William P. Johnson

Avian cholera in waterfowl: the role of lesser snow and ross's geese as disease carriers in the Playa Lakes Region.

We collected samples from apparently healthy geese in the Playa Lakes Region (USA) during the winters of 2000–01 and 2001–02 to determine whether carriers of Pasteurella multocida, the bacterium that causes avian cholera, were present in wild populations. With the use of methods developed in laboratory challenge trials (Samuel et al., 2003a) and a serotype-specific polymerase chain reaction method for identification of P. multocida serotype 1, we found that a small proportion of 322 wild birds (<5%) were carriers of pathogenic P. multocida. On the basis of serology, an additional group of these birds (<10%) were survivors of recent avian cholera infection. Our results confirm the hypothesis that wild waterfowl are carriers of avian cholera and add support for the hypothesis that wild birds are a reservoir for this disease. In concert with other research, this work indicates that enzootic infection with avian cholera occurs in lesser snow goose (Chen caerulescens caerulescens) populations throughout their annual cycle. Although fewer Rosss geese (Chen rossii) were sampled, we also found these birds were carriers of P. multocida. Even in the absence of disease outbreaks, serologic evidence indicates that chronic disease transmission and recent infection are apparently occurring year-round in these highly gregarious birds and that a small portion of these populations are potential carriers with active infection.

Nest Success of Snowy Plovers (Charadrius nivosus) in the Southern High Plains of Texas

Abstract. Snowy Plovers (Charadrius nivosus) nesting on edges of saline lakes within the Southern High Plains (SHP) of Texas are threatened by habitat degradation due to reduced artesian spring flow, making many saline lakes unsuitable for nesting and migrating shorebirds. Factors influencing nest success were evaluated, current nest success estimates in the SHP of Texas were compared to estimates obtained ten years prior, and causes and timing of nest failures determined. Overall, 215 nests were monitored from three saline lakes in 2008–2009, with nest success estimates from Program MARK ranging from 7–33% ( = 22%). The leading causes of nest failures were attributed to predation (40%) and weather (36%). Nest success was negatively influenced by number of plants within 707-cm2 plot, positively influenced by percent surface water availability, and at one saline lake, negatively influenced by day during the nesting season (i.e., nest success declined later in the nesting season). When compared to estimates ten years prior (1998–1999), mean nest success has declined by 31%. If nesting Snowy Plovers continue to experience increased predation rates, decreased hydrological integrity, and habitat alterations, populations will continue to decline throughout this region.

Foraging Ecology of Migrant Shorebirds in Saline Lakes of the Southern Great Plains

Abstract. Tens of thousands of shorebirds use saline lakes as migratory stopovers in the Southern Great Plains, USA. To assess their foraging strategies and understand how they replenish energy reserves during spring and summer/fall migrations, we examined diets, prey taxa selection, and prey size selection of American Avocets (Recurvirostra americana), Least Sandpipers (Calidris minutilla), Wilsons Phalaropes (Phalaropus tricolor), and Lesser Yellowlegs (Tringa flavipes). Migrant shorebirds foraged opportunistically by taking most prey taxa according to their availability. Least Sandpipers preferred small prey (2–5 mm), whereas American Avocet, Wilsons Phalaropes, and Lesser Yellowlegs generally preferred intermediate and large prey (6–20 mm). By consuming prey taxa according to their availability and prey sizes that require minimum energy to capture and ingest, shorebirds increase their ability to replenish energy reserves while migrating through interior North America. Drought and drying of freshwater springs will reduce availability of prey in saline lakes for migrating shorebirds. To preserve the saline lakes as important habitats where shorebirds replenish nutrient reserves while migrating through the Great Plains, it is important to conserve groundwater so that freshwater springs continue to discharge into the lakes.

Migration Chronology of Waterfowl in the Southern High Plains of Texas

Abstract Migration chronology was quantified for 15 waterfowl species on 58 playa wetlands in the Southern High Plains of Texas from February 2004 through April 2006. Abundance of each species was estimated on playas once every two weeks during the nonbreeding season (16 August to 30 April); presence of ice was also recorded. Dabbling ducks were most common (N = 250,668) and most tended to exhibit either a bimodal migration pattern (lower abundance in winter than during fall and spring passage) or a unimodal pattern (one defined peak). Abundance of the most common dabbling ducks was skewed toward late winter and spring. Most species of diving ducks (N = 15,128) tended to exhibit irregular migration patterns. Canada Geese (both Branta canadensis and B. hutchinsii, N = 15,347) had an abundance pattern that gradually increased, peaking in midwinter, and then decreased, which is typical for a terminal wintering area. Ice was most common on playas during the first half of December, which coincided with the lowest winter abundance in dabbling ducks. Data from this study will support management efforts focused on playa wetlands, including the development of population goals and habitat objectives that span the entire non-breeding season.

Comparison of methods to detect Pasteurella multocida in carrier waterfowl.

We conducted laboratory challenge trials using mallard ducks (Anas platyrhynchos) to compare methods for detecting carriers of Pasteurella multocida, the bacterium that causes avian cholera, in wild birds. Birds that survived the initial infection were euthanized at 2–4 wk intervals up to 14 wk post challenge. Isolates of P. multocida were obtained at necropsy from 23% of the birds that survived initial infection. We found that swab samples (oral, cloacal, nasal, eye, and leg joint) were most effective for detecting carrier birds up to 14 wk post infection. No detectable differences in isolation were observed for samples stored in either 10% dimethysulfoxide or brain heart infusion broth. The frequency of detecting carriers in our challenge trials appeared to be related to mortality rates observed during the trial, but was not related to a number of other factors including time after challenge, time delays in collecting tissues postmortem, and route of infection. In our trials, there was little association between antibody levels and carrier status. We concluded that swabs samples collected from recently dead birds, stored in liquid nitrogen, and processed using selective broth provide a feasible field method for detecting P. multocida carriers in wild waterfowl.

Behavior of Migrant Shorebirds in Saline Lakes of the Southern Great Plains

Abstract We recorded and compared diurnal and nocturnal time-activity budgets of American Avocet (Recurvirostra americana), Lesser Yellowlegs (Tringa flavipes), Least Sandpiper (Calidris minutilla), and Wilson’s Phalarope (Phalaropus tricolor) on 21 saline lakes in the Southern Great Plains, USA, during spring and summer/fall 2002 and 2003 to examine importance of saline lakes as migratory stopover sites. All four species spent most of their time feeding (47-70%) and resting (7-37%) by day and at night during spring and fall migrations. Little time was spent in other behaviors. Time budgets differed among species and between seasons, likely due to different energy needs. Time spent foraging varied seasonally between saline lakes and freshwater playas for American Avocets and Least Sandpipers, likely due to differences in vegetation cover and availability of prey between these wetland types. For most species, time spent foraging and resting differed between day and night. Therefore, extrapolating diurnal activity budgets to the entire 24-hour period and from one type of habitat to another within the same region is not recommended. Saline lakes are used by migrant shorebirds as stopover sites where they replenish lipid stores. Conservation efforts should focus on preserving these unique wetlands and the freshwater springs that discharge in them.

Recent Declines in Apparent Survival and Survey Counts of Snowy Plovers Breeding in the Southern High Plains of Texas

Abstract We quantified changes in long-term Snowy Plover (Charadrius nivosus) survey counts and return rates, estimated current sex and age-specific apparent survival and encounter rates, and calculated recruitment thresholds needed to maintain a stable population in the Southern High Plains of Texas. Mean survey counts of adult Snowy Plovers decreased by 78% at one saline lake (from 80 adults/survey to 18 adults/survey) from 1999–2000 to 2008–2010 but remained consistent at an alternate lake (from 45 adults/survey to 41 adults/survey). Adult and juvenile return rates have similarly declined within this time frame by 25 and 62%, respectively. Long-term declines in return rates may be the result of increased mortality from declining habitat conditions either within or outside the breeding season. Current estimates of adult (65%) and juvenile (12%) apparent survival are lower than most other estimates for Snowy Plovers throughout their range. Current estimates of adult and juvenile apparent survival and return rates indicate 5.8–10.0 hatchlings per adult per year are needed to maintain the current population without immigration, a 3–5 fold increase in the past 10 years.

Seasonal Variation in Offspring Sex Ratio in the Snowy Plover

ABSTRACT. The Snowy Plover (Charadrius nivosus) is unique in being a determinate layer of an odd modal clutch size and in having a variable mating system in which female brood desertion occurs regularly. These traits make determining Snowy Plover offspring sex ratios important not only for long-term population stability, as the species is of conservation concern, but also for application to sex allocation theory. In this study, we determined Snowy Plover offspring sex ratios, examined differential costs of producing male and female offspring, and evaluated sex ratio variation in relation to maternal condition, habitat condition, and time during the nesting season on saline lakes of the Southern High Plains of Texas. Examination of 245 chicks from 118 clutches during 1999–2000 and 2008–2009 showed that male offspring were more costly to produce than female offspring; however, offspring sex ratio did not differ from parity, but was slightly male-biased in most years. The probability of producing a male offspring was greater both earlier and later in the breeding season than in the middle. As the availability of saline lake surface water and the subsequent availability of food vary unpredictably throughout the breeding season, depending on precipitation events, we suggest that sex ratio adjustment in unpredictable environments may not be straightforward and may follow nonlinear models and/or vary annually The effects such changes in sex ratios may have on population growth and stability remain unknown.