Geology topics
J.A. Sedgwick
Publications and source records attributed to J.A. Sedgwick.
Geographic variation in the song of Willow Flycatchers: Differentiation between Empidonax traillii adastus and E. t. extimus
The vocal signatures of the primary song form (“ fitz-bew ”) of the endangered Southwestern Willow Flycatcher ( Empidonax traillii extimus ) and its northern counterpart, E. t. adastus, are distinctive. Songs of the extimus subspecies are longer (total song, note, internote) and frequencies at maximum amplitude are lower than those of adastus. I used vocal evidence to clarify the distributional limits of the Southwestern Willow Flycatcher and that of the geographically adjacent subspecies, E. t. adastus. Unweighted pair-group method using averaging (UPGMA) cluster analysis and canonical discriminant analysis revealed that (1) low elevation, southerly desert populations (Arizona, New Mexico, and southern Utah) have a unique vocal identity corresponding to populations in the range of E. t. extimus ; (2) northerly song groups (Oregon, Colorado, and northern Utah) share a different song type corresponding to populations in the range of E. t. adastus; and (3) a departure from vocal and morphological congruence occurs for a population of high-elevation Arizona birds that, although in the currently accepted range of E. t. extimus, sings songs acoustically similar to more northern populations ( E. t. adastus ). Multiple regression of song distance on latitude and elevation, and a comparison of a matrix of song distances with a matrix of latitude and elevation dissimilarities, demonstrated that song populations sort out by both latitude and elevation: birds with the vocal identity of extimus occur as far north as 37°N if at low elevation, and those acoustically similar to adastus occur as far south as 33.7°N if at high elevation. The vocal background of northern New Mexico birds appears to be intermediate between that of extimus and adastus, suggesting that northern New Mexico is a zone of intermixing and intergradation between the subspecies. Pure forms of E. t. extimus apparently do not occur in Colorado because even the southernmost populations are acoustically similar to more northerly populations of adastus. A low-elevation population in western Colorado, however, stands apart from other adastus populations, suggesting moderate introgression of extimus genes into the adastus gene pool.
A spatial and genetic analysis of Cowbird host selection
Our study of brood parasitism patterns in forest communities revealed the egg-laying frequency and host selection patterns of female cowbirds. By integrating molecular genetics and spatial data, we have the first published estimate on cowbird laying rates in field studies. The 29 females in the study laid only 1-5 eggs each, much lower than previous estimates from captive cowbirds and extrapolations from ovarian development in capture/recapture studies that had suggested that as many as 40 eggs could be laid per individual cowbird. Cowbird females also were shown for the first time to lay significantly more eggs within the home range areas they established rather than outside the home range. No patterns were uncovered for individual females preferentially parasitizing particular host species
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 mused in 1986; 38.5% of 122 cavities occupied in 1986 were mused 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.
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.