Peter C. Frederick

METHYLMERCURY ACCUMULATION IN TISSUES AND ITS EFFECTS ON GROWTH AND APPETITE IN CAPTIVE GREAT EGRETS

To test the hypothesis that fledging wading birds would be more at risk from mercury toxicosis than younger nestlings, captive great egret nestlings were maintained as controls or were dosed from 1- to 14-wk-old with 0.5 or 5 mg methylmercury chloride/kg wet weight in fish. Birds dosed with 5 mg/kg suffered from subacute toxicosis at wk 10–12. Growing feather concentrations were the most closely correlated with cumulative mercury consumed per weight. Blood concentrations of mercury increased more rapidly after 9 wk in all groups when feathers stopped growing. Total mercury accumulated in tissues in concentrations in the following order: growing scapular feathers > powderdown > mature scapular feathers > liver > kidney > blood > muscle > pancreas > brain > bile > fat > eye. The proportion of total mercury that was methylated depended upon tissue type and dose group. Selenium accumulated in liver in direct proportion to liver mercury concentrations. After wk 9, appetite and weight index (weight/bill length) declined significantly in both dosed groups. At current exposure levels in the Everglades (Florida, USA) mercury deposited in rapidly growing feathers may protect nestlings from adverse effects on growth until feathers cease growing.

Altered pairing behaviour and reproductive success in white ibises exposed to environmentally relevant concentrations of methylmercury

Methylmercury (MeHg) is the most biologically available and toxic form of mercury, and can act as a powerful teratogen, neurotoxin and endocrine disruptor in vertebrates. However, mechanisms of endocrine impairment and net effects on demography of biota are poorly understood. Here, we report that experimental exposure of an aquatic bird over 3 years to environmentally relevant dietary MeHg concentrations (0.05–0.3 ppm wet weight) resulted in dose-related increases in male–male pairing behaviour (to 55% of males), and decreases in egg productivity (to 30%). Dosed males showed decreased rates of key courtship behaviours, and were approached less by courting females in comparison to control males. Within dosed groups, homosexual males showed a similar reduction when compared with dosed heterosexual males. We found an average 35 per cent decrease in fledgling production in high-dose birds over the study duration. These results are of interest because (i) MeHg exposure is experimentally tied to demographically important reproductive deficits, (ii) these effects were found at low, chronic exposure levels commonly experienced by wildlife, and (iii) effects on reproductive behaviour and sexual preference mediated by endocrine disruption represent a novel and probably under-reported mechanism by which contaminants may influence wild populations of birds.

Histologic, neurologic, and immunologic effects of methylmercury in captive great egrets.

Captive great egret (Ardea albus) nestlings were maintained as controls or were dosed with methylmercury chloride at low (0.5), and high doses (5 mg/kg, wet weight) in fish. Low dosed birds were given methylmercury at concentrations comparable to current exposure of wild birds in the Everglades (Florida, USA). When compared with controls, low dosed birds had lower packed cell volumes, dingy feathers, increased lymphocytic cuffing in a skin test, increased bone marrow cellularity, decreased bursal wall thickness, decreased thymic lobule size, fewer lymphoid aggregates in lung, increased perivascular edema in lung, and decreased phagocytized carbon in lung. High dosed birds became severely ataxic and had severe hematologic, neurologic, and histologic changes. The most severe lesions were in immune and nervous system tissues. By comparing responses in captive and wild birds, we found that sublethal effects of mercury were detected at lower levels in captive than in wild birds, probably due to the reduced sources of variation characteristic of the highly controlled laboratory study. Conversely, thresholds for more severe changes (death, disease) occurred at lower concentrations in wild birds than in captive birds, probably because wild birds were exposed to multiple stressors. Thus caution should be used in applying lowest observed effect levels between captive and wild studies.

The use of genetic markers to estimate the frequency of successful alternative reproductive tactics

SummaryThis paper outlines a method for estimating rates of successful alternative reproductive tactics from parental exclusions known through the use of genetic markers. We review a method for calculating the probability of excluding a putative father when he is not the actual father. We adapt this method to model two mating tactics of concern to sociobiologists: extrapair copulations (EPCs) and intra-specific egg parasitism (egg-dumping). Four different types of parental exclusions are possible (both male and female, male only, female only, and ambiguous). The two models predict different proportions of each type of exclusion. Models are also generated for the case when the putative mothers or fathers genotypes are not available.We used parental exclusions from an electrophoretic study of indigo buntings (Westneat 1987b) to demonstrate these methods. The distribution of parental exclusions in the buntings departed significantly from the predictions of the egg-dumping model, but agreed well with those of the EPC model. The probability of detection for the EPC model (0.401) was then used to estimate the actual rate of extra-pair fertilizations (0.421 of all the young sampled). We present a method for calculating a confidence interval on this estimate, which ranged from 0.247 to 0.659. We concluded that these methods will allow the quantitative study of the success of alternative reproductive tactics in a wide variety of species.

Extrapair Copulations in the Mating System of White Ibis (Eudocimus Albus)

Mating behavior of white ibis was intensively observed in a large breeding colony in coastal South Carolina, U.S.A. Extrapair copulations (EPCs) were found to be a frequent and regular feature of the mating system. EPCs appeared to be fully capable of fertilizing eggs, on the basis of their form, timing, and male and female behavior in response to them. Females appeared to cooperate fully in some EPCs; this cooperation included cloacal flaring, and sperm was clearly transferred during some EPCs. Females were able to reject EPCs, and EPCs were not forced by males. Based on the relative frequency of apparently successful extrapair and withinpair copulations, EPCs were estimated to fertilize 6.05% of all eggs. EPC involvement among males was based on abilities to remain on the colony, to displace other males, and to take only short foraging trips. Female EPC involvement was based on the amount of time they were left unguarded. A males EPC involvement was inversely correlated with that of his mate. Extrapair males were of distinctly different quality than the females nestmate. Among study groups, EPC rate was not correlated with degree of local nesting density, sex ratio, clutch size. Female cooperation and the efficiency of male mate guarding were concluded to be important to the evolution of this mating strategy in white ibis. It is concluded that EPC is an important strategy in this otherwise monogamous species. The exact costs and benefits of this strategy to both sexes are discussed.

Accuracy and variation in estimates of large numbers of birds by individual observers using an aerial survey simulator

Abstract The accuracy of aerial estimates of avian aggregation sizes is variable across studies, and the relative importance of techniques and inter-observer error to this variation are poorly understood. Using a scaled physical model of a wading bird colony, we examined accuracy and variation in observer counts of simulated large numbers (200–6000) of densely nesting birds in vegetated situations. Observer estimates averaged 29% less than true numbers (under- and overestimates averaged together), and the mean absolute value of observer errors was 49% of true values. We found no effects of the size of the aggregation, the experience of the observer, the size of the previous aggregations surveyed by observers, the use of corrective lenses, or fatigue on degree of individual error. Over- and underestimates by individuals did not tend to cancel out in estimates by individuals of a “population” of colonies. Photographic counts of the same trials were significantly more accurate than observer estimates. We suggest that many studies using estimates of large numbers of birds may be confounded by similar errors. We urge that researchers use caution in interpreting the results of past surveys and develop ways to minimize, measure, and correct for visual estimation error within individuals and among observers.

SUBCHRONIC EFFECTS OF METHYLMERCURY ON PLASMA AND ORGAN BIOCHEMISTRIES IN GREAT EGRET NESTLINGS

In recent years, high concentrations of mercury have been found in wading birds in Florida, USA. Great egret (Ardea alba) chicks (2 weeks old) were dosed orally daily with the equivalent of 0, 0.5, or 5 microg/g Hg as methylmercury chloride in the diet for up to 12 weeks. Weakness of the legs or paralysis occurred in all high-dosed birds. Geometric mean blood Hg concentrations were 0.17, 10.3, and 78.5 microg/g (wet wt), respectively. Mercury concentrations for organs (microg/g wet wt), including brain (0.22, 3.4, and 35, respectively), liver (0.34, 15.1, 138, respectively), and kidney (0.28, 8.1, and 120, respectively), increased in a dose-dependent manner. Total glutathione (GSH) peroxidase activity was significantly lower in the plasma, brain, liver, and kidney of the high-dosed group. Plasma aspartate aminotransferase activity increased with mercury treatment, whereas lactate dehydrogenase activity decreased. Four other plasma chemistries were decreased significantly in the high-dosed group and included uric acid, total protein, albumin, and inorganic phosphorus. Lipid peroxidation increased in liver (low and high dose) and brain (high dose). Tissue changes in concentrations of reduced thiols included decreased total thiols and protein-bound thiols in liver, decreased protein-bound thiols in kidney, and increased GSH in kidney and brain. Activities of GSH S-transferase and oxidized glutathione reductase increased in liver. In kidney, GSH S-transferase and glucose-6-phosphate dehydrogenase activities increased with mercury dose. These findings, including apparent compensatory changes, are compared to other Hg studies where oxidative stress was reported in egrets, herons, and diving ducks in the field and mallards in the laboratory.

The Role of Predation in Determining Reproductive Success of Colonially Nesting Wading Birds in the Florida Everglades

In a sample of 1,609 marked nests of five species of Ciconiiformes in 21 colonial nesting aggregations in the Everglades, evidence of abandonment without destruction of nest contents accounted for 31.3% of failures. In 66.9% of the failures, evidence at the nest suggested either predation resulting in nest failure or postabandonment scavenging of nest contents. In a sample of 106 nests isolated by a nonrepelling tracking medium, we found predation by snakes to account for 23% of nest failures; mammals accounted for an additional 20%. Failures due to these two categories accounted for 12% of the treated nests; abandonments may have been considerably underrepresented in this sample of nests. Mammalian predators rarely visited widely distributed baited tracking stations in the marsh, and we hypothesize that even 5-10 cm of water can substantially restrict travel by raccoons, foxes, and rats. Visitation by mammals to colonies occurred only when the water surrounding them receded, and was not related to the presence of alligators or distance from permanently dry land. We found little evidence of avian predation on wading bird nests, though birds readily scavenged abandoned nest contents. We discuss several attributes of the Everglades marshes which may limit access of predators to nesting colonies.

FEEDING PATTERNS AND AGGRESSIVE BEHAVIOR IN JUVENILE AND ADULT AMERICAN FLAMINGOS

We studied the feeding and aggressive behavior of adult and juvenile American Flamingos (Phoenicopterus ruber ruber) at a coastal salina in Venezuela. Most birds fed in large flocks in early morning, roosted at mid-day, and resumed feeding in late afternoon- early evening. Flamingos rarely flew, except when disturbed. Adults in our study stepped at the same rates while feeding as did Chilean Flamingos (P. chilensis) feeding in the Chilean and Bolivian Andes. Paired observations of birds within mixed-age flocks revealed that adults stepped more during feeding bouts, but less between bouts, and spent more time overall with their bills in the water filter-feeding, than did juveniles. We calculate that the food-intake rate of juveniles was, at most, 82% that of adults. Both adults and juveniles walk-fed and stamp-fed. Juveniles, but not adults, were twice as likely to be supplanted when stamp-feeding than when walk-feeding. Juveniles were more often involved in ag- gression, especially as recipients, than were adults. Aggressive encounters significantly af- fected the amount of time flamingos spent filter feeding.

PULSED BREEDING OF LONG-LEGGED WADING BIRDS AND THE IMPORTANCE OF INFREQUENT SEVERE DROUGHT CONDITIONS IN THE FLORIDA EVERGLADES

At the scale of ecosystems and regions, numbers of nesting long-legged wading birds are often highly variable from year to year, and much of this variation is thought to reflect variation in production or availability of prey animals in wetlands. Based on observations during and following a severe drought in the Florida Everglades (1989–1992), we predicted that large nesting events would be more likely immediately following droughts than at other times. Using a 38-year history of wading bird nesting events in the Florida Everglades, we tested the hypothesis that “supranormal” annual nesting events (numbers of nests >1 standard deviation above the long-term mean) would occur more frequently during the period of up to two years after severe droughts (stages <1 standard deviation below the mean) than after non-drought years. Within this database, we identified 8 supranormal events and 8 severe droughts; 7 of the nesting events occurred immediately after a drought, and 7 of the droughts were followed by a supranormal nesting event. There was a highly significant association between the two types of events. Because many studies suggest that wading bird reproduction is food-limited, this result implies that post-drought conditions somehow result in exceptional productivity and/or availability of small fishes and macroinvertebrates. We propose two biological mechanisms for this pattern and suggest that rare, severe droughts in the Everglades are a forcing function for wading bird population cycles and large-scale movements through the action of pulsed productivity in the aquatic food web.