Timing and magnitude of net methylmercury effects on waterbird reproductive output are dependent on food availability.

Mercury (Hg) is a globally distributed pollutant. Its sub-lethal effects on reproduction of birds have been used as indicators of contamination and of potential demographic effects. However, studies typically used single endpoints that might not be representative of entire reproductive cycle. To estimate timing and net cumulative effects of Hg exposure under field conditions, we used observational data over 11 years from >1200 nests of great egrets breeding under temporally and spatially varying food availability and Hg exposures in the Florida Everglades. We collected measures of fish biomass and availability (>100 locations annually) and used four avian reproductive endpoints that represented the entire breeding cycle. We calculated net reproductive loss by adding estimated Hg effects on failures prior to egg laying, clutch size, hatching success and nestling survival in response to food availability and Hg exposure. To validate and assess results of the observational egret study, we ran the same analyses with data of captive breeding white ibises experimentally exposed to Hg with ad libitum food over 3 years. We found large (>50 %) reductions in great egret offspring with high Hg exposure (18 µg/g dw THg nestling feather, ~0.7 µg/g ww whole egg THg) and high food availability, and even larger reductions (up to 100 %) with high Hg exposure and low food. Timing and the relative contribution of different endpoints to overall reproductive failure varied with food availability. Failures prior to egg laying were relevant at all food availabilities and proportionally most important during high food availability (~70 % of total losses). Under high food, post-hatching failures increased moderately with increasing exposure (~10 % of total losses), and under low food, hatching failures became dominant (~50 % of total losses). Patterns of failure of captive white ibis fed ad libitum resembled those of great egrets under high food availability but differed in total magnitude. We suggest that, a) net reproductive effects of Hg in free-ranging animals are probably much higher than generally reported in studies using single endpoints, b) Hg effect sizes vary considerably among different endpoints and c) food availability is a strong driver of timing and net effects of Hg exposure.

Accounting for food availability reveals contaminant-induced breeding impairment, food-modulated contaminant effects, and endpoint-specificity of exposure indicators in free ranging avian populations.

It remains unclear how sub-lethal effects of contaminants play out in relation to other stressors encountered by free-ranging populations. Effects may be masked or influenced by interactions with field stressors such as food availability. We predicted that (1) including food availability, and particularly its interaction with Hg, would reveal or enhance associations between Hg and breeding endpoints. We further predicted that (2) breeding impairment associated with Hg would be higher under food stress conditions. We monitored Hg and nest success of great egrets (Ardea alba) in eight breeding colonies in the Florida Everglades over 11 years. We characterized variation in local food availability among colonies and years using fish biomass and recession range -a proxy to fish vulnerability. We used two Hg exposure indicators (egg albumen Hg and nestling feather Hg) and six breeding endpoints (clutch-size, brood-size, fledged-size, hatching success, post-hatching success and fledglings per egg) to assess whether variation in food availability influenced associations between Hg and these endpoints. Accounting for interactions between Hg and food availability, we identified statistically significant associations in all 12 indicator-endpoint combinations, while only three were detectable without food. Further, 10 combinations showed interactions between Hg and components of food availability. Our results also indicated an endpoint-specific affinity, with albumen [Hg] explaining more variation in hatching success while nestling feather [Hg] explained more variation in post-hatching survival. Both Hg indicators accounted for relevant (6-10%) amounts of variation in fledglings produced per egg laid, an integrative endpoint. Increased Hg exposure resulted in overall reduced reproductive success when food availability was low, but our models predicted low or no effects of increasing Hg exposure when food availability was high. Our results indicate that Hg induced impairment is strongly driven by food availability, providing a framework that accommodates previously contradictory results in the literature.

Migratory behaviour and survival of Great Egrets after range expansion in Central Europe.

Great Egret Ardea alba is one of few Western Palearctic species that underwent a rapid range expansion in the recent decades. Originally breeding in central and eastern Europe, the species has spread in northern (up to the Baltic coast) and western (up to the western France) directions and established viable breeding populations throughout almost entire continent. We monitored one of the first Great Egrets colonies established in Poland to infer migratory patterns and survival rates directly after range expansion. For this purpose, we collected resightings from over 200 Great Egret chicks marked between 2002-2017 in central Poland. Direction of migration was non-random, as birds moved almost exclusively into the western direction. Wintering grounds were located mainly in the western Europe (Germany to France) within 800-950 km from the breeding colony. First-year birds migrated farther than adults. We found some, although relatively weak, support for age-dependent survival of Great Egrets and under the best-fitted capture-recapture model, the estimated annual survival rate of adults was nearly twice higher than for first-year birds (φ ad  = 0.85 ± 0.05 vs. φ fy  = 0.48 ± 0.15). Annual survival rate under the constant model (no age-related variation) was estimated at φ = 0.81 ± 0.05. Our results suggest that Great Egrets rapidly adapted to novel ecological and environmental conditions during range expansion. We suggest that high survival rate of birds from central Poland and their western direction of migration may facilitate further colonization processes in western Europe.

Variation in nestling feather mercury concentrations at individual, brood, and breeding colony levels: Implications for sampling mercury in birds.

Nestling feathers are often used to monitor and estimate Hg exposure in birds. Decision-making and results of studies on effects of exposure to Hg depend on adequate estimation of [Hg] at the studied level, which in turn is sensitive to variation in [Hg] in sampling units. However, there is little information on how feather [Hg] varies within nestlings, between nest mates, or between broods. We sampled feathers from sibling great egret nestlings in three breeding colonies of the Everglades where birds were exposed to variable Hg levels through diet. We compared the deviation of samples from average [Hg] within four levels of aggregation: individual (different scapular feathers of the same individual); brood (samples from siblings in the same nest), breeding colonies; and the entire wetland ecosystem. We found, on average, little intra-individual (<2%) and intra-brood (<6%) differences in [Hg], and no effects of chick hatch order in feather [Hg]. Further, intra-brood variability was not statistically different from intra-individual variability in [Hg]. In contrast, there was much higher variation between broods within colonies (>20%) and within the entire wetland (>30%) that could bias estimates of exposure to Hg obtained through small sample sizes. We simulated the influence of inter-brood variability in estimates of exposure to Hg under different sample sizes, and we found that uncertainty decreases markedly with samples sizes >6, and suggest a sample size of 10 individuals from different nests for monitoring programs and to estimate Hg contamination in areas where no previous knowledge is available.

Free-Ranging Synanthropic Birds (Ardea alba and Columba livia domestica) as Carriers of Salmonella spp. and Diarrheagenic Escherichia coli in the Vicinity of an Urban Zoo.

The presence of free-ranging urban birds is a risk factor for transmitting pathogens to captive animals and humans alike, including Salmonella spp. and diarrheagenic Escherichia coli. Cloacal swabs from 156 synanthropic Great egrets (Ardea alba) and feral pigeons (Columba livia domestica) that inhabit the surroundings of an urban zoo were processed for the identification of Salmonella spp. and diarrheagenic E. coli pathotypes. Bacterial species identification and genotypic characterization employed the matrix-assisted laser desorption ionization-time of flight mass spectrometry and PCR techniques, respectively, comparing their phylogenetic profiles through amplified fragment length polymorphism (AFLP) analysis. A total of 11 birds were positive for Salmonella Typhimurium (7%) and 9 individuals (5.8%) for diarrheagenic E. coli (enteropathogenic E. coli/Shiga-toxin producing E. coli [EPEC/STEC]) strains. S. Typhimurium strains presented highly similar AFLP profiles (85-100%), whereas EPEC/STEC strains showed more polymorphism. The results show free-ranging birds as carriers for both microorganisms in a zoo environment in Brazil for the first time and suggest these species as possible sources of infection to other animals as well as exposing personnel and visitors to potential zoonotic microorganisms. The presence of carriers highlights the importance of a surveillance system and the need for preventive measures to avoid attracting growing number of synanthropic avian species.

POTENTIAL FOR GREAT EGRETS (ARDEA ALBA) TO TRANSMIT A VIRULENT STRAIN OF AEROMONAS HYDROPHILA AMONG CHANNEL CATFISH (ICTALURUS PUNCTATUS) CULTURE PONDS.

Aeromonas hydrophila is a gram-negative, rod-shaped, facultative, anaerobic bacterium that is ubiquitous in freshwater and slightly brackish aquatic environments and infects fish, humans, reptiles, and birds. Recent severe outbreaks of disease in commercial channel catfish (Ictalurus punctatus) aquaculture ponds have been associated with a highly virulent A. hydrophila strain (VAH), which is genetically distinct from less-virulent strains. The epidemiology of this disease has not been determined. Given that A. hydrophila infects birds, we hypothesized that fish-eating birds may serve as a reservoir for VAH and spread the pathogen by flying to uninfected ponds. Great Egrets (Ardea alba) were used in this transmission model because these wading birds frequently prey on farmed catfish. Great Egrets that were fed VAH-infected catfish shed VAH in feces demonstrating their potential to spread VAH.