Geology topics

R.A. Stehn

Publications and source records attributed to R.A. Stehn.

7 recordsLinked to original sources

USGS2004

Glaucous gull predation of goslings on the Yukon-Kuskokwim Delta, Alaska

Glaucous Gulls ( Larus hyperboreus ) nesting on the Yukon-Kuskokwim (Y-K) Delta frequently prey on juvenile waterfowl. We collected 434 Glaucous Gulls from late June to early August 1994 to examine diet. Identification of undigested prey tissue, based on DNA microsatellite loci, showed three species of goslings in gull stomachs: Emperor Goose ( Chen canagica ), White-fronted Goose (Anser albifrons), and Cackling Canada Goose ( Branta canadensis minima ). Gulls that nested inland and were collected > 1.6 km from the coast accounted for approximately 70% of the total gull predation on Emperor and Canada Geese, and 96% on White-fronted Geese. Our stratified sample of gull stomachs and aerial survey estimates of population size and distribution of gulls and juvenile geese enabled extrapolation of species-specific predation rates to the entire Y-K Delta. We estimated that a minimum of 21 000 Emperor Goose, 34 000 Canada Goose, and 16 000 White-fronted Goose goslings were consumed by 12 600 Glaucous Gulls during the brood-rearing period on the Y-K Delta in 1994. Minimum estimated take by gulls represented 33% of Cackling Canada Goose, 47% of Emperor Goose, and 39% of White-fronted Goose eggs estimated to have hatched in the same area as gull collections. Gulls selected the three species of geese approximately in proportion to their abundance. Although gull predation caused significant gosling mortality, its role in regulating goose populations on Y-K Delta remains unresolved.

Condor↗

USGS1999

Response of fall-staging brant and Canada geese to aircraft overflights in southwestern Alaska

Because much of the information concerning disturbance of waterfowl by aircraft is anecdotal, we examined behavioral responses of Pacific brant (Branta bernicla nigricans) and Canada geese (B. canadensis taverneri) to experimental overflights during fall staging at Izembek Lagoon, Alaska. These data were used to develop predictive models of brant and Canada goose response to aircraft altitude, type, noise, and lateral distance from flocks. Overall, 75% of brant flocks and 9% of Canada goose flocks flew in response to overflights. Mean flight and alert responses of both species were greater for rotary-wing than for fixed-wing aircraft and for high-noise than for low-noise aircraft. Increased lateral distance between an aircraft and a flock was the most consistent predictive parameter associated with lower probability of a response by geese. Altitude was a less reliable predictor because of interaction effects with aircraft type and noise. Although mean response of brant and Canada geese generally was inversely proportional to aircraft altitude, greatest response occurred at intermediate (305-760 m) altitudes. At Izembek Lagoon and other areas where there are large concentrations of waterfowl, managers should consider lateral distance from the birds as the primary criterion for establishing local flight restrictions, especially for helicopters.

Journal of Wildlife Management↗

USGS1997

Aerial videography for estimating goose populations staging in Izembek Lagoon, Alaska

We conducted experimental photographic surveys of black brant and Canada geese in a pre-migration staging area on the Alaska Peninsula to compare precision and efficiency of this technique to currently employed ocular estimates. Video and digital cameras sensitive to near infrared and visible radiation were tested at various altitudes and with lenses of different focal length. Using information from these tests, we constructed an artificial population of geese to determine sampling effort and efficiency of the technique.

Conference Paper↗

USGS1996

A bayesian approach to classification criteria for spectacled eiders

To facilitate decisions to classify species according to risk of extinction, we used Bayesian methods to analyze trend data for the Spectacled Eider, an arctic sea duck. Trend data from three independent surveys of the Yukon-Kuskokwim Delta were analyzed individually and in combination to yield posterior distributions for population growth rates. We used classification criteria developed by the recovery team for Spectacled Eiders that seek to equalize errors of under- or overprotecting the species. We conducted both a Bayesian decision analysis and a frequentist (classical statistical inference) decision analysis. Bayesian decision analyses are computationally easier, yield basically the same results, and yield results that are easier to explain to nonscientists. With the exception of the aerial survey analysis of the 10 most recent years, both Bayesian and frequentist methods indicated that an endangered classification is warranted. The discrepancy between surveys warrants further research. Although the trend data are abundance indices, we used a preliminary estimate of absolute abundance to demonstrate how to calculate extinction distributions using the joint probability distributions for population growth rate and variance in growth rate generated by the Bayesian analysis. Recent apparent increases in abundance highlight the need for models that apply to declining and then recovering species.

Alaska↗

USGS1995

GIS for mapping waterfowl density and distribution from aerial surveys

We modified standard aerial survey data collection to obtain the geographic location for each waterfowl observation on surveys in Alaska during 1987-1993. Using transect navigation with CPS (global positioning system), data recording on continuously running tapes, and a computer data input program, we located observations with an average deviation along transects of 214 m. The method provided flexibility in survey design and data analysis. Although developed for geese nesting near the coast of the Yukon-Kuskokwim Delta, the methods are widely applicable and were used on other waterfowl surveys in Alaska to map distribution and relative abundance of waterfowl. Accurate location data with GIS analysis and display may improve precision and usefulness of data from any aerial transect survey.

Wildlife Society Bulletin↗

USGS1995

Locating waterfowl observations on aerial surveys

Wildlife Society Bulletin↗

USGS1994

Navigating aerial transects with a laptop computer

SUMMARY: A comparison is made of different methods of determining size of home range from grid trapping data. Studies of artificial populations show that a boundary strip method of measuring area and an adjusted range length give sizes closer to the true range than do minimum area or observed range length methods. In simulated trapping of artificial populations, the known range size increases with successive captures until a level is reached that approximates the true range. The same general pattern is followed whether traps are visited at random or traps nearer the center of the range are favored; but when central traps are favored the curve levels more slowly. Range size is revealed with fewer captures when traps are far apart than when they are close together. The curve levels more slowly for oblong ranges than for circular ranges of the same area. Fewer captures are required to determine range length than to determine range area. Other examples of simulated trapping in artificial populations are used to provide measurements of distances from the center of activity and distances between successive captures. These are compared with similar measurements taken from Peromyscus trapping data. The similarity of range sizes found in certain field comparisons of area trapping, colored scat collections, and trailing is cited. A comparison of home range data obtained by area trapping and nest box studies is discussed. It is shown that when traps are set too far apart to include two or more in the range of each animal, calculation of average range size gives biased results. The smaller ranges are not expressed and cannot be included in the averages. The result is that range estimates are smaller at closer spacings and greater at wider spacings, purely as a result of these erroneous calculations and not reflecting any varying behavior of the animals. The problem of variation in apparent home range with variation in trap spacing is considered further by trapping in an artificial population. It is found that trap spacing can alter the apparent size of range even when biological factors are excluded and trap visiting is random. The desirability of excluding travels outside the normal range from home range calculations is discussed. Effects of varying the trapping plan by setting alternate rows of traps, or setting two traps per site, are discussed briefly.

Wildlife Society Bulletin↗