Synthesis of Maternal Transfer of Mercury in Birds: Implications for Altered Toxicity Risk.

Maternal transfer is a predominant route of methylmercury (MeHg) exposure to offspring. We reviewed and synthesized published and unpublished data on maternal transfer of MeHg in birds. Using paired samples of females' blood (n = 564) and their eggs (n = 1814) from 26 bird species in 6 taxonomic orders, we conducted a meta-analysis to evaluate whether maternal transfer of MeHg to eggs differed among species and caused differential toxicity risk to embryos. Total mercury (THg) concentrations in eggs increased with maternal blood THg concentrations; however, the proportion of THg transferred from females to their eggs differed among bird taxa and with maternal THg exposure. Specifically, a smaller proportion of maternal THg was transferred to eggs with increasing female THg concentrations. Additionally, the proportion of THg that was transferred to eggs at the same maternal blood THg concentration differed among taxonomic orders, with waterfowl (Anseriformes) transferring up to 382% more THg into their eggs than songbirds (Passeriformes). We provide equations to predict THg concentrations in eggs using female blood THg concentrations, and vice versa, which may help translate toxicity benchmarks across tissues and life stages. Our results indicate that toxicity risk of MeHg can vary among bird taxa due to differences in maternal transfer of MeHg to offspring.

Mercury Exposure and Altered Parental Nesting Behavior in a Wild Songbird.

Methylmercury is a neurotoxin and endocrine disruptor and may impair avian reproduction directly through embryotoxicity or by altering parental care behaviors. We studied mercury exposure and incubation behavior of free-living tree swallows ( Tachycineta bicolor) nesting in artificial nest boxes. Using small temperature dataloggers, we measured incubation constancy (the proportion of each day the female spent incubating eggs), the number of incubation recesses taken per day, and the duration of incubation recesses. We also assessed maternal mercury exposure by measuring mercury concentrations in both blood and eggs. Females with higher mercury concentrations exhibited lower incubation constancy, took more frequent and shorter incubation recesses, and were more likely to take incubation recesses that caused nest temperature decreases that were likely to slow embryonic development. Overall, females that laid eggs with the highest observed mercury concentration (0.53 µg/g fww) spent an average of 12% less time incubating their eggs over the 14-day incubation period than females that laid eggs with the lowest mercury concentration (0.07 µg/g fww). Because less time spent incubating can lower egg temperatures, slow embryonic development, and potentially lengthen the incubation period, these results suggest that environmentally relevant mercury concentrations may negatively influence reproduction by altering parental nesting behaviors of wild songbirds.

It's what's inside that counts: egg contaminant concentrations are influenced by estimates of egg density, egg volume, and fresh egg mass.

In egg contaminant studies, it is necessary to calculate egg contaminant concentrations on a fresh wet weight basis and this requires accurate estimates of egg density and egg volume. We show that the inclusion or exclusion of the eggshell can influence egg contaminant concentrations, and we provide estimates of egg density (both with and without the eggshell) and egg-shape coefficients (used to estimate egg volume from egg morphometrics) for American avocet (Recurvirostra americana), black-necked stilt (Himantopus mexicanus), and Forster's tern (Sterna forsteri). Egg densities (g/cm(3)) estimated for whole eggs (1.056 ± 0.003) were higher than egg densities estimated for egg contents (1.024 ± 0.001), and were 1.059 ± 0.001 and 1.025 ± 0.001 for avocets, 1.056 ± 0.001 and 1.023 ± 0.001 for stilts, and 1.053 ± 0.002 and 1.025 ± 0.002 for terns. The egg-shape coefficients for egg volume (K v ) and egg mass (K w ) also differed depending on whether the eggshell was included (K v  = 0.491 ± 0.001; K w  = 0.518 ± 0.001) or excluded (K v  = 0.493 ± 0.001; K w  = 0.505 ± 0.001), and varied among species. Although egg contaminant concentrations are rarely meant to include the eggshell, we show that the typical inclusion of the eggshell in egg density and egg volume estimates results in egg contaminant concentrations being underestimated by 6-13 %. Our results demonstrate that the inclusion of the eggshell significantly influences estimates of egg density, egg volume, and fresh egg mass, which leads to egg contaminant concentrations that are biased low. We suggest that egg contaminant concentrations be calculated on a fresh wet weight basis using only internal egg-content densities, volumes, and masses appropriate for the species. For the three waterbirds in our study, these corrected coefficients are 1.024 ± 0.001 for egg density, 0.493 ± 0.001 for K v , and 0.505 ± 0.001 for K w .

Estimating Mercury Exposure of Piscivorous Birds and Sport Fish Using Prey Fish Monitoring.

Methylmercury is a global pollutant of aquatic ecosystems, and monitoring programs need tools to predict mercury exposure of wildlife. We developed equations to estimate methylmercury exposure of piscivorous birds and sport fish using mercury concentrations in prey fish. We collected original data on western grebes (Aechmophorus occidentalis) and Clark's grebes (Aechmophorus clarkii) and summarized the published literature to generate predictive equations specific to grebes and a general equation for piscivorous birds. We measured mercury concentrations in 354 grebes (blood averaged 1.06 ± 0.08 µg/g ww), 101 grebe eggs, 230 sport fish (predominantly largemouth bass and rainbow trout), and 505 prey fish (14 species) at 25 lakes throughout California. Mercury concentrations in grebe blood, grebe eggs, and sport fish were strongly related to mercury concentrations in prey fish among lakes. Each 1.0 µg/g dw (∼0.24 µg/g ww) increase in prey fish resulted in an increase in mercury concentrations of 103% in grebe blood, 92% in grebe eggs, and 116% in sport fish. We also found strong correlations between mercury concentrations in grebes and sport fish among lakes. Our results indicate that prey fish monitoring can be used to estimate mercury exposure of piscivorous birds and sport fish when wildlife cannot be directly sampled.

Experimental dosing of wetlands with coagulants removes mercury from surface water and decreases mercury bioaccumulation in fish.

Mercury pollution is widespread globally, and strategies for managing mercury contamination in aquatic environments are necessary. We tested whether coagulation with metal-based salts could remove mercury from wetland surface waters and decrease mercury bioaccumulation in fish. In a complete randomized block design, we constructed nine experimental wetlands in California's Sacramento-San Joaquin Delta, stocked them with mosquitofish (Gambusia affinis), and then continuously applied agricultural drainage water that was either untreated (control), or treated with polyaluminum chloride or ferric sulfate coagulants. Total mercury and methylmercury concentrations in surface waters were decreased by 62% and 63% in polyaluminum chloride treated wetlands and 50% and 76% in ferric sulfate treated wetlands compared to control wetlands. Specifically, following coagulation, mercury was transferred from the filtered fraction of water into the particulate fraction of water which then settled within the wetland. Mosquitofish mercury concentrations were decreased by 35% in ferric sulfate treated wetlands compared to control wetlands. There was no reduction in mosquitofish mercury concentrations within the polyaluminum chloride treated wetlands, which may have been caused by production of bioavailable methylmercury within those wetlands. Coagulation may be an effective management strategy for reducing mercury contamination within wetlands, but further studies should explore potential effects on wetland ecosystems.

Egg Mercury Concentration and Egg Size Varies with Position in the Laying Sequence in two Songbird Species.

In birds, mercury embryotoxicity can occur through the transfer of mercury from the female to her eggs. Maternal transfer of mercury can vary by egg position in the laying sequence, with first-laid eggs often exhibiting greater mercury concentrations than subsequently laid eggs. We studied egg mercury concentration, mercury burden (total amount of mercury in the egg), and egg morphometrics by egg position in the laying sequence for two songbirds: tree swallows (Tachycineta bicolor) and house wrens (Troglodytes aedon). Egg mercury concentration in the second egg laid was 14% lower for tree swallows and 6% lower for house wrens in comparison with the first egg laid. These results indicate that in both species, after an initial relatively high transfer of mercury into the first egg laid, a smaller amount of mercury was transferred to the second egg laid. This lower mercury concentration persisted among all subsequently laid eggs (eggs three to eight) in tree swallows (all were 14%-16% lower than egg 1), but mercury concentrations in subsequently laid house wren eggs (eggs three to seven) returned to levels observed in the first egg laid (all were 1% lower to 3% greater than egg 1). Egg size increased with position in the laying sequence in both species; the predicted volume of egg 7 was 5% and 6% greater than that of egg 1 in tree swallows and house wrens, respectively. This change was caused by a significant increase in egg width, but not egg length, with position in the laying sequence. The percentage of decline in mercury concentration with position in the laying sequence was considerably lower in tree swallows and house wrens compared with other bird taxonomic groups, suggesting that there are key differences in the maternal transfer of mercury into songbird eggs compared with other birds. Finally, we performed simulations to evaluate how within-clutch variation in egg mercury concentrations affected estimates of mean mercury concentrations in each clutch and the overall sampled population, which has direct implications for sampling designs. Environ Toxicol Chem 2024;00:1-11. Published 2024. This article is a U.S. Government work and is in the public domain in the USA.

Marsh wrens as bioindicators of mercury in wetlands of Great Salt Lake: do blood and feathers reflect site-specific exposure risk to bird reproduction?

Nonlethal sampling of bird blood and feathers are among the more common ways of estimating the risk of mercury exposure to songbird reproduction. The implicit assumption is that mercury concentrations in blood or feathers of individuals captured in a given area are correlated with mercury concentrations in eggs from the same area. Yet, this assumption is rarely tested. We evaluated mercury concentrations in blood, feathers, and eggs of marsh wrens in wetlands of Great Salt Lake, Utah, and, at two spatial scales, specifically tested the assumption that mercury concentrations in blood and feather samples from birds captured in a defined area were predictive of mercury concentrations in eggs collected in the same area. Mercury concentrations in blood were not correlated with mercury concentrations in eggs collected within the same wetland unit, and were poorly correlated with mercury concentrations in eggs collected at the smaller home range spatial scale of analysis. Moreover, mercury exposure risk, as estimated via tissue concentrations, differed among wetland units depending upon whether blood or egg mercury concentrations were sampled. Mercury concentrations in feathers also were uncorrelated with mercury concentrations in eggs, and were poorly correlated with mercury concentrations in blood. These results demonstrate the potential for contrasting management actions that may be implemented based solely on the specific avian tissue that is sampled, and highlight the importance of developing avian tissues as biomonitoring tools for assessing local risk of mercury exposure to bird reproduction.

Nest attendance, incubation constancy, and onset of incubation in dabbling ducks.

In birds, parents must provide their eggs with a safe thermal environment suitable for embryonic development. Species with uniparental incubation must balance time spent incubating eggs with time spent away from the nest to satisfy self-maintenance needs. Patterns of nest attendance, therefore, influence embryonic development and the time it takes for eggs to hatch. We studied nest attendance (time on the nest), incubation constancy (time nests were at incubation temperatures), and variation in nest temperature of 1,414 dabbling duck nests of three species in northern California. Daily nest attendance increased from only 1-3% on the day the first egg was laid to 51-57% on the day of clutch completion, and 80-83% after clutch completion through hatch. Variation in nest temperature also decreased gradually during egg-laying, and then dropped sharply (33-38%) between the day of and the day after clutch completion because increased nest attendance, particularly at night, resulted in more consistent nest temperatures. During the egg-laying stage, nocturnal nest attendance was low (13-25%), whereas after clutch completion, nest attendance was greater at night (≥87%) than during the day (70-77%) because most incubation recesses occurred during the day. Moreover, during egg-laying, nest attendance and incubation constancy increased more slowly among nests with larger final clutch sizes, suggesting that the number of eggs remaining to be laid is a major driver of incubation effort during egg-laying. Although overall nest attendance after clutch completion was similar among species, the average length of individual incubation bouts was greatest among gadwall (Mareca strepera; 779 minutes), followed by mallard (Anas platyrhynchos; 636 minutes) and then cinnamon teal (Spatula cyanoptera; 347 minutes). These results demonstrate that dabbling ducks moderate their incubation behavior according to nest stage, nest age, time of day, and clutch size and this moderation likely has important implications for egg development and overall nest success.

Predator movements in relation to habitat features reveal vulnerability of duck nests to predation.

Nest predation is the main cause of nest failure for ducks. Understanding how habitat features influence predator movements may facilitate management of upland and wetland breeding habitats that reduces predator encounter rates with duck nests and increases nest survival rates. For 1618 duck nests, nest survival increased with distance to phragmites (Phragmites australis), shrubs, telephone poles, human structures, and canals, but not for four other habitat features. Using GPS collars, we tracked 25 raccoons (Procyon lotor) and 16 striped skunks (Mephitis mephitis) over 4 years during waterfowl breeding and found marked differences in how these predators were located relative to specific habitat features; moreover, the probability of duck nests being encountered by predators differed by species. Specifically, proximity to canals, wetlands, trees, levees/roads, human structures, shrubs, and telephone poles increased the likelihood of a nest being encountered by collared raccoons. For collared skunks, nests were more likely to be encountered if they were closer to canals, trees, and shrubs, and farther from wetlands and human structures. Most predator encounters with duck nests were attributable to a few individuals; 29.2% of raccoons and 38.5% of skunks were responsible for 95.6% of total nest encounters. During the central span of duck nesting (April 17-June 14: 58 nights), these seven raccoons and five skunks encountered >1 nest on 50.8 ± 29.2% (mean ± SD) and 41.5 ± 28.3% of nights, respectively, and of those nights individual raccoons and skunks averaged 2.60 ± 1.28 and 2.50 ± 1.09 nest encounters/night, respectively. For collared predators that encountered >1 nest, a higher proportion of nests encountered by skunks had evidence of predation (51.9 ± 26.6%) compared to nests encountered by raccoons (22.3 ± 17.1%). Because duck eggs were most likely consumed as raccoons and skunks opportunistically discovered nests, managing the habitat features those predators most strongly associated with could potentially reduce rates of egg predation.

Nocturnal incubation recess and flushing behavior by duck hens.

Incubating birds must balance the needs of their developing embryos with their own physiological needs, and many birds accomplish this by taking periodic breaks from incubation. Mallard (Anas platyrhynchos) and gadwall (Mareca strepera) hens typically take incubation recesses in the early morning and late afternoon, but recesses can also take place at night. We examined nocturnal incubation recess behavior for mallard and gadwall hens nesting in Suisun Marsh, California, USA, using iButton temperature dataloggers and continuous video monitoring at nests. Fourteen percent of all detected incubation recesses (N = 13,708) were nocturnal and took place on 20% of nest-days (N = 8,668). Video monitoring showed that hens covered their eggs with down feathers when they initiated a nocturnal recess themselves as they would a diurnal recess, but they left the eggs uncovered in 94% of the nocturnal recesses in which predators appeared at nests. Thus, determining whether or not eggs were left uncovered during a recess can provide strong indication whether the recess was initiated by the hen (eggs covered) or a predator (eggs uncovered). Because nest temperature decreased more rapidly when eggs were left uncovered versus covered, we were able to characterize eggs during nocturnal incubation recesses as covered or uncovered using nest temperature data. Overall, we predicted that 75% of nocturnal recesses were hen-initiated recesses (eggs covered) whereas 25% of nocturnal recesses were predator-initiated recesses (eggs uncovered). Of the predator-initiated nocturnal recesses, 56% were accompanied by evidence of depredation at the nest during the subsequent nest monitoring visit. Hen-initiated nocturnal recesses began later in the night (closer to morning) and were shorter than predator-initiated nocturnal recesses. Our results indicate that nocturnal incubation recesses occur regularly (14% of all recesses) and, similar to diurnal recesses, most nocturnal recesses (75%) are initiated by the hen rather than an approaching predator.

Interrupted incubation: How dabbling ducks respond when flushed from the nest.

Nesting birds must provide a thermal environment sufficient for egg development while also meeting self-maintenance needs. Many birds, particularly those with uniparental incubation, achieve this balance through periodic incubation recesses, during which foraging and other self-maintenance activities can occur. However, incubating birds may experience disturbances such as predator or human activity which interrupt natural incubation patterns by compelling them to leave the nest. We characterized incubating mallard Anas platyrhynchos and gadwall Mareca strepera hens' responses when flushed by predators and investigators in Suisun Marsh, California, USA. Diurnal incubation recesses initiated by investigators approaching nests were 63% longer than natural diurnal incubation recesses initiated by the hen (geometric mean: 226.77 min versus 142.04 min). Nocturnal incubation recesses, many of which were likely the result of predators flushing hens, were of similar duration regardless of whether the nest was partially depredated during the event (115.33 [101.01;131.68] minutes) or not (119.62 [111.96;127.82] minutes), yet were 16% shorter than natural diurnal incubation recesses. Hens moved further from the nest during natural diurnal recesses or investigator-initiated recesses than during nocturnal recesses, and the proportion of hen locations recorded in wetland versus upland habitat during recesses varied with recess type (model-predicted means: natural diurnal recess 0.77; investigator-initiated recess 0.82; nocturnal recess 0.31). Hens were more likely to take a natural recess following an investigator-initiated recess earlier that same day than following a natural recess earlier that same day, and natural recesses that followed an investigator-initiated recess were longer than natural recesses that followed an earlier natural recess, suggesting that hens may not fulfill all of their physiological needs during investigator-initiated recesses. We found no evidence that the duration of investigator-initiated recesses was influenced by repeated visits to the nest, whether by predators or by investigators, and trapping and handling the hen did not affect investigator-initiated recess duration unless the hen was also fitted with a backpack-harness style GPS-GSM transmitter at the time of capture. Hens that were captured and fitted with GPS-GSM transmitters took recesses that were 26% longer than recesses during which a hen was captured but a GPS-GSM transmitter was not attached. Incubation interruptions had measurable but limited and specific effects on hen behavior.

Avian eggshell thickness in relation to egg morphometrics, embryonic development, and mercury contamination.

Eggshell thickness is important for physiological, ecological, and ecotoxicological studies on birds; however, empirical eggshell thickness measurements for many species and regions are limited. We measured eggshell thickness at the equator and the egg poles for 12 avian species and related eggshell thickness to egg morphometrics, embryonic development, egg status, and mercury contamination. Within an egg, eggshells were approximately 5.1% thicker at the equator than the sharp pole of the egg, although this difference varied among species (0.6%-9.8%). Within Forster's tern (Sterna forsteri), where eggshell thickness was measured at 5 equally spaced positions along the longitude of the egg, eggshell thickness changed more rapidly near the sharp pole of the egg compared to near the blunt pole of the egg. Within species, eggshell thickness was related to egg width and egg volume for six of the 12 species but was not related to egg length for any species. Among species, mean eggshell thickness was strongly related to species mean egg width, egg length, egg volume, and bird body mass, although species mean body mass was the strongest predictor of species mean eggshell thickness. Using three species (American avocet [Recurvirostra americana], black-necked stilt [Himantopus mexicanus], and Forster's tern), whose nests were carefully monitored, eggshell thickness (including the eggshell membrane) did not differ among viable, naturally abandoned, dead, or failed-to-hatch eggs; was not related to total mercury concentrations of the egg content; and did not decrease with embryonic age. Our study also provides a review of all existing eggshell thickness data for these 12 species.

Mercury exposure in mammalian mesopredators inhabiting a brackish marsh.

Bioaccumulation of environmental contaminants in mammalian predators can serve as an indicator of ecosystem health. We examined mercury concentrations of raccoons (Procyon lotor; n = 37 individuals) and striped skunks (Mephitis mephitis; n = 87 individuals) in Suisun Marsh, California, a large brackish marsh that is characterized by contiguous tracts of tidal marsh and seasonally impounded wetlands. Mean (standard error; range) total mercury concentrations in adult hair grown from 2015 to 2018 were 28.50 µg/g dw (3.05 µg/g dw; range: 4.46-81.01 µg/g dw) in raccoons and 4.85 µg/g dw (0.54 µg/g dw; range: 1.53-27.02 µg/g dw) in striped skunks. We reviewed mammalian hair mercury concentrations in the literature and raccoon mercury concentrations in Suisun Marsh were among the highest observed for wild mammals. Although striped skunk hair mercury concentrations were 83% lower than raccoons, they were higher than proposed background levels for mercury in mesopredator hair (1-5 µg/g). Hair mercury concentrations in skunks and raccoons were not related to animal size, but mercury concentrations were higher in skunks in poorer body condition. Large inter-annual differences in hair mercury concentrations suggest that methylmercury exposure to mammalian predators varied among years. Mercury concentrations of raccoon hair grown in 2017 were 2.7 times greater than hair grown in 2015, 1.7 times greater than hair grown in 2016, and 1.6 times greater than hair grown in 2018. Annual mean raccoon and skunk hair mercury concentrations increased with wetland habitat area. Furthermore, during 2017, raccoon hair mercury concentrations increased with the proportion of raccoon home ranges that was wetted habitat, as quantified using global positioning system (GPS) collars. The elevated mercury concentrations we observed in raccoons and skunks suggest that other wildlife at similar or higher trophic positions may also be exposed to elevated methylmercury bioaccumulation in brackish marshes.

Timing, frequency, and duration of incubation recesses in dabbling ducks.

Nest attendance is an important determinant of avian reproductive success, and identifying factors that influence the frequency and duration of incubation recesses furthers our understanding of how incubating birds balance their needs with those of their offspring. We characterized the frequency and timing (start time, end time, and duration) of incubation recesses for mallard (Anas platyrhynchos) and gadwall (Mareca strepera) hens breeding in Suisun Marsh, California, USA, and examined the influences of day of year, ambient temperature at the nest, incubation day, and clutch size on recess frequency and timing using linear mixed models. Mallard, on average, took more recesses per day (1.69 ± 0.80, mean ± standard deviation) than did gadwall (1.39 ± 0.69), and 45% of mallard nest-days were characterized by two recesses, while only 27% of gadwall nest-days were characterized by two recesses. Mallard morning recesses started at 06:14 ± 02:46 and lasted 106.11 ± 2.01 min, whereas mallard afternoon recesses started at 16:39 ± 02:11 and lasted 155.39 ± 1.99 min. Gadwall morning recesses started at 06:30 ± 02:46 and lasted 91.28 ± 2.32 min, and gadwall afternoon recesses started at 16:31 ± 01:57 and lasted 192.69 ± 1.89 min. Mallard and gadwall started recesses earlier in the day with increasing ambient temperature, but later in the day as the season progressed. Recess duration decreased as the season progressed and as clutch size increased, and increased with ambient temperature at the nest. The impending darkness of sunset appeared to be a strong cue for ending a recess and returning to the nest, because hens returned to their nests earlier than expected when recesses were expected to end after sunset. Within hens, the timing of incubation recesses was repeatable across incubation days and was most repeatable for mallard afternoon recesses and on days in which hens took only one recess. Hens were most likely to be away from nests between 04:00 and 07:00 and between 16:00 and 19:00; therefore, investigators should search for nests between 07:00 and 16:00. Our analyses identified important factors influencing incubation recess timing in dabbling ducks and have important implications for nest monitoring programs.

Mercury contamination in resident and migrant songbirds and potential effects on body condition.

Methlymercury is a significant risk to environmental health globally. We examined the ecological drivers of methylmercury bioaccumulation in songbirds and its effect on body condition while experimentally removing the potentially confounding and predominant effects of site and habitat. We measured blood and feather mercury concentrations and body condition in nearly 1200 individuals representing resident or migrant songbirds of 52 species and 5 foraging guilds. Songbird mercury concentrations differed among species, foraging guilds, residency status, dates, and ages, but not sexes. Blood mercury concentrations 1) ranged from 0.003 in house finch to 0.85 µg/g ww in American robin, 2) were 125 times greater in insectivores than granivores and 3.6 times greater in insectivores than omnivores, 3) were 3.3 times greater in summer residents than in migrating songbirds, 4) increased by 25% throughout spring and summer, and 5) were 45% higher in adults than juveniles. Songbird mercury concentrations were negatively correlated with body condition, with blood mercury concentrations decreasing by 44% and 34% over the range of standardized body masses and fat scores, respectively. Our results highlight the importance of foraging and migration ecology in determining methylmercury contamination in birds, and the potential for reduced body condition with methylmercury exposure in songbirds.

Sitting ducklings: Timing of hatch, nest departure, and predation risk for dabbling duck broods.

For ground-nesting waterfowl, the timing of egg hatch and duckling departure from the nest may be influenced by the risk of predation at the nest and en route to wetlands and constrained by the time required for ducklings to imprint on the hen and be physically able to leave the nest. We determined the timing of hatch, nest departure, and predation on dabbling duck broods using small video cameras placed at the nests of mallard (Anas platyrhynchos; n = 26), gadwall (Mareca strepera; n = 24), and cinnamon teal (Anas cyanoptera; n = 5). Mallard eggs began to hatch throughout the day and night, whereas gadwall eggs generally started to hatch during daylight hours (mean 7.5 hr after dawn). Among all species, duckling departure from the nest occurred during daylight (98%), and 53% of hens typically left the nest with their broods 1-4 hr after dawn. For mallard and gadwall, we identified three strategies for the timing of nest departure: (a) 9% of broods left the nest the same day that eggs began to hatch (6-12 hr later), (b) 81% of broods left the nest the day after eggs began to hatch, and (c) 10% of broods waited 2 days to depart the nest after eggs began to hatch, leaving the nest just after the second dawn (27-42 hr later). Overall, eggs were depredated at 10% of nests with cameras in the 2 days prior to hatch and ducklings were depredated at 15% of nests with cameras before leaving the nest. Our results suggest that broods prefer to depart the nest early in the morning, which may best balance developmental constraints with predation risk both at the nest and en route to wetlands.

Prey fish returned to Forster's tern colonies suggest spatial and temporal differences in fish composition and availability.

Predators sample the available prey community when foraging; thus, changes in the environment may be reflected by changes in predator diet and foraging preferences. We examined Forster's tern (Sterna forsteri) prey species over an 11-year period by sampling approximately 10,000 prey fish returned to 17 breeding colonies in south San Francisco Bay, California. We compared the species composition among repeatedly-sampled colonies (≥ 4 years), using both relative species abundance and the composition of total dry mass by species. Overall, the relative abundances of prey species at seven repeatedly-sampled tern colonies were more different than would be expected by chance, with the most notable differences in relative abundance observed between geographically distant colonies. In general, Mississippi silverside (Menidia audens) and topsmelt silverside (Atherinops affinis) comprised 42% of individuals and 40% of dry fish mass over the study period. Three-spined stickleback (Gasterosteus aculeatus) comprised the next largest proportion of prey species by individuals (19%) but not by dry mass (6%). Five additional species each contributed ≥ 4% of total individuals collected over the study period: yellowfin goby (Acanthogobius flavimanus; 10%), longjaw mudsucker (Gillichthys mirabilis; 8%), Pacific herring (Clupea pallasii; 6%), northern anchovy (Engraulis mordax; 4%), and staghorn sculpin (Leptocottus armatus; 4%). At some colonies, the relative abundance and biomass of specific prey species changed over time. In general, the abundance and dry mass of silversides increased, whereas the abundance and dry mass of three-spined stickleback and longjaw mudsucker decreased. As central place foragers, Forster's terns are limited in the distance they forage; thus, changes in the prey species returned to Forster's tern colonies suggest that the relative availability of some fish species in the environment has changed, possibly in response to alteration of the available habitat.

Maternal transfer of mercury to songbird eggs.

We evaluated the maternal transfer of mercury to eggs in songbirds, determined whether this relationship differed between songbird species, and developed equations for predicting mercury concentrations in eggs from maternal blood. We sampled blood and feathers from 44 house wren (Troglodytes aedon) and 34 tree swallow (Tachycineta bicolor) mothers and collected their full clutches (n = 476 eggs) within 3 days of clutch completion. Additionally, we sampled blood and feathers from 53 tree swallow mothers and randomly collected one egg from their clutches (n = 53 eggs) during mid to late incubation (6-10 days incubated) to evaluate whether the relationship varied with the timing of sampling the mother's blood. Mercury concentrations in eggs were positively correlated with mercury concentrations in maternal blood sampled at (1) the time of clutch completion for both house wrens (R2 = 0.97) and tree swallows (R2 = 0.97) and (2) during mid to late incubation for tree swallows (R2 = 0.71). The relationship between mercury concentrations in eggs and maternal blood did not differ with the stage of incubation when maternal blood was sampled. Importantly, the proportion of mercury transferred from mothers to their eggs decreased substantially with increasing blood mercury concentrations in tree swallows, but increased slightly with increasing blood mercury concentrations in house wrens. Additionally, the proportion of mercury transferred to eggs at the same maternal blood mercury concentration differed between species. Specifically, tree swallow mothers transferred 17%-107% more mercury to their eggs than house wren mothers over the observed mercury concentrations in maternal blood (0.15-1.92 µg/g ww). In contrast, mercury concentrations in eggs were not correlated with those in maternal feathers and, likewise, mercury concentrations in maternal blood were not correlated with those in feathers (all R2 < 0.01). We provide equations to translate mercury concentrations from maternal blood to eggs (and vice versa), which should facilitate comparisons among studies and help integrate toxicity benchmarks into a common tissue.

A critical evaluation of the utility of eggshells for estimating mercury concentrations in avian eggs.

Eggshells are a potential tool for nonlethally sampling contaminant concentrations in bird eggs, yet few studies have examined their utility to represent mercury exposure. We assessed mercury concentrations in eggshell components for 23 bird species and determined whether they correlated with total mercury (THg) in egg contents. We designed a multi-experiment analysis to examine how THg is partitioned into eggshell components, specifically hardened eggshells, material adhered to the eggshells, and inner eggshell membranes. The THg concentrations in eggshells were much lower than in egg contents, and almost all of the THg within the eggshell was contained within material adhered to eggshells and inner eggshell membranes, and specifically not within calcium-rich hardened eggshells. Despite very little mercury in hardened eggshells, THg concentrations in hardened eggshells had the strongest correlation with egg contents among all eggshell components. However, species with the same THg concentrations in eggshells had different THg concentrations in egg contents, indicating that there is no global predictive equation among species for the relationship between eggshell and egg content THg concentrations. Furthermore, for all species, THg concentrations in eggshells decreased with relative embryo age. Although the majority of mercury in eggshells was contained within other eggshell components and not within hardened eggshells, THg in hardened eggshells can be used to estimate THg concentrations in egg contents, if embryo age and species are addressed. Environ Toxicol Chem 2017;36:2417-2427. Published 2017 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Season, molt, and body size influence mercury concentrations in grebes.

We studied seasonal and physiological influences on mercury concentrations in western grebes (Aechmophorus occidentalis) and Clark's grebes (A. occidentalis) across 29 lakes and reservoirs in California, USA. Additionally, at three of these lakes, we conducted a time series study, in which we repeatedly sampled grebe blood mercury concentrations during the spring, summer, and early fall. Grebe blood mercury concentrations were higher among males (0.61 ± 0.12 µg/g ww) than females (0.52 ± 0.10 µg/g ww), higher among Clark's grebes (0.58 ± 0.12 µg/g ww) than western grebes (0.51 ± 0.10 µg/g ww), and exhibited a strong seasonal pattern (decreasing by 60% from spring to fall). Grebe blood THg concentrations exhibited a shallow, inverse U-shaped pattern with body size, and was lowest among the smallest and largest grebes. Further, the relationship between grebe blood mercury concentrations and wing primary feather molt exhibited a shallow U-shaped pattern, where mercury concentrations were highest among birds that had not yet begun molting, decreased approximately 24% between pre-molt and late molt, and increased approximately 19% from late molt to post-molt. Because grebes did not begin molting until mid-summer, lower grebe blood mercury concentrations observed in late summer and early fall were consistent with the onset of primary feather molt. However, because sampling date was a much stronger predictor of grebe mercury concentrations than molt, other seasonally changing environmental factors likely played a larger role than molt in the seasonal variation in grebe mercury concentrations. In the time series study, we found that seasonal trends in grebe mercury concentrations were not consistent among lakes, indicating that lake-specific variation in mercury dynamics influence the overall seasonal decline in grebe blood mercury concentrations. These results highlight the importance of accounting for sampling date, as well as ecological processes that may influence mercury concentrations, when developing monitoring programs to assess site-specific exposure risk of mercury to wildlife.