I. Lehr Brisbin

Consequences of sexual dimorphism in sand fiddler crabs,Uca pugilator: differential vulnerability to avian predation

The predatory behaviour of seven captive white ibises,Eudocimus albus, on sand fiddler crabs,Uca pugilator, was studied to assess the degree to which the enlarged claw of male crabs affects the relative vulnerabilities of male versus female crabs. When given a simultaneous choice between a female and male crab or between a male crab with its major claw removed and an intact male, all seven ibises showed a preference for the animal without the major claw. Overall, when offered crabs on an exposed surface, ibises chose female and declawed male crabs four times as often as intact male crabs. When probing into artificial burrows containing either a male or female crab, all seven ibises rejected male crabs almost twice as often as they did female crabs. During encounters both with crabs on the surface and with crabs in burrows, ibises appeared to be especially wary of the enlarged claw of male crabs. These data suggest that sexual dimorphism in sand fiddler crabs substantially affects the relative vulnerabilities of male and female crabs to avian predators.

Factors affecting the analysis of growth patterns of large mammals

Growth of large mammals under natural conditions often is studied by regressing the mass of collected individuals against their estimated ages; however, this practice might lead to biased estimates of growth. Monte Carlo simulations were used to assess effects of nonuniform sample distribution and of imprecise and erroneous age determination on estimates of growth for male white-tailed deer ( Odocoileus virginianus ) using the Richards sigmoid growth equation. Effects of errors in age determination and sample distribution also were evaluated over a range of values of growth parameters, and biases were compared between estimates obtained from Richards, Gompertz, and von Bertalanffy equations. Variation in birth dates of individuals tended to increase the variance, but not the bias of growth estimates. Assigning individuals to the wrong year class resulted in biased estimates of all three growth parameters, with the shape parameter m being affected most. This bias was most pronounced when the sampling distribution was nonuniform and was not consistent for growth curves of different shapes. It appears that little confidence should be placed in estimates of m from data that may contain animals assigned incorrect ages. Asymptotic mass ( W ∞) and growth period ( T ) were relatively insensitive to this effect; however, a nonuniform sample distribution increased the bias of these estimates. For our data, estimates of W ∞ and T obtained from the Richards equation were less biased than estimates obtained from Gompertz or von Bertalanffy equations.

FLIGHT BEHAVIOR OF BLACK AND TURKEY VULTURES: IMPLICATIONS FOR REDUCING BIRD–AIRCRAFT COLLISIONS

Abstract Mid-air collisions with black vultures (Coragyps atratus) and turkey vultures (Cathartes aura) regularly cause substantial damage to military and civilian aircraft. Information concerning the flight behavior of black and turkey vultures potentially could improve predictive models designed to reduce bird strikes by aircraft. We examined the flight behavior of black and turkey vultures at the Savannah River Site (SRS) in South Carolina, USA, and determined whether flight characteristics were predictable with respect to weather and time variables. We captured birds at their primary roost and subsequently relocated them via aerial telemetry from 11 February 2002 through 29 January 2003. One hundred eighty of 326 locations (55%) for 8 black vultures and 129 of 206 locations (63%) for 5 turkey vultures were of flying birds. Black vultures flew at an average altitude of 169 ± 115 (SD) m above ground level, whereas turkey vulture flight altitude averaged 163 ± 92 m. Our results contrast with those of previous studies that reported less frequent and lower altitude flights. The flight behavior of both species appeared to be influenced minimally by weather and time variables. However, we were unable to construct useful models predicting aspects of flight behavior using the variables we measured (all models had R2 or pseudo R2 values <0.10). We suggest that other factors, such as food availability, inter- and intra-specific interactions, and physiological demands play a larger role in vulture flight behavior than the variables we measured. Our results suggest that the development of bird avoidance strategies by aircraft operators should consider the variability of flight behaviors of black and turkey vultures across their ranges. Future research emphases should shift from examinations of the effects of local conditions on flight behavior to the elucidation of factors contributing to differences in flight behavior among regions.

A Determination of Live-Weight Caloric Conversion Factors for Laboratory Mice

Regression equations are described which allow the calculation of the calories of energy consumed by a carnivore fed on whole laboratory mice (Mus musculus) given the individual weights of the mice eaten. The live—weight caloric density of mice weighing 4.8 g or less may be determined by the linear relationship: kcal/g live weight = 0.657 + 0.306 (g live body weight). Mice weighing more than 4.8 g may be considered to have a uniform live—weight caloric density of 2.14 ° 0.08 (SE) kcal/g. These conversion factors may be used for either frozen or freshly killed laboratory mice or laboratory raised Peromyscus polionotus. Fat indices (g fat/g lean dry weight) were not correlated with body weight. However, water indices (g water/g lean dry weight) decreased exponentially with increasing body weight. There was no change in the caloric value of the lean dry weight of the mice with increasing body weight. See full-text article at JSTOR

A Spatially Explicit Model of the Wild Hog for Ecological Risk Assessment Activities at the Department of Energy's Savannah River Site

Abstract In North America, wild hogs (Sus scrofa) are both sought after as prime game and despised due to their detrimental impacts to the environment from their digging and rooting behavior. They are also a potentially useful indicator species for environmental health for both ecological- and human-based risk assessments. An inductive approach was used to develop probabilistic resource selection models using logistic regression to quantify the likelihood of hogs being in any area of the Department of Energys 805 km2 Savannah River Site (SRS) in west-central South Carolina. These models were derived by using available SRS hog hunt data from 1993–2000 and a Geographic Information System database describing the habitat structure of the SRS. The models significant parameters indicated that wild hogs preferred hardwoods and avoided pine and shrubby areas. Further, landscape metric analyses revealed that hogs preferred areas with large complex patch areas and low size variation. These resource selection models were then utilized to better estimate exposure of wild hogs to radionuclides and metals in a disturbed riparian ecosystem on the SRS using two different possible diets based on food availability. Contaminant exposure can be better estimated using these resource selection models than has been previously possible, because past practices did not consider home range and habitat utilization probability in heterogeneously contaminated habitats. Had these models not been used, risk calculations would assume that contaminated areas were utilized 100% of the time, thus overestimating exposure by a factor of up to 25.

Mercury patterns in wood duck eggs from a contaminated reservoir in South Carolina, USA

Mercury contamination of wildlife populations has been documented widely in recent years as biomonitoring has become an important tool for assessing environmental contamination. Avian eggs provide an ideal assay material for Hg biomonitoring, particularly when the collection of eggs is simplified by using cavity-nesting species that nest in easily monitored nest boxes. However, studies are needed that address the dynamics of how Hg is distributed within eggs, and how Hg is deposited naturally within clutches laid by a single female and among clutches laid by different females occupying the same contaminated environment. We collected 138 eggs from 13 complete clutches of box-nesting wood ducks (Aix sponsa) during 1991 and 1992 at a contaminated reservoir of the U.S. Department of Energys Savannah River Site in South Carolina, USA. Total Hg residues in egg components and clutches were determined, partitioning of Hg among egg components was examined, and effects of egg-laying sequence on egg component Hg levels were determined. Mean albumen Hg was 0.22 ppm wet mass, mean yolk Hg was 0.04 ppm, and mean shell Hg was 0.03 ppm. On average, 86.1% of total egg Hg was concentrated in the albumen, 11.2% in the yolk, and 2.7% in the shell. Mercury concentrations in all egg components varied significantly among clutches and between successive clutches laid by the same female in the same year. Laying sequence significantly affected Hg concentrations in the albumen and shell, but not in the yolk. Declines of albumen Hg due to laying sequence were more pronounced for clutches that contained higher average Hg levels. Our results suggest that collection of first-laid eggs may be preferable for assessing maximal Hg exposure to developing embryos, and that monitoring Hg levels through the use of empty eggshells following brood departure from nests may be valid only if the laying sequence is known.

Low levels of genetic variability in north American populations of the Wood Stork (Mycteria americana)

The breeding population of Wood Storks (Mycteria americana) in the southeastern United States has declined since the 1930s, resulting in the listing of all United States populations of the species as endangered in 1984. We assessed genetic structure within and among nine colonies of Wood Storks from Georgia and Florida. Levels of band sharing based on multilocus oligonucleotide DNA fingerprinting of individuals from seven colonies ranged from 58% among nests within the same tree to approximately 55% within the same colony. Levels of band sharing were similar to those reported for other populations of birds that have experienced drastic reductions in population size. A more thorough analysis of four polymorphic microsatellite loci for 136 individuals from nine colonies indicated low levels of allelic diversity and low genetic divergence among colonies. Genetic differentiation (F ST = 0.015) was similar to levels detected from allozymes for 15 colonies of Wood Storks in Florida (F ST = 0.019). These data, together with demographic studies of these populations, indicated high levels of gene flow among colonies (N m = 16.4). In agreement with previous studies, we recommend that all colonies of Wood Storks in the southeastern United States be managed on a regional basis as a single interbreeding population. Similar genetic surveys of the disjunct breeding populations of Wood Storks in South America and Central America would be beneficial in understanding the total genetic differentiation in the species.

Niche differences between male and female white-tailed deer on Ossabaw Island, Georgia

Abstract Niche differences between male and female white-tailed deer ( Odocoileus virginianus ) were studied on Ossabaw Island, Georgia, U.S.A. Most deer seen in pastures were males, but males and females were seen in approximately equal numbers in forest habitat. Pastures contained more abundant and apparently higher quality forage than did forest habitat. The use of higher quality areas by males is not consistent with observations of most other ungulate species. Resource limitation and the relatively high population density of deer on Ossabaw Island may have contributed to this pattern of habitat use. No dietary differences between the sexes were observed within habitats, as indicated by a lack of significant differences in rumen contents, in vitro digestion of forages, and foraging behavior.

A Determination of the Caloric Density and Major Body Components of Large Birds

A procedure is described for determining the live—weight caloric density and major body components of large birds by using a series of random homogeneous aliquots of the total biomass. The procedure is illustrated with data from 6 adult mourning doves. Statistical analyses indicate that the procedure described is capable of producing uniform aliquots with respect to the percentage composition of the major body components of water, fat and lean dry weight. In future biomass analyses of large birds, it may be possible to obtain reliable estimates of the live—weight caloric density and major body components by processing 10% or less of the birds total live weight, providing that the carcass is sufficiently homogenized to produce a series of uniform aliquots. The lean dry biomass of the 6 doves had an average energy content of 5.14kcal/g,and using this figure, an average caloric density of 2.61kcal/g live weight was calculated. See full-text article at JSTOR

MERCURY IN BALD EAGLE NESTLINGS FROM SOUTH CAROLINA, USA

Bald eagles (Haliaeetus leucocephalus) may be at risk from contaminants in their diet and young birds may be particularly sensitive to contaminant exposure. To evaluate potential risks from dietary mercury exposure to eagle nestlings in South Carolina (USA), we surveyed mercury concentrations in 34 nestlings over two breeding seasons (1998 and 1999). Samples were also obtained from several post-fledging eagles in the region. Nestling feather mercury ranged from 0.61–6.67 μg Hg/g dry weight, nestling down mercury from 0.50–5.05 μg Hg/g dry weight, and nestling blood mercury from 0.02–0.25 μg Hg/g wet weight. We did not detect significant differences in tissue mercury between nestlings from coastal and inland regions in contrast to some other studies of piscivorous birds. Mercury concentrations were much higher in the post fledging birds we sampled. Our data show that nestling eagles in South Carolina are accumulating mercury, and that concentrations in older birds may exceed regulatory guidelines.