Robert R. Cox

Mallard duckling growth and survival in relation to aquatic invertebrates

Identification and assessment of the relative importance of factors affecting duckling growth and survival are essential for effective management of mallards on breeding areas. For each of 3 years (1993-95), we placed F 1 -generation wild mallard (Anas platyrhynchos) females on experimental wetlands and allowed them to mate, nest, and rear broods for 17 days. We manipulated invertebrate densities by introducing fathead minnows (Pimephales promelas) at high densities in half of the wetlands on which broods were confined. Day-17 body mass of surviving ducklings (n = 183) was greater for ducklings that were heavier at hatch; the difference averaged 1.7 g at day 17 for each 1.0 g at hatch (P = 0.047). Growth ratio (the proportion of body mass attained by ducklings when they were last measured relative to that predicted for wild female mallard ducklings) also was positively related to body mass at hatch (P = 0.004). Mean day-17 body mass and mean growth ratio of ducklings per brood (each adjusted for body mass at hatch) were positively related to numbers of aquatic invertebrates (Ps < 0.001) and negatively related to variance in the daily minimum air temperature during the exposure period (Ps < 0.020). Early growth of mallards was more sensitive to variation in numbers of invertebrates than to air temperature or biomass of invertebrates. Duckling survival was positively related to growth ratio (P < 0.001). Our study provides parameter estimates that are essential for modeling growth and survival of mallard ducklings. We emphasize the need for conserving brood-rearing wetlands in the Prairie Pothole Region that are capable of supporting high densities of aquatic invertebrates.

The Farm Bill and Duck Production in the Prairie Pothole Region: Increasing the Benefits

Abstract The Food Security Act of 1985 contained provisions that affected wildlife conservation nationwide. Two provisions that most benefited waterfowl populations in the Prairie Pothole Region (PPR) were the Conservation Reserve Program (CRP) and “Swampbuster” (wetland conservation). Permanent cover established under the CRP provides attractive nesting habitat for upland-nesting ducks that is more secure than other major habitats. Swampbuster has prevented drainage of wetlands vital to breeding duck pairs. In 2007 many CRP contracts will expire. Deliberations will begin in late 2006 regarding the next Farm Bill. The United States Department of Agriculture needs sound biological information and scientific analyses to help establish wildlife priorities in the Farm Bill. We used data from breeding duck population and wetland habitat surveys to develop models for 5 species of upland-nesting ducks and applied these models to >2.6 million wetlands in a digital database for the PPR in North and South Dakota, USA. We used geographic information systems techniques to identify locations in the PPR where CRP cover would be accessible to the greatest number of nesting hens. We then summarized distribution of current CRP contracts relative to distribution of upland-breeding ducks. We also used our models to predict change in the breeding duck population (landscape carrying capacity) that might occur if certain wetlands were exempt from the Swampbuster provision. Our analyses showed that 75% of CRP contracts as of July 2005 were in areas accessible to high or medium numbers of breeding ducks and 25% were in areas of low populations. We suggest a method to prioritize CRP extensions and reenrollment of current contracts or target new contracts to maintain or increase duck production. Additionally, our models suggested that if the Swampbuster provision were removed from future Farm Bills and protected wetland were drained, this area of the PPR could experience a 37% decline in the waterfowl populations we studied.

Less waste corn, more land in soybeans, and the switch to genetically modified crops: trends with important implications for wildlife management

Abstract American agriculture has provided abundant high-energy foods for migratory and resident wildlife populations since the onset of modern wildlife management. Responding to anecdotal evidence that corn residues are declining in cropland, we remeasured waste corn post-harvest in the Central Platte River Valley (CPRV) of Nebraska during 1997 and 1998 to compare with 1978. Post-harvest waste corn averaged 2.6% and 1.8% of yield in 1997 and 1998, respectively. After accounting for a 20% increase in yield, waste corn in 1997 and 1998 was reduced 24% and 47% from 1978. We also evaluated use of soybeans by spring-staging sandhill cranes (Grus canadensis) and waterfowl during spring 1998 and 1999. Despite being widely available in the CPRV, soybeans did not occur in esophageal contents of sandhill cranes (n= 174), northern pintails (Anas acuta, n= 139), greater white-fronted geese (Anser albifrons, n= 198), or lesser snow geese (Chen caerulescens, n = 208) collected with food in their esophagi. Lack of soybean consumption by cranes and waterfowl in Nebraska in early spring builds upon previously published findings, suggesting that soybeans are poorly suited for meeting nutrient needs of wildlife requiring a high-energy diet. Given evidence that high-energy food and numerous populations of seed-eating species found on farmland are declining, and the enormous potential risk to game and nongame wildlife populations if high-energy foods were to become scarce, a comprehensive research effort to study the problem appears warranted. Provisions under the Conservation Security subtitle of The Farm Security and Rural Investment Act of 2002 offer a potential mechanism to encourage producers to manage cropland in ways that would replace part of the high-energy foods that have been lost to increasing efficiency of production agriculture.

Factors affecting gadwall brood and duckling survival in prairie pothole landscapes

Waterfowl biologists need reliable predictors of brood and duckling survival to accurately estimate recruitment rates. We examined 30-day survival rates of gadwall (Anas strepera) broods (1992-1994) and ducklings (1990-1994) in eastern North Dakota, USA, during years when water conditions ranged from extremely dry to extremely wet. We evaluated effects of several variables on brood survival: (1) percent of seasonal wetland basins containing water, (2) occurrence of rain on the current or 2 previous exposure days, (3) minimum ambient temperature averaged over the current and 2 previous exposure days, (4) hatch date, (5) duckling age, and (6) brood size. Only 9 of 58 radiomarked females lost their entire broods; Kaplan-Meier 30-day survival rate for broods was 0.84. Brood size (adjusted for hatch date) was a better brood-survival predictor than were any of the environmental variables. Risk of tool brood loss decreased by 24% for each additional duckling in the brood. We monitored fates of 212 radiomarked ducklings from 94 broods. Daily risk of death for ducklings was twice as high when seasonal ponds were scarce as when ponds were abundant. Duckling survival rate was lower during the first 7 days of life whether it rained or not; survival rate was greater for 8- to 30-day-old ducklings, but was reduced by rain. Thirty-day duckling survival was greatest when minimum daily temperatures exceeded 10°C and no rain occurred. We attributed 86% of 87 deaths of radiomarked ducklings to predation; American mink (Mustela vison) accounted for ≥68% of the 40 deaths for which predator type could be ascertained. Despite apparent resilience of gadwall populations during drought, our study documented a positive effect of seasonal wetland availability on gadwall duckling survival. Management efforts to improve recruitment will be more effective in years when most seasonal basins contain water.

Use of Habitats by Female Northern Pintails Wintering in Southwestern Louisiana

The breeding population of northern pintails (Anas acuta) in 1996 was 39% below the long-term average. Because winter habitat quality may influence subsequent breeding population size in pintails, identification of habitats used by wintering pintails and factors influencing use of habitats may be important for managing for population increase. We examined variation in diel use of habitats by radiotagged female pintails (n = 272) in southwestern Louisiana in relation to age (imm and ad), winter (1991-92 and 1992-93), and time period within winters (pre-hunting season, first hunting season, time between split hunting seasons, second hunting season, and post-hunting season). Diurnal use of refuges was significantly greater during hunting seasons than during immediately preceding or succeeding nonhunting seasons. Consequently, we reject Tamisiers (1976) hypothesis that high diurnal use of refuges by pintails in southwestern Louisiana occurs independent of hunting. Nocturnal use of refuges was low (<14%) during all time periods, and declined later in winters. Time-period differences in diurnal and nocturnal use of habitats (large permanent pools, marsh, rice, fallow [idle], and other agriculture [primarily soybeans]) were not consistent between winters. Diel use of refuges or habitats did not differ in relation to female age. Females used fallow and rice agriculture extensively, particularly at night, and these habitats collectively accounted for 68-93% of nocturnal use. Differential use of habitats between winters was related to annual differences in relative abundances of rice and fallow agriculture. Proximity of refuges to agricultural areas should be an important management consideration for wintering pintails and other waterfowl.

EVENING FLIGHTS OF FEMALE NORTHERN PINTAILS FROM A MAJOR ROOST SITE

We monitored evening flights of female Northern Pintails (Anas acuta) from Lacassine National Wildlife Refuge (NWR) in southwestern Louisiana during winters of 1991-1992 and 1992-1993. We analyzed the influence of female age, winter, and date within wintering period on three flight parameters: distance, duration, and departure time. Flight distance and duration increased with date within wintering period, and age differences in flight distance and duration were not consistent between winters. Females departed 12 min later, on average, on clear, moonlit evenings than on overcast, moonless evenings, and 4 min later when winds were light rather than heavy. After controlling for variation due to environmental conditions, immature females departed Lacassine NWR 1.3 min earlier, on average, than did adults. Flight parameters of females did not differ between hunting and non-hunting time periods. Estimated daily transit costs ranged from 27-54% of basal met- abolic rate, 7-19% of daily energy expenditure, and 8-20% of daily dietary intake of rice (Oryza sativa). Our findings that flight distance and duration increased with date within wintering period were consistent with predictions of refuging theory, but alternative hy- potheses also could explain these results. Evening flights of Northern Pintails roosting on Lacassine NWR were greater in distance and duration than those reported for most other species of wintering waterfowl. We recommend that proximity of refuges to feeding sites be considered in conservation and management plans for wintering Northern Pintails and other refuging waterfowl.

Mallard Brood Movements, Wetland Use, and Duckling Survival During and Following a Prairie Drought

Abstract We used radiotelemetry to study mallard (Anas platyrhynchos) brood movements, wetland use, and duckling survival during a major drought (1988–1992) and during the first 2 years of the subsequent wet period (1993–1994) at 4 51-km2 sites in prairie pothole landscapes in eastern North Dakota, USA. About two-thirds of 69 radiomarked mallard broods initiated moves from the nest to water before noon, and all left the nest during daylight. On average, broods used fewer wetlands, but moved greater distances during the dry period than the wet period. Broods of all ages were more likely to make inter-wetland moves during the wet period and probabilities of inter-wetland moves decreased as duckling age increased, especially during the dry period. Brood use of seasonal wetlands nearly doubled from 22% to 43% and use of semi-permanent wetlands declined from 73% to 50% from the dry to the wet period. Eighty-one of 150 radiomarked ducklings died during 1,604 exposure days. We evaluated survival models containing variables related to water conditions, weather, duckling age, and hatch date. Model-averaged risk ratios indicated that, on any given date, radiomarked ducklings were 1.5 (95% CI = 0.8–2.8) times more likely to die when the percentage of seasonal basins containing water (WETSEAS) was ≤18% than when WETSEAS was >40%. An interaction between duckling age and occurrence of rain on the current or 2 previous days indicated that rain effects were pronounced when ducklings were 0–7 days old but negligible when they were 8–30 days old. The TMIN (mean daily minimum temperature on the current and 2 previous days) effects generally were consistent between duckling age classes, and the risk of duckling death increased 9.3% for each 1°C decrease in TMIN across both age classes. Overall, the 30-day survival rate of ducklings equipped with radiotransmitters was about 0.23 lower than the survival rate of those without radiotransmitters. Unmarked ducklings were 7.6 (95% CI = 2.7–21.3) times more likely to die on any given day when WETSEAS was ≤18% than when WETSEAS was >40%. Higher duckling survival and increased use of seasonal wetlands during the wet period suggest that mallard production will benefit from programs that conserve and restore seasonal wetland habitat. Given adverse effects of low temperatures on duckling survival, managers may want to include this stochastic variable in models used to predict annual production of mallards in the Prairie Pothole Region.

DOES PRESENCE OF PERMANENT FRESH WATER AFFECT RECRUITMENT IN PRAIRIE-NESTING DABBLING DUCKS?

Abstract In the Prairie Pothole Region (PPR) of North Dakota, USA, American mink (Mustela vison) are a major predator of ducklings. Mink populations plummet during severe droughts, but some mink survive where permanent fresh water is available. In 1992–1993, we evaluated whether development of a permanent water body, the 125-km McClusky Canal (MC), had affected survival of gadwall (Anas strepera) and mallard (A. platyrhynchos) broods and ducklings in surrounding wetland complexes. Twelve of 25 radiomarked gadwall and mallard hens experienced total brood loss, and 148 of 199 radiomarked ducklings from 58 broods died by day 30. Gadwall broods (n = 18 radiomarked hens) survived to 30 days at a lower rate (0.52) than predicted for similar areas in the region with limited permanent fresh water (0.85; P = 0.009). Observed (n = 162 radiomarked ducklings from 48 broods) survival rates also were lower than predicted for gadwall ducklings 0–7 days old (0.42 vs. 0.60; P < 0.001) and 8–30 days old (0.41 vs. 0.80; P < 0.001). We attempted to include mallards in models constructed to predict brood and duckling survival rates in the Koenig Study Area (KSA), but data were too sparse. Rates of survival to 30 days for gadwall and mallard ducklings declined from an estimated 0.83 and 0.68 in 1976–1981 (Lokemoen et al. 1990), when the MC was first filling with water, to 0.36 and 0.31 (adjusted for radiotransmitter effects) in 1992–1993 after the MC had become a permanent freshwater body. Estimated gadwall recruitment rate (females fledged per hen) during 1992–1993 was 0.5, <50% of the estimated recruitment rate in 1976–1981. Of 130 radiomarked ducklings (both species) for which we determined cause of death, 114 mortalities were attributed to predation; at least 65% of 62 deaths in which the predator type could be discerned were caused by mink. Environmental planners and waterfowl managers should be aware of potential risks to waterfowl production from development of permanent freshwater bodies in prairie pothole landscapes and may wish to refine duck productivity models to consider negative effects of permanent water on duckling survival.

Survival of female northern pintails wintering in Southwestern Louisiana

The North American breeding population of northern pintails (Anas acuta) has reached previoasly unprecedented low numbers 4 times since 1983. Because pintails show high fidelity to wintering areas, regional survival estimates and identification of factors influencing survival are needed to guide management of wintering pintails. We used radiotelemetry to estimate survival rates of female pintails wintering in southwestern Louisiana. We tested for variation in survival and hunting mortality rates in relation to age (immature or adult), winter (1990-91, 1991-92, 1992-93), time period (prehunting season, first hunting season, time between split hunting seasons, second hunting season, posthunting season), body condition (body mass when released, adjusted for body size), and region (southwestern Louisiana or elsewhere on the Texas-Louisiana Gulf Coast or Mississippi Alluvial Valley). Within southwestern Louisiana, the 147-day (5 Oct-28 Feb) survival rate of adults (0.714 ± 0.045; x ± SE) was greater (P = 0.02) than that of immatures (0.550 ± 0.068), primarily because immatures had higher hunting mortality. Female survival was lower (Ps 0.25). Survival did not differ in relation to winter or condition (Ps > 0.12). Hunting mortality did not differ (Ps > 0.13) in relation to winter, condition, or region, but hunting mortality of immatures (0.287 ± 0.046) was greater (P < 0.001) than that of adults (0.130 ± 0.025). Despite conservative hunting regulations (30-day seasons and 1 pintail daily), hunting mortality rates of pintails in southwestern Louisiana were high. Pintails in southwestern Louisiana rely on a small number of key refuges for protection but feed almost exclusively in privately owned agricultural fields. If increased winter survival of female pintails in southwestern Louisiana is desired, we recommend that managers increase food availability on refuges.

Survival of wood duck ducklings and broods in Mississippi and Alabama

Abstract Although North American wood ducks (Aix sponsa) are well-studied throughout their range, researchers know little about demographic and environmental factors influencing survival of ducklings and broods, which is necessary information for population management. We studied radiomarked female and duckling wood ducks that used nest boxes and palustrine wetlands at Noxubee National Wildlife Refuge (NNWR) in Mississippi, USA, in 1996–1999, and riverine wetlands of the Tennessee-Tombigbee Rivers and Waterway (TTRW) system in Alabama in 1998–1999. We estimated survival of ducklings and broods and evaluated potentially important predictors of duckling survival, including age and body mass of brood-rearing females, hatch date of ducklings, duckling mass, brood size at nest departure, inter-day travel distance by ducklings, site and habitat use, and daily minimum air temperature and precipitation. At NNWR, survival of 300 radiomarked ducklings ranged from 0.15 (95% CI = 0.04–0.27) to 0.24 (95% CI = 0.13–0.38) and was 0.21 (95% CI = 0.15–0.28) for 1996–1999. Our overall estimate of brood survival was 0.64 (n = 91; 95% CI = 0.54–0.73). At TTRW, survival of 129 radiomarked ducklings was 0.29 in 1998 (95% CI = 0.20–0.41) and 1999 (95% CI = 0.13–0.45) and was 0.29 (95% CI = 0.20–0.40) for 1998–1999. Our overall estimate of brood survival was 0.71 (n = 38; 95% CI = 0.56–0.85). At NNWR, models that included all predictor variables best explained variation in duckling survival. Akaike weight (wi) for the best model was 0.81, suggesting it was superior to other models (<0.01 ≤ wi ≤0.18). We detected 4 competing models for duckling survival at TTRW. Inter-day distance traveled by ducklings was important as this variable appeared in all 4 models; duckling survival was positively related to this variable. Patterns of habitat-related survival were similar at both study areas. Ducklings in broods that used scrub-shrub habitats disjunct from wetlands containing aggregations of nest boxes had greater survival probabilities than birds remaining in wetlands with such nest structures. Managers may increase local wood duck recruitment by promoting availability of suitable brood habitats (e.g., scrub-shrub wetlands) without aggregations of nest boxes that may attract predators and by dispersing nest boxes amid or adjacent to these habitats. We did not determine an optimal density of nest boxes relative to local or regional population goals, which remains important research and conservation needs.