Michael R. Riggs

Winter severity, survival, and cause-specific mortality of female white-tailed deer in north-central Minnesota

Knowledge of age-specific survival of deer (Odocoileus spp.) and the underlying causes of mortality are essential to our understanding of their population dynamics. We examined age-specific survival and cause-specific mortality of female white-tailed deer (O. virginianus) in an area where wolves (Canis lupus) recently had become reestablished and assessed the influence of winter severity during 1991-1996 using new survival analysis procedures. Sufficient data permitted rigorous survival analyses on 153 of 179 radiocollared females ≥0.6 years old, whose age distribution remained stable with annual median ages of 5.9-6.7 years old during the last 5 years of the study. Winter severities ranged from unusually mild to historically severe; 84% (81 of 97) of all mortalities occurred during November-May. The median age of deer survival according to the iterative Nelson estimator (INE) was 3.6 years (90% confidence limits [CL] = 2.4, 4.0). The smoothed estimate of the hazard function was a U-shaped curve with an elevated risk of mortality at 0.6 years old, which declined progressively until 5 years old, then increased steadily with the highest age-specific risk of death for deer ≥10 years old. Fits of the Anderson-Gill (AG) proportional hazards model to our data indicated that survival was negatively associated (P < 0.0001) with winter severity (primarily snow depth), but not with site of winter capture or body mass at capture. The risk of death increased progressively over the course of each winter, with maximum risk occurring in winters of greatest snow depth. Estimated life-time mortality rates (90% CL) due to hunting, wolf predation, and miscellaneous causes were 43.3% (29.5, 57.1%), 33.1% (20.1, 45.3%), and 23.6% (13.2, 34.0%), respectively. Within 5-11 years of reestablishment in our area, wolf predation became a leading cause of mortality for female deer, but its importance relative to hunting depended on annual fluctuations in winter severity. Competing risk analyses permitted comparison of age-specific hazards associated with hunter harvest, wolf predation, and miscellaneous causes of mortality. The nature of the observed interaction between the cause-specific hazards and the effects of winter severity appears to validate the practice of regulating the issuance of antlerless deer permits based on population condition. Collective evidence suggests that as agencies formulate management strategies relative to wolf reintroduction or range expansion, the frequency of severe winters, snow depth, deer population goals, and the use of antlerless permits should be primary considerations.

Potential effects of fish predation on Wetland invertebrates: A comparison of wetlands with and without fathead minnows

We evaluated potential effects of fathead minnows (Pimephales promelas) on wetland invertebrates by comparing activity trap catches from semipermanent wetlands with and without fathead minnows during 1991 and 1992. Indices of aquatic invertebrate abundance, biomass, and taxon richness were all lower in wetlands containing fathead minnows. Reductions in abundance and biomass of insects and crustaceans in wetlands with fathead minnows were pronounced during both years. Our results indicate that at high densities, fathead minnows may severely depress abundance, biomass, and taxon richness of aquatic invertebrates in semipermanent prairie wetlands, thereby reducing the suitability of these habitats as seasonal foraging areas for waterfowl.

A Long-Term Age-Specific Survival Analysis of Female White-Tailed Deer

Abstract We conducted a 13-year survival (i.e., time survived since birth) and cause-specific mortality study, divided into 2 phases (Phase I = years 1–6; Phase II = years 7–13), of 302 female white-tailed deer (Odocoileus virginianus) ≥0.6 years old at capture. The study spanned a period of extreme variability in winter severity (maximum winter severity indexes [WSI] of 45–195) and hunting pressure. Most studies of survival and cause-specific mortality of northern deer have assumed constant survival rates for adults of each sex (≥1.0 yr old pooled) and examined fawns (0.6 ≤ x ≤ 1.0 yr old) separately. We observed U-shaped hazard (i.e., instantaneous risk of death) curves for both phases of the study, indicating that risk of death is highest for younger and older individuals. The estimated hazard for Phase II was generally lower and relatively constant for adults 2–10 years old compared to Phase I, where the instantaneous risk of death began to increase at age 6 years. This difference likely reflected differences in winter severities, associated changes in magnitude of wolf (Canis lupus) predation, and changes in hunting pressure between the 2 phases. The age distribution of our study cohort was relatively stable over the study period. Subsequently, when we included 76 neonates (i.e., ≤0.6 yr old) in the study cohort, the descending arm of the all-causes hazard began its descent at a hazard rate of 2.3 (vs. 1.0 without neonates), clearly demonstrating that the greatest risk of mortality occurs in the first year of life. We compared cumulative survival estimates for these data using the generalized Kaplan–Meier (GKM) and the iterative Nelson estimator (INE), and we illustrate the potential for bias when applying the GKM to left-truncated data. Median age of survival for females was 0.83 years old (90% CI = 0.79–1.45 yr old) using the INE and 0.43 years old (90% CI = 0.17–0.78 yr old) using the GKM. Lastly, we used a simulation approach to examine the potential for bias resulting from pooling adults. These simulations suggest that models using the constructed discrete time variable give nearly unbiased survival estimates and provide support for researchers and managers applying age-specific hazards derived during study periods to determine the reliability of adult age-pooled survival estimates. As indicated by our data, it is important to consider environmental variation and its interactions with natural mortality forces (e.g., predation) and age distribution of the population when setting harvest goals.

Common Goldeneye nest attendance patterns

Common Goldeneye (Bucephala clangula) nest attendance was recorded on three Minnesota lakes, 1982-1985. Data were from 22 nests monitored 545 days. Hens increased time at nests and frequency of overnight sessions as incubation approached. Incubation recesses were diurnal; most occurred between 9:00 and 19:00 CST. Recesses were fewest and longest in early incubation, but total recess time was greatest in late incubation. Daily incubation constancy was highly variable, ranging from 36.7 to 96.3%. Lake and year effects influenced the number of daily recesses and total daily recess time. Mean recess length did not differ among lakes and years. Three females monitored in both 1984 and 1985 recessed more daily (χ = 55 min) in 1984, and their yearly ranks were the same for all parameters suggesting strong hen effects. Incubation ranged from 28 to 30 days and was correlated (r = 0.72) with mean daily recess time. Nests incubated 29 and 30 days had mean total incubation times that differed by 13 min. While brooding young, incubation-like patterns were maintained but absences were fewer and shorter. Our results differed slightly from those reported for a recent Ontario study, but the differences may be due to analytical approaches

A SURFACE-ASSOCIATED ACTIVITY TRAP FOR CAPTURING WATER-SURFACE AND AQUATIC INVERTEBRATES IN WETLANDS

We developed a surface-associated activity trap (SAT) for sampling aquatic invertebrates in wetlands. We compared performance of this trap with that of a conventional activity trap (AT) based on nondetection rates and relative abundance estimates for 13 taxa of common wetland invertebrates and for taxon richness using data from experiments in constructed wetland. Taxon-specific non-detection rates for ATs generally exceeded those of SATs, and largest improvements using SATs were for Chironomidae and Gastropoda. SATs were efficient at capturing cladocera, Chironomidae, Gastropoda, total Crustacea, and multiple taxa (taxon richness) but were only slightly better than ATs at capturing Dytiscidae. Temporal differences in capture rates were observed only for cladocera, Chironomidae, Dytiscidae, and total Crustacea, with capture efficiencies of SATs usually decreasing from mid-June through mid-July for these taxa. We believe that SATs may be useful for characterizing wetland invertebrate communities and for developing improved measures of prey available to foraging waterfowl and other aquatic birds.

Habitat use and nest success of overwater nesting ducks in westcentral Minnesota

Where several duck species coexist, managers need knowledge of species-specific similarities and differences in patterns of nest habitat use and nest success. We searched overwater habitat (i.e., rooted stands of cattail [Typha spp.], bulrush [Scirpus spp.], or phragmites [Phragmites australis], floating mats of sedge [Carex spp.] and/or cattail, wet sedge/grass [Poaceae] meadows, and willow [Salix spp.] swamps) in westcentral Minnesota and located 155 overwater nests of 5 duck species. Nests were in wetlands of 0.2 to 32.0 ha that were 20 to 100% covered by overwater habitat. A canonical discriminant function analysis clarified some similarities and differences among the 5 species in their nest habitat use. Some individuals of each species, but especially redheads (Aythya americana) and ruddy ducks (Oxyura jamaicensis), nested in relatively more open, larger, deeper type 4 wetlands with cover near nests dominated by cattails and lacking sedges. Most mallards (Anas platyrhynchos) and ring-necked ducks (Aythya collaris) nested in smaller, shallower type 2 and 3 wetlands which had a high proportion of the basin covered by overwater habitat and at sites where overwater vegetation, particularly sedges, was dense and nests were well screened. Mallards and ring-necked ducks were the only species nesting in type 2 wetlands or on floating sedge mats and except for 2 canvasback (Aythya valisineria) nests, were the only species to use floating cattail mats as nest sites. Canvasbacks typically nested in type 3 and 4 wetlands of intermediate size, depth, and proportion of basin covered by overwater habitat. Their nests were at sites of moderate vegetation density and were less well screened by vegetation than those of the other species. Mallards had lower nest success (3.8%) than ring-necked ducks (34.1%) or all diving ducks combined (26.5%) (α = 0.10). Of 105 nests that failed to hatch, 74.3% were depredated while 13.3% were flooded. Multiple logistic regression analyses indicated that none of the 11 habitat characteristics measured at our nest sites predicted whether a nest would hatch or be depredated (all P > 0.20).

Black Tern Nest Habitat Selection and Factors Affecting Nest Success in Northwestern Minnesota

Abstract We documented nest habitat selection, nests success, and factors affecting nest success of Black Terns (Chlidonias niger) at Agassiz National Wildlife Refuge in northwestern Minnesota. During 1992-1994, 289 Black Tern nest sites and 400 random sites were sampled on five search areas totaling 1,325 ha. Four habitat characteristics were measured at each nest and random site: (1) mean water depth, (2) distance to open water, (3) dominant vegetation within a 2-m radius, and (4) amount of open water within a 2-m radius. Habitat variables were highly correlated with each other, making it difficult to estimate independent effects of each habitat variable on nest-site selection. However, conditional logistic regression models indicated that locations closer to open water and in deeper water were more likely to be associated with nest sites. Locations in bulrush (Scirpus acutus) and sedge/grass were preferred, although 68% of nests were in cattail (Typha spp.) reflecting the greater availability of that habitat in the study area. Nest success ranged from 48-69% (Apparent) and 33-62% (Mayfield) among years. Except for five nests that were abandoned or had infertile eggs, nests that failed to hatch apparently were depredated. Nest success was higher for nests with larger clutch sizes and for nests located farther away from other nests. Nests with 3-egg clutches were 2.8 times as likely to hatch as 2-egg nests. The odds of a nest being successful increased by 25% for each 5 m increase in distance to the nearest nest. Earlier nests were also more successful (the odds of a nest being successful was estimated to decrease 7% for each additional day that passed before the nest was initiated). Nest success was not related to nest cluster size and was negatively related to habitat factors associated with nest site-selection.

Analysis of Covariance Models for Data From Observational Field Studies

Abstract We outline the features of a general class of statistical models (i.e., analysis of covariance [ANCOVA] models) that has proven to be effective for the analysis of data from observational studies. In observational studies, treatments are assigned by Nature in a decidedly nonrandom manner; consequently, many of the crucial assumptions and safeguards of the classic experimental design either fail or are absent. Hence, inferences (causal or associative) are more difficult to justify. Typically, investigators can expect the primary factors of interest, which are usually called environmental exposures rather than treatments, to be involved in complex interactions with each other and with other factors, and these factors will be confounded with still other factors. We provide examples illustrating the application of ANCOVA models to adjust for confounding factors and complex interactions, thereby providing relatively clean estimates of association between exposure and response. We summarize information on available software and supporting literature for implementing ANCOVA models for the analysis of cross-sectional and longitudinal observational field data. We conclude with a brief discussion of critical model fitting issues, including proper specification of the functional form of continuous covariates and problems associated with overfitted models and misspecified models that lack important covariates.

Factors influencing incubation egg-mass loss for three species of waterfowl

Abstract Many bird eggs lose ∼15% of their fresh mass before pipping, but individual species have been reported to lose 10–23%. Most published estimates have been imprecise due to small sample sizes. Moreover, published estimates of within- or among-species variance components of mass loss are virtually unknown. We modeled the influence of nest type, clutch size, and egg size on daily mass loss of Mallard (Anas platyrhynchos), Common Goldeneye (Bucephala clangula), and Hooded Merganser (Lophodytes cucullatus) eggs and compared fractional mass loss among species. Mallard eggs in artificial nest cylinders lost more mass than those in ground nests, but were unaffected by nest initiation date. Average-sized eggs in Mallard ground nests, Mallard cylinder nests, and Common Goldeneye and Hooded Merganser nest boxes lost 7.9 g (15.2%), 10.8 g (20.3%), 10.3 g (15.5%), and 9.2 g (15.8%) of fresh mass, respectively. For all species, daily mass loss increased as incubation progressed and was affected by an interaction between egg size and incubation time, but was not influenced by clutch size. Depending on species, smallest eggs lost 1.0–4.0% more of their fresh mass than did the largest. Egg-mass variability was partitioned into years, nests within years, and eggs within nests and years. Variability was evenly distributed among the variance components in Mallard ground nests; however, among-eggs within-nest variance predominated in nest cylinders. In contrast, among-nests variation was the dominant source for goldeneyes and mergansers. Nest-site selection and egg size likely involve trade-offs among optimum egg-mass loss and nest and hatchling survival. Factores que Influencian la Pérdida de Peso de los Huevos durante la Incubación en Tres Especies de Aves Acuáticas Resumen. Muchas aves pierden aproximadamente el 15% de su peso fresco antes de iniciar la ruptura del cascarón, pero se ha reportado que esto varía entre especies entre el 10 y el 23%. La mayoría de los estimados publicados han sido imprecisos debido a tamaños de muestra pequeños. Más aún, los estimados de los componentes de la varianza dentro de especies o entre especies en la pérdida de peso son virtualmente desconocidos. En este estudio modelamos la influencia del tipo de nido, el tamaño de la nidada y tamaño del huevo sobre la pérdida diaria de peso en huevos de Anas platyrhynchos, Bucephala clangula y Lophodytes cucullatus, y comparamos la fracción de peso perdida entre especies. Los huevos de A. platyrhynchos perdieron más peso en nidos cilíndricos artificiales que en nidos en el suelo, pero no fueron afectados por la fecha de iniciación de la nidificación. Huevos de tamaño promedio de A. platyrhynchos puestos en nidos en el suelo y en cilindros, y huevos de B. clangula y L. cucullatus puestos en cajas de nidificación, perdieron 7.9 g (15.2%), 10.8 g (20.3%), 10.3 g (15.5%) y 9.2 g (15.8%) de su peso fresco, respectivamente. Para todas las especies, la pérdida diaria de peso se incrementó a medida que progresó la incubación y fue influenciada por una interacción entre el tamaño de los huevos y el tiempo de incubación, pero no por el tamaño de la nidada. Dependiendo de la especie, los huevos más pequeños perdieron entre 1.0 y 4.0% más de su peso fresco que los huevos más grandes. La variabilidad en el peso de los huevos estuvo repartida entre años, entre nidos de un mismo año y entre huevos de un mismo nido en cada año. La variabilidad estuvo igualmente distribuida entre sus distintos componentes en los nidos de A. platyrhynchos del suelo. Sin embargo, la varianza entre huevos de un nido predominó en los nidos puestos en cilindros. En contraste, la variación entre nidos fue la más importante en B. clangula y L. cucullatus. La selección de sitios de nidificación y el tamaño de los huevos probablemente involucran una solución de compromiso entre los niveles óptimos de pérdida de peso de los huevos y de supervivencia de los nidos y pichones.

Effect of winter temperature on wild turkey metabolism

We used indirect calorimetry to measure the effects of air temperature (T a ), age class, and body mass on metabolic rates of 9 adult and 7 juvenile female eastern wild turkeys (Meleagris gallopavo silvestris) during winter. Previous studies produced disparate results on this important aspect of winter ecology of wild turkeys. Standard metabolic rates (SMRs) of adult and juvenile hens were not different (P = 0.122) and averaged 28.69 mL O 2 .min -1 .bird -1 . Wild turkey metabolism increased with decreasing T a (P < 0.001) below the lower critical temperature (T lc ) of 10.9°C. Metabolic rates were not related to body mass (P = 0.571), and age-specific metabolic rates were not distinguishable (P = 0.998). We estimated that a flock of 20 hens would need to find 400 g/day of additional food to meet thermoregulatory demands for each 10°C drop in T a below 10.9°C.