masked information about the frequency with which each is parasitized. Whereas several studies of the superspecies (Traills Flycatcher) have focused on or included details of cowbird parasitism, most were of eastern populations, and most reported frequencies of

Sequential cavity use in a cottonwood bottomland

I studied the patterns and frequency of cavity reuse in a community of cavity-nesting birds in a cottonwood bottomland along the South Platte River in northeastern Colorado from 1985-1987. Of 100 cavities occupied in 1985, 56% were reused in 1986; 38.5% of 122 cavities occupied in 1986 were reused in 1987. Of 81 old cavities monitored in both 1986 and 1987, 65.4% were reused at least once. Similar proportions of secondary cavity-nesting bird (SCNB) and primary cavity-nesting bird (PCNB) cavities were reused in both years. Reoccupancy by the same species was 27% and 20.5% in 1986 and 1987, respectively, and was greater for SCNB than for PCNB cavities in both years. Conversely, reoccupancy by different species was greater for PCNB than for SCNB cavities in both years. Thus, old cavities of PCNB were more available to other species of cavity-nesting birds, whereas old SCNB cavities tended to be reused by the same species that previously occupied the cavity. SCNB used a greater proportion of old cavities than did PCNB in both 1986 and 1987. House Wrens (Troglodytes aedon) and Northern Flickers (Colaptes auratus) reoccupied most of the old cavities.

Regional Correspondence among Shrubsteppe Bird Habitats

Habitats of Brewer’s Sparrows (Spizella breweri) and Green-tailed Towhees (Pipilo chlorurus) were compared (as biological replicates) in Colorado, Nevada, and Utah to identify regional commonalities in vegetation species, structure, and vigor among habitats of shrubsteppe passerines. Sparrow habitats were in landscapes dominated by sagebrush (Artemisia spp.) whereas towhee habitats were better described as ecotones between sagebrush and other shrub species. Individuals of both species selected bushes of comparable vegetative vigor across the three states even though species populations were allopatric in two of the states. For each species, measures of vegetation structure and vigor were compared between breeding habitats in Colorado and Nevada (near the longitudinal extremes of the continental distribution of each species at 42”N latitude) in 1983. Sparrow habitats in the two states were similar in four measures of vigor but differed in all structural variables measured. Towhee habitats were similar between Colorado and Nevada for three measures of vigor and three measures of structure. Independent tests of the Colorado/Nevada conclusions were conducted near the center of the species’ ranges (Utah) in the subsequent breeding season (1984). Those tests confirmed the validity of all measures of vegetation vigor as descriptors of sparrow habitat and one measure of vigor plus two of vegetation structure as descriptors of towhee habitat. Thus, sparrow habitats were best characterized regionally by measures of sagebrush vigor, whereas towhee habitats were characterized by shrub vigor and structure. Our tests and conclusions generally support recent, correlational studies that identify vegetative physiognomy as the primary descriptor of passerine habitats at a regional scale and floristic composition as the primary descriptor at a local scale. We add that individuals of these two species selected habitats from a mosaic of patches of varying vegetation vigor (supplemented by structural information for towhees) within a locale. Measures of vegetation vigor are rarely used to described passerine habitats, although vigor surely correlates better with food abundance and cover for nests than vegetation structure. These observations suggest an hierarchical approach for viewing habitat selection by shrubsteppe birds.

Prescribed grazing as a secondary impact in a western riparian floodplain.

The effect of late-autumn cattle grazing on plant biomass was examined in a western Great Plains cottonwood riparian zone prone to catastrophic flooding every 5-8 years. Following 1 year of pre-treatment data collection in 1982, five 16-ha pastures were grazed from 1982 to 1984 and compared to 5 control pastures within the South Platte River floodplain in northeastern Colorado. At a prescribed grazing level of 0.46 ha/AUM, riparian vegetation proved to be resilient to the impacts of grazing. We detected only a few significant treatment effects for above-ground biomass after succeeding growing seasons. Willows (Salix spp.) responded negatively to grazing whereas biomass of prairie cordgrass (Spartina pectinata Link) was greater on grazed plots. Yearly changes in above-ground biomass, especially dramatic following a severe flood in 1983, suggest that periodic, catastrophic flooding is a major perturbation to the ecosystem, and in conjunction with our results on grazing impacts, indicate that dormant-season grazing within Soil Conservation Service (SCS) guidelines is a comparatively minor impact within the floodplain. In addition, grazing impacts were probably further mitigated by a major forage supplement of cottonwood leaves which was available at the time of cattle introductions. This local forage supplement ultimately created a lighter grazing treatment than that originally prescribed.

Latent population responses of summer birds to a catastrophic, climatological event

Catastrophic events can create ecological crunches for avian populations. Ornithologists generally assume that such events elicit immediate, dramatic declines in populations followed by rapid recolonization of a site with habitat recovery. Despite total inundation of habitats within the South Platte floodplain of eastern Colorado during the 1983 breeding season, populations of Brown Thrashers (Toxostoma rufum) and Rufoussided Towhees (Pipilo erythrophthalmus) did not decline significantly from 1982 densities. However, populations of both species declined significantly in 1984 and towhee populations remained low in 1985. These observations support speculations from recent studies of shrubsteppe bird populations that site tenacity may play a stronger role in determining annual densities of breeding birds than previously realized.