Frank C. Rohwer

Egg Viability, Nest Predation, and the Adaptive Significance of Clutch Size in Prairie Ducks

Waterfowl begin incubating their eggs before the clutch is completed. No current hypotheses can explain this phenomenon. We show that egg viability declines during the period of egg laying if eggs remain unincubated, and we suggest that early nest attendance may slow the rate of decline in viability. We then develop the hypothesis that declining egg viability associated with delayed incubation plays an important role in determining the most productive clutch size in temperate-breeding waterfowl. The benefit of laying additional eggs is offset by the reduced value of the first-laid eggs, which suffer lowered viability and greater risk of predation. Our model combining egg viability and the risks of egg predation can account for most of the selection pressure determining clutch size in prairie ducks.

Composition of blue-winged teal eggs in relation to egg size, clutch size, and the timing of laying

Blue-winged Teal (Anas discors) eggs from 172 nests were analyzed to determine how organic composition was affected by clutch size, laying sequence, nest attempt, laying date, and egg size. On average, eggs weighed 28.1 g and were 46% yolk and 44% albumen. Eggs contained 3.6 g lipid and 3.2 g protein almost equally distributed between yolk and albumen. Yolk increased a disproportionate amount and the proportion of albumen tended to decrease with increased egg mass. Consequently, large eggs contained proportionately more lipid than small eggs. Shell decreased in proportion with increasing egg mass. For most egg traits 60 to 80% of the variation occurred between clutches rather than within clutches. Teal eggs did not vary in composition in relation to clutch size, laying sequence, nest attempt, or the timing of laying on either a seasonal or yearly basis. Eggs in renests weighed 0.8 g less than eggs from the first clutch of the same females; however, this disparity was not reflected in differences in lipid or lean dry mass for the collected eggs of known renesters.

Laying-stage nest attendance and onset of incubation in Prairie nesting ducks

Abstract We used microprocessor data loggers to document patterns of nest attendance during the laying stage and to quantify temperatures of dummy eggs during laying for Mallard (Anas platyrhynchos), Blue-winged Teal (A. discors), Northern Shoveler (A. clypeata), Northern Pintail (A. acuta), Gadwall (A. strepera), Green-winged Teal (A. crecca), American Wigeon (A. americana), and Lesser Scaup (Aythya affinis) nesting in southern Manitoba in 1994 and in northeastern North Dakota in 1995–1997 and 2000–2002. Females of all species increased the time they spent on the nest as laying progressed, but species differed in their pattern of increased attendance. Female Blue-winged Teal and Northern Shoveler that laid smaller clutches increased the time they spent on the nest more rapidly than conspecifics that laid larger clutches, but large- and small-clutch conspecifics had similar attendance at the end of the laying period. Attendance during laying was not influenced by low ambient temperature, precipitation, or nest initiation date. For all species combined, maximum egg temperatures increased as laying progressed. Eggs were heated to temperatures sufficient for embryonic development as early as the day that the second egg was laid. Our findings contradict the prevailing paradigm that incubation in waterfowl begins after clutch completion and raise questions about how hatching synchrony is achieved. We relate our findings to two hypotheses (nutrient limitation and viability–predation) that have been proposed to explain the limits to clutch size in ducks.

HABITAT RELATIONSHIPS OF BIRDS OVERWINTERING IN A MANAGED COASTAL PRAIRIE

Abstract Grassland birds are considered to be rapidly declining in North America. Management approaches for grassland birds frequently rely on prescribed burning to maintain habitat in suitable condition. We evaluated the relationships among years since burn, vegetation structure, and overwintering grassland bird abundance in coastal prairie. Le Contes Sparrows (Ammodramus leconteii) were most common in areas that had: (1) been burned within the previous 2 years, (2) medium density herbaceous vegetation, and (3) sparse shrub densities. Savannah Sparrows (Passerculus sandwichensis) were associated with areas: (1) burned within 1 year, (2) with sparse herbaceous vegetation, and (3) with sparse shrub densities. Sedge Wrens (Cistothorus platensis) were most common in areas that had: (1) burned greater than 2 years prior and (2) dense herbaceous vegetation. Swamp Sparrows (Melospiza georgiana): (1) were most common in areas of dense shrubs, (2) not related to time since burnings, and (3) demonstrated no relationship to herbaceous vegetation densities. The relationships to fire histories for all four bird species could be explained by the associated vegetation characteristics indicating the need for a mosaic of burn rotations and modest levels of woody vegetation.

From the Field: Evaluation of nest temperatures to assess female nest attendance and use of video cameras to monitor incubating waterfowl

Wildlife Society Bulletin 2004, 32(2):581–587 Peer refereed Quantifying avian incubation provides insight into reproductive strategies and potential constraints on reproduction. Methods to assess nest attendance by parent birds include direct observations, mechanical devices, time-lapse photography, radiotelemetry, weight scales, changes in temperature in nests and eggs, and a multiple-sensor system of weight scales and infrared beams (e.g.,Afton and Paulus 1992). In recent years, monitoring changes in nest and egg temperatures has been used to study waterfowl attendance patterns in redheads (Aythya americana;Yerkes 1998), blue-winged teal (Anas discors; Loos 1999), northern shovelers (A. clypeata; MacCluskie and Sedinger 1999), spectacled eiders (Somateria fischeri; Flint and Grand 1999),wood ducks (Aix sponsa;Manlove and Hepp 2000), and greater snow geese (Chen caerulescens atlantica; Poussart et al. 2000). Researchers discretely hid temperature-sensing probes, or thermistors, inside artificial eggs placed in the nest bowl and recorded temperature data at specified time intervals on microcomputers or data loggers. Data loggers can collect data for extended periods of time; their small size allows them to be hidden next to nests or hundreds of meters away. Low cost,ease of setup in the field, minimal investigator disturbance to the incubating female, and ability to monitor many nests during a breeding season likely have led to the increased use of temperature sensors with data loggers to monitor nest attendance. However, detecting movements of incubating females on and off the nest with temperature data appears subjective and may contain observer error. Female movements off the nest may not be detected, especially when ambient temperature approaches incubation temperature (Afton and Paulus 1992). Furthermore, despite the method’s recent popularity, the reliability of temperature data to quantify nest attendance has not been adequately tested. Limited validation of temperature recordings involved flushing females from their nests (Loos 1999), and direct observations of females leaving and returning to nests (Manlove and Hepp 2000, Poussart et al. 2000) to provide comparisons of these times to temperature changes recorded by data loggers. Although these studies failed to find differences between real movements of the female and temperature data, they have not provided a thorough validation of the use of temperature data to assess nest attendance for prairie-nesting dabbling ducks. Manlove and Hepp (2000) validated female wood ducks using nest boxes,Poussart et al. (2000) monitored colonially nesting greater snow geese, and Loos (1999) provided only a known exit time from the nest. Our primary objective was to examine the accuracy of using temperature data to assess female nest attendance by monitoring northern pintail (A. acuta, hereafter pintail) and mallard (A. platyrhynchos) nests on the prairie breeding grounds. We continuously monitored nests using closed-circuit video recordings to provide true times of female presence or absence, while simultaneously monitoring the same nests with 2 different types of temFrom the Field: Evaluation of nest temperatures to assess female nest attendance and use of video cameras to monitor incubating waterfowl

Effects of brood size and age on survival of female wood ducks

During a 13-year study of Wood Ducks (Aix sponsa) nesting in eastern Massachusetts, 43.3% of nests were parasitized by conspecifics. Early season nests were more frequently parasitized than were late nests. First-nesting females were less frequently parasitized than experienced breeders, partially because first-nesting females nest later in the season when parasitism declines. Nest parasitism created clutch size variation that allowed us to investigate the effect of clutch size and brood size on survival of Wood Ducks. Females that hatched large broods had a minor delay in nesting the following year. Brood size had no influence on the survival rate of female Wood Ducks. Survival rates averaged 52.8% over all year and age classes. Survival rates declined by 6.1% per year of breeding experience.

Metabolizable energy in Chinese tallow fruit for Yellow-rumped Warblers, Northern Cardinals, and American Robins

Abstract The invasive exotic Chinese tallow tree (Triadica sebifera) produces an abundant fruit crop, which is primarily bird-dispersed. The fruit pulp of tallow is lipid-rich, high in saturated fatty acids, and consumed by many bird species. Long-chained fatty acids can be difficult for many birds to digest and we investigated the ability of tallow consumers to assimilate energy in the pulp. We used the total collection method and compared apparent metabolizable energy (AME) of tallow fruit for three species of birds with differing fruit composition in their natural diets. All birds exhibited nitrogen deficits and lost body mass during the trials. Northern Cardinals (Cardinalis cardinalis) lost more mass (8.73%/day) than Yellow-rumped Warblers (Dendroica coronata) (5.29%/day) and American Robins (Turdus migratorius) (5.48%/day), and had larger nitrogen deficits (−120.1 mg N/g diet) than both species as well (−36.4 mg N/g diet and −68.9 mg N/g diet, respectively). Food intake relative to metabolic body mass was highest in Yellow-rumped Warblers (0.70 g-dry/g0.75·day). Northern Cardinal and American Robin food intake was lower and did not differ from each other (both species: 0.13 g-dry/g0.75·day). Nitrogen corrected values of AME were used to make species comparisons. Yellow-rumped-Warblers exhibited the highest values of AME (30.00 kJ/g), followed by American Robins (23.90 kJ/g), and Northern Cardinals (14.34 kJ/g). We suggest tallow may be an important winter food source for Yellow-rumped Warblers where their ranges overlap.

Breeding Habitat Requirements and Colony Formation by Royal Terns (Thalasseus Maximus) and Sandwich Terns (T. Sandvicensis) on Barrier Islands in the Gulf of Mexico

ABSTRACT. Restoration and maintenance of barrier islands to preserve structural integrity and protect against erosional forces is a common goal of coastal protection. An assessment of restored barrier islands for their suitability as wildlife habitat is crucial for improvement of restoration methods and conservation of barrier-island-dwelling species, especially ground-nesting waterbirds. During the 2008 and 2009 breeding periods, we conducted a quantitative assessment of colonial waterbird use of the Isles Dernieres Barrier Island Refuge (IDBIR), Louisiana, which has experienced several restoration projects since the early 1990s, to understand the breeding ecology of two terns in the genus Thalasseus and investigated why some restored areas have not been colonized. Our objectives were to determine hatching success of the two species, identify important habitat characteristics for their reproduction and colony formation, and evaluate the success of past restoration efforts in providing suitable nesting habitat. Habitat characteristics were important for hatching success, including spatial attributes of nest sites and substrate composition. Discriminant function analysis revealed that suitable habitat for colony formation was available at some inactive restored areas, but the majority of inactive areas were unsuitable. The paucity of nesting activity at potentially suitable restored areas may be attributable to the greater activity of mammalian predators that we detected at inactive areas than at active colony sites. Management of restored barrier islands for specific waterbird habitat requirements, creation of new islands containing suitable ground-nesting habitat, and appropriate control of mammalian predators are critical factors for effective waterbird conservation and ecosystem function in these disturbance-prone regions.