Mark J. Petrie

DIET AND NUTRITION OF NORTHERN PINTAILS WINTERING ALONG THE SOUTHERN COAST OF TEXAS

Abstract Most northern pintails (Anas acuta; hereafter pintails) in the Central Flyway winter within the Gulf Coast and adjacent rice prairies of Texas, USA. However, wintering habitat has declined in this region as a result of decreased rice production and changes in land use. Because pintails exhibit high winter site fidelity, more pintails are likely to rely on adjacent coastal habitats during winter as freshwater habitats along the Texas coast disappear. However, few studies have investigated the diet of pintails in estuarine environments. We estimated the composition and quality of the diet of pintails wintering along the lower Texas coast, and we compared our estimates to those for pintails in freshwater habitats. Proximate composition and true metabolizable energy (TME) were estimated for 4 foods in the diet of 253 pintails collected along the lower Texas coast during October–February 1997–1998 and 1998–1999. Shoalgrass (Halodule wrightii) rhizomes, wigeongrass (Ruppia maritima) seeds, dwarf surf clams (Mulinia lateralis), marine gastropods, and Gammarus amphipods comprised most of the pintail diet. Pintail diets in coastal habitats contained smaller proportions of protein and fat and a large proportion of ash compared to diets of pintails from freshwater habitats. As a result, the diet of pintails wintering along the lower Texas coast provided about half the TME of diets of pintails wintering in freshwater habitats. Because pintails rely on endogenous reserves acquired during winter and spring migration to support egg production, pintails wintering in Texas may experience greater reductions in recruitment and survival if displaced to coastal habitats.

Depletion of rice as food of waterfowl wintering in the Mississippi Alluvial Valley.

Abstract Waterfowl habitat conservation strategies in the Mississippi Alluvial Valley (MAV) and several other wintering areas assume carrying capacity is limited by available food, and increasing food resources is an effective conservation goal. Because existing research on winter food abundance and depletion is insufficient to test this hypothesis, we used harvested rice fields as model foraging habitats to determine if waste rice seed is depleted before spring migration. We sampled rice fields (n  =  39 [winter 2000–2001], n  =  69 [2001–2002]) to estimate seed mass when waterfowl arrived in late autumn and departed in late winter. We also placed exclosures in subsets of fields in autumn (n  =  8 [2000–2001], n  =  20 [2001–2002]) and compared seed mass inside and outside exclosures in late winter to estimate rice depletion attributable to waterfowl and other processes. Finally, we used an experiment to determine if the extent of rice depletion differed among fields of varying initial abundance and if the seed mass at which waterfowl ceased foraging or abandoned fields differed from a hypothesized giving-up value of 50 kg/ha. Mean seed mass was greater in late autumn 2000 than 2001 (127.0 vs. 83.9 kg/ha; P  =  0.018) but decreased more during winter 2000–2001 than 2001–2002 (91.3 vs. 55.7 kg/ha) and did not differ at the end of winter (35.8 vs. 28.3 kg/ha; P  =  0.651). Assuming equal loss to deterioration inside and outside exclosures, we estimated waterfowl consumed 61.3 kg/ha (48.3%) of rice present in late autumn 2000 and 21.1 kg/ha (25.1%) in 2001. When we manipulated late-autumn rice abundance, mean giving-up mass of rice seed was similar among treatments (48.7 kg/ha; P  =  0.205) and did not differ from 50 kg/ha (P  =  0.726). We integrated results by constructing scenarios in which waterfowl consumed rice at different times in winter, consumption and deterioration were competing risks, and consumption occurred only above 50 kg/ha. Results indicated waterfowl likely consumed available rice soon after fields were flooded and the amount consumed exceeded our empirical estimates but was ≤48% (winters pooled) of rice initially present. We suggest 1) using 50 kg/ha as a threshold below which profitability limits waterfowl feeding in MAV rice fields; 2) reducing the current estimate (130 kg/ha) of rice consumed in harvested fields to 47.2 kg/ha; and 3) increasing available rice by increasing total area of fields managed, altering management practices (e.g., staggered flooding), and exploring the potential for producing second or ratoon rice crops for waterfowl.

True metabolizable energy for seeds of common moist-soil plant species

Abstract Understanding the true metabolizable energy (TME) value of food is important for constructing bioenergetic models. We estimated gross energy, nutrient composition, and TME values for the seeds of 3 native and 1 invasive exotic wetland plant that occur in the diet of waterfowl. True metabolizable energy values were 0.50 ± 0.080 kcal/g for spike rush (Eleocharis palustris), 0.65 ± 0.080 kcal/g for alkali bulrush (Schoenoplectus maritimus), 1.31 ± 0.090 kcal/g for perennial pepperweed (Lepidium latifolium), and 2.52 ± 0.080 kcal/g for lambs quarters (Chenopodium album). The TME value for 3 of 4 species, including the invasive species, was low relative to the TME value of other seeds consumed by waterfowl.

True metabolizable energy estimates of Canada goose foods

We determined true metabolizable energy (TME) and digestibility of 8 foods commonly available to Canada geese (Branta canadensis) in order to provide managers with a better understanding of the nutritional value of these foods. True metabolizable energy estimates were highest for chufa tubers (Cyperus esculentus ; 16.86 kJ/g), corn (Zea mays; 16.32 kJ/g), and milo (Sorghum vulgare; 15.72 kJ/g), while estimates for soybeans (Glycine max; 14.85 kJ/g), wild millet (Echinochloa crusgalli; 13.77 kJ/g), and rice (Oryza sativa; 11.76 kJ/g) were intermediate in value. Winter wheat foliage (Triticum aestivum; 10.04 kJ/g) and largeseed smartweed seeds (Polygonum pensylvanicum; 6.65 kJ/g) provided the least TME of foods tested. Crude fiber accounted for 76% of the variation in TME. Digestibility values of the 8 foods ranged from 35.1 to 88.5%, with digestibility varying widely among both agricultural and natural foods. To supply Canada geese with a balanced diet and foods high in metabolizable energy (ME), managers should provide a variety of agricultural and natural foods that are low in fiber content.

Population Dynamics of Mallards Breeding in Agricultural Environments in Eastern Canada

Abstract Population dynamics of mallards (Anas platyrhynchos) in eastern North America likely differ relative to the midcontinent population, but independent management has been hampered by lack of data. We used data from radiomarked females from 4 sites in southern Ontario and 1 site in New Brunswick during 1992–2000 to assess demographic performance during the breeding season and to inform regional management strategies. For each site, we estimated recruitment of female young to 30 days posthatch per breeding female and projected the finite rate of population growth λ, assuming annual survival typical of the region. We constructed a matrix population model for females and conducted perturbation analyses (i.e., analytic sensitivities and elasticities) to predict response of λ to change in vital rates. Excluding 1 site, we estimated mean recruitment of 0.89 (range = 0.79–0.98). At 1 site, duckling survival approaching zero resulted in extremely low estimated recruitment (0.08). Perturbation analyses indicated that λ was highly sensitive to changes in nest survival and nonbreeding survival and moderately sensitive to changes in survival of breeding females. Recruitment for most of our sites was >2 times the mean for the Prairie Pothole Region of Canada and likely was sufficient to maintain populations despite relatively high hunting kill in the region. Despite high sensitivity of λ to nest survival, intensive management to increase nest survival likely would be inefficient because of uniform nest survival among habitats and low nest densities. Because recruitment levels were generally high but densities of breeding pairs low, we recommend protection, enhancement, and restoration of wetlands to maintain and increase habitat suitability for breeding mallards as likely the most cost-efficient management strategy to benefit populations. Relatively high hunting kill and high sensitivity of λ to nonbreeding survival imply that harvest is important to regional population dynamics and suggest the potential effectiveness of adaptive harvest management.

The Effects of Seasonal Flooding on Seed Availability for Spring Migrating Waterfowl

Abstract We hypothesized the seed biomass available to migrating waterfowl would be higher in spring- versus fall-flooded wetlands. To test this hypothesis we conducted an experiment using 5 pairs of wetland impoundments in northern Missouri, USA (2000–2002). We strategically assigned one impoundment of each pair to either a fall or spring treatment group. We estimated seed biomass in fall and in spring by clipping seed heads and collecting soil cores at 20 random locations within each impoundment. We placed exclosures near each fall sample site in spring-flooded impoundments to estimate seed loss from granivorous birds and rodents. Despite similar biomass in fall between treatments (P = 0.64), overwinter seed loss was greater in fall-flooded (79%; 1,324 ± 195 kg/ha) than in spring-flooded (31%; 653 ± 130 kg/ha) impoundments (P = 0.009). Considering barnyard grass or millet (Echinochloa spp.) only, seed loss was higher in fall-flooded than in spring-flooded impoundments (P = 0.05). Spring biomass estimates were similar inside versus outside exclosures (P = 0.63) indicating loss to granivorous birds and rodents was limited. Our results suggest that fall flooding reduces seed availability for spring migrating waterfowl. We recommend spring flooding be used in areas where impoundment water levels can be manipulated to increase seed availability for spring migrating waterfowl.

Waterfowl distribution relative to wetland acidity

We measured the influence of water acidity on the presence of fish and abundance of aquatic invertebrates to better understand how those variables influence the distribution of waterfowl broods on unmanaged freshwater wetlands in southern New Brunswick. Broods of piscivorous waterfowl were most common on wetlands with pH≥5.5; whereas wetland selection by broods of insectivores was unaffected by acidity. Wetlands with the most invertebrates and most broods and ducklings of insectivores contained no fish. Abundance of aquatic invertebrates was most important in influencing use of wetlands by insectivorous wartefowl, especially American black duck (Anas rubripes). Biomass (dried mass) and diversity of aquatic invertebrates were greater (P<0.005) on wetlands where black duck broods 1-36 days old were observed than on wetlands where broods of those ages were not seen

Mallard and black duck breeding parameters in New Brunswick: a test of the reproductive rate hypothesis.

Divergent population trends of mallards (Anas platyrhynchos) and black ducks (Anas rubripes) in eastern North America have been attributed to differences in reproductive rates. In order to test this hypothesis, we compared breeding parameters of radiomarked mallards and black ducks in an agricultural environment in midwestern New Brunswick. No differences in clutch size, nest success, hen success, duckling survival, or hen survival were detected during the breeding season. Reproductive success of both species appeared to be at or above the level needed to maintain population size. These results support a growing body of evidence that mallards are not more productive than black ducks in agricultural or forested environments. Moreover, earlier studies indicate no difference in annual survival rates between the 2 species. Given the failure of this and other studies to detect differences in vital rates of the 2 species, divergent population trends of mallards and black ducks present somewhat of a paradox. We hypothesize that differences in the population status of mallards and black ducks are the result of differences in breeding propensity that may arise from competition for breeding resources.

Predicted eelgrass response to sea level rise and its availability to foraging Black Brant in Pacific coast estuaries

Managers need to predict how animals will respond to habitat redistributions caused by climate change. Our objective was to model the effects of sea level rise on total eelgrass (Zostera marina) habitat area and on the amount of that area that is accessible to Brant geese (Branta bernicla), specialist grazers of eelgrass. Digital elevation models were developed for seven estuaries from Alaska, Washington, California (USA), and Mexico. Scenarios of future total eelgrass area were derived from combinations of estuarine specific sediment and tectonic rates (i.e., bottom change rate) with three rates of eustatic sea level rise (ESLR). Percentages of total eelgrass areas that were accessible to foraging Brant were determined for December when the birds overwinter at more southerly sites and in April as they move north to sites where they build body stores on their way to nesting areas in Alaska. The modeling showed that accessible eelgrass area could be lower than total area due to how daytime low-tide height, eelgrass shoot length, and the upper elevation of eelgrass determined Brant-reaching depth. Projections of future eelgrass area indicated that present-day ESLR (2.8 mm/yr) and bottom change rates should sustain the current pattern of estuarine use by Brant except in Morro Bay, where use should decrease because eelgrass is being ejected from this estuary by a positive bottom change rate. Higher ESLR rates (6.3 and 12.7 mm/yr) should result in less Brant use of estuaries at the northern and southern ends of the flyway, particularly during the winter, but more use of mid-latitude estuaries. The capacity of mid-latitude estuaries to function as Brant feeding refugia, or for these estuaries and Izembek Lagoon to provide drift rather than attached leaves, is eventually limited by the decrease in total eelgrass area, which is a result of a light extinction affect on the eelgrass, or the habitat being pushed out of the estuary by positive tectonic rates. Management responses are limited to the increase or decrease of sediment supply and the relocation of levees to allow for upslope migration of eelgrass habitat.

Predicting effects of environmental change on a migratory herbivore

Changes in climate, food abundance and disturbance from humans threaten the ability of species to successfully use stopover sites and migrate between non-breeding and breeding areas. To devise successful conservation strategies for migratory species we need to be able to predict how such changes will affect both individuals and populations. Such predictions should ideally be process-based, focusing on the mechanisms through which changes alter individual physiological state and behavior. In this study we use a process-based model to evaluate how Black Brant (Branta bernicla nigricans) foraging on common eelgrass (Zostera marina) at a stopover site (Humboldt Bay, USA), may be affected by changes in sea level, food abundance and disturbance. The model is individual-based, with empirically based parameters, and incorporates the immigration of birds into the site, tidal changes in availability of eelgrass, seasonal and depth-related changes in eelgrass biomass, foraging behavior and energetics of the birds, a...