Effects of mercury exposure on the reproductive success of tree swallows (Tachycineta bicolor)

An experimental tree swallow population was established in the headwaters of the Shenandoah River, Virginia, USA to assess the accumulation and effects of mercury contamination on birds that eat emergent aquatic insects. One tributary, the South River, was contaminated with mercury before 1950. Reproductive success of swallows nesting within 50 m of this river was compared to that of three uncontaminated reference tributaries in 2005 and 2006. Female swallows on the contaminated stretch of river had significantly elevated blood and feather total mercury (blood: 3.56 ± 2.41 ppm ww vs. 0.17 ± 0.15 ppm reference; feather: 13.55 ± 6.94 ppm vs. 2.34 ± 0.87 ppm reference), possibly the highest ever reported for an insectivorous songbird. Insects collected by the swallows to be fed to nestlings averaged 0.97 ± 1.11 ppm dw total mercury, significantly higher than on reference sites. Swallows in the contaminated area produced fewer fledglings than those in reference areas. The effect of mercury contamination on productivity was detectable only for young females in the contaminated area that were breeding for the first time in 2006, a segment of the population that may already have been stressed by inexperience. Tree swallows served as practical and effective biomonitors for mercury levels and effects and have great potential as proxy biomonitors for more logistically challenging birds such as loons or eagles.     

Productivity of tree swallows (Tachycineta bicolor) exposed to PCBs at the Kalamazoo River superfund site

A 123-km stretch of the Kalamazoo River in Michigan, was designated a Superfund site in 1990 due to historical releases of effluent containing polychlorinated biphenyl (PCB)-contaminated paper waste. Risk to bird species in the river ecosystem was evaluated using the tree swallow (Tachycineta bicolor) as a monitor for possible effects due to PCB exposure at two nesting locations, one in the Superfund site and one in an upstream reference location that is less contaminated with PCBs. In 2 of the 3 years of the study, clutch size at the contaminated location was 3.7 +/- 1.4 and 4.8 +/- 0.73 eggs per nest (mean +/- SD), which was significantly less than the clutch size at the reference location (5.0 +/- 1.1 and 5.3 +/- 1.1 eggs per nest). However, there were no statistically significant differences in fledging success, predicted brood size, predicted number of fledglings, or growth of nestlings between the Kalamazoo River Superfund site and an upstream reference location with lesser concentrations of PCBs in the sediments and riparian soils. Productivity and hatching success comparisons between these same sites were also not significantly different; however, the power of these conclusions was less (p < .10). The reduction in clutch size at the co-contaminated location could not be attributed to PCBs due to a number of confounding factors, including Co-cocontaminants, habitat structure, and food availability. Other reproductive parameters were not significantly impaired, and the size of the newly established colony at the Kalamazoo River Superfund site continued to grow over the period of the study. These site-specific observations, combined with multiple lines of evidence approach that considered results reported for the effects of both total PCBs and 2,3,7,8 tetrachlorodibenzo-p-dioxin equivalents (TEQ) on tree swallows at other locations, suggest that there were no significant population-level effects of PCBs on tree swallows at the Kalamazoo River Superfund site.     

Effects of exposure to naphthenic acids in tree swallows (Tachycineta bicolor) on the Athabasca oil sands, Alberta, Canada

Naphthenic acids (NAs) are a group of carboxylic acids that are of particular concern to the steadily growing oil sands mining industry of Alberta, Canada, because they become highly concentrated in the water used for oil sands extraction and are toxic to aquatic biota and mammals. Upon mine closure, vast amounts of process-affected water will need to be reclaimed and proven safe for wildlife colonizing reclaimed areas. The effects of exposure to NAs have not been investigated in avian species. To address this void, tree swallow (Tachycineta bicolor) nestlings were dosed with NAs while being reared normally by their free-ranging parents on a site in the vicinity of the oil sands. Nestlings received 1.5 mg NAs/day (approximately 0.075 g/kg body mass) from d 7 to d 13 of age, which represented a 10-fold "worst exposure" scenario. Nestling growth, hematocrit, blood biochemistry, organ weights, and ethoxyresorufin O-deethylase (EROD) activity were unaffected by NAs. The only change detected on histopathological evaluation of major organs was an increase in extramedullary erythropoiesis in the liver. These findings indicate that nestling tree swallows can successfully tolerate short-term exposures to environmentally realistic concentrations of NAs. However, this study did not investigate the chronic or reproductive toxicity of NAs. More research needs to be conducted to complete this initial assessment, to determine environmental risks on reclaimed areas where birds will be breeding and where their exposure to NAs could extend for several weeks.     

The corticosterone stress response and mercury contamination in free-living tree swallows, Tachycineta bicolor

We determined mercury concentrations in tree swallows, Tachycineta bicolor, from Massachusetts and Maine with different levels of contamination. Baseline and stress-induced plasma corticosterone concentrations from adults and nestlings (Massachusetts only) were compared with mercury concentrations. In Massachusetts, adult baseline corticosterone was negatively correlated with blood mercury, but showed a nearly-significant positive correlation with feather mercury. There was a negative relationship between baseline corticosterone and blood mercury in nestlings and between baseline corticosterone and egg mercury. There was no relationship between mercury and stress-induced corticosterone in any of the groups, or with baseline corticosterone in Maine sites where mercury levels were lower. The findings suggest blood and egg mercury may be a better indicator of current condition than feather mercury. Further, mercury contamination may not alter stress-induced corticosterone concentrations in tree swallows but appears to have a significant impact on baseline circulating corticosterone.     

Scale-dependence in polychlorinated biphenyl (PCB) exposure effects on waterbird habitat occupancy

Spatial scale is rarely considered in population-level assessments of contaminant impacts on wild animals; as a result misinterpretation of the relationship between contaminant exposure and population status may occur. We assessed the strength of expression of polychlorinated biphenyl (PCB) exposure effects at local vs. regional spatial scales on population status in five species of waterbirds, “bioaccumulators” often promoted as indicators of contaminant effects in aquatic ecosystems. Our focus was the upper Hudson River where PCBs occur at levels reported to have adverse impacts on wild birds. At the local scale, waterbird habitat occupancy was estimated from 220 repeat surveys made between 2001 and 2010 along the same survey route divided into 25 contiguous river segments with markedly different PCB concentrations. At the regional scale, waterbird habitat occupancy in relation to proximity to the upper Hudson River was estimated across 1248 Breeding Bird Atlas survey blocks while controlling for region-wide variation in habitat availability. At the local scale, many associations of habitat and sampling covariates with species detection probabilities were evident but none, including PCB concentration, with habitat occupancy, extinction or colonization of a given river segment. At the regional scale, survey effort and habitat factors not related to PCB exposure were the most important drivers of waterbird occurrence although two species were more likely to occur farther from the contaminated river segment. Spatial scale clearly mediates expression of contaminant impacts on wild bird populations; large-scale, expert-generated databases provide an underused opportunity for better delineating the spatial scales at which population impacts occur and risk assessments should be performed.     

Pathology of chronic polychlorinated biphenyl (PCB) feeding in rats

The hepatocarcinogenic effect of Clophen A 30 and Clophen A 60 was tested in male weanling rats by long-term feeding over a period of 832 days. The mortality rate was investigated in 100-day intervals. In the first 800 days liver carcinoma accounted for 21% of necropsies in the Clophen A 60 group but only 2% of the necropsies in the Clophen A 30 group and none in the control animals. The tumors were first observed after 700 days. After 800 days hepatocellular carcinoma was the most common lesion observed in the Clophen A 60 animals (61%) whereas it was only observed in 3% of animals in the Clophen A 30 group and 2% in the controls. Preneoplastic lesions, such as foci of hepatocellular alterations and neoplastic nodules, were first observed after Day 500. The incidence of foci predominated in all time intervals, but an increase in neoplastic nodules and hepatocellular carcinomas was observed with increased time. There was a marked trend from foci to neoplastic nodule to hepatocellular carcinoma with time. The total mortality rate and the incidence of thymoma, inflammatory lesions of the urogenital tract, in the experiment were significantly reduced by Clophen administration. Whether this protective effect could be induced by polychlorinated biphenyls (PCBs) is discussed.     

Dopamine-dependent behavior in adult rats after perinatal exposure to purity-controlled polychlorinated biphenyl congeners (PCB52 and PCB180)

Since knowledge about toxic effects of non-dioxinlike (NDL) PCBs is fragmentary, regulatory panels have concluded that risk assessment of these congeners is hampered or impossible. As the dopaminergic system is one of the main targets in PCB-related neurotoxic effects after developmental exposure, we selected catalepsy induced by the dopamine receptor blocker haloperidol to characterize effects of the NDL congeners PCB52 and PCB180 in adult offspring from exposed rat dams. Rat dams were treated with PCB congeners by gavage using six dose levels (total doses: PCB52 – 0, 30, 100, 300, 1000 or 3000 mg/kg body wt.; PCB180 – 0, 10, 30, 100, 300, or 1000 mg/kg body wt.) to allow benchmark dose analysis of the results. Testing of adult offspring (starting at 180 days of age) for catalepsy induced by injection with haloperidol revealed slightly prolonged latencies to movement onset in female offspring exposed to PCB52. Exposure to PCB180 resulted in more pronounced effects, with generally reduced latencies in male offspring. These results indicate reduced dopaminergic activity after PCB52 exposure, whereas the outcome for PCB180 may be related to increased extracellular dopamine as reported in the literature. Benchmark dose analyses revealed that both PCB congeners exerted effects mainly at moderate exposure levels. Together, these results underline the importance of effects on the dopaminergic system as indicated by studies in human females after occupational PCB exposure.     

Exposure and effects of perfluoroalkyl compounds on tree swallows nesting at Lake Johanna in east central Minnesota, USA

Tree swallow (Tachycineta bicolor) samples were collected at a reference lake and a nearby lake (Lake Johanna) in east central Minnesota, USA contaminated with perfluorinated carboxylic and sulfonic acids. Tissues were analyzed for a suite of 13 perfluoroalkyl compounds (PFCs) to quantify exposure and to determine if there was an association between egg concentrations of PFCs and reproductive success of tree swallows. Concentrations of perfluoroocatane sulfonate (PFOS) were elevated in all tree swallow tissues from Lake Johanna compared to tissues collected at the reference lake. Other PFCs, except for two, were elevated in blood plasma at Lake Johanna compared to the reference lake. PFOS was the dominant PFC (>75%) at Lake Johanna, but accounted for <50% of total PFCs at the reference lake. There was a negative association between concentrations of PFOS in eggs and hatching success. Reduced hatching success was associated with PFOS levels as low as 150ng/g wet weight.     

Mercury exposure and survival in free-living tree swallows (<Emphasis Type="Italic">Tachycineta bicolor</Emphasis>)

Mercury has become a ubiquitous contaminant in food chains worldwide. A large body of literature detailing bioaccumulation and effects on birds has revealed the potential for mercury to adversely impact avian physiology and reproduction. However, the extent to which these effects impair survival remains poorly understood. The objective of this study was to determine whether mercury exposure was associated with reduced annual survivorship in tree swallows (Tachycineta bicolor) breeding at a site with legacy industrial contamination. From 2005 to 2008, we captured and marked 932 adult swallows. We used Cormack-Jolly-Seber models and an information-theoretic approach to test our hypotheses that adult survival varied by sex, breeding location, and cumulative individual mercury exposure. Blood mercury was significantly elevated on contaminated sites (2005–2007 combined mean ± SE: 2.84 ± 0.09 μg/g; reference: 0.17 ± 0.01 μg/g). Model-averaged estimates of female apparent survival ranged from 0.483 to 0.488 on reference sites and 0.473 to 0.477 on contaminated sites. For males, apparent survival ranged from 0.451 to 0.457 on reference sites and 0.444 to 0.448 on contaminated sites. Thus, we observed approximately a 1% difference in survival between mercury-contaminated and reference sites. Such a small difference is unlikely to impact population viability in this short-lived species; however, some songbirds accumulate mercury to a greater degree than tree swallows and do not possess the migratory behavior that removes swallows to less contaminated areas for the majority of the year. Identifying whether such species are at risk of suffering biologically significant reductions in survival should become a focus of future research.     

Distribution and elimination in vivo of polychlorinated biphenyl (PCB) isomers and congeners in the pigeon

1. Pigeons were injected with a single dose of commercial PCB mixtures (Aroclor 1248 plus Aroclor 1260), killed 120 h later and the abundance of individual PCBs was determined in adipose tissue, gonads, liver, brain, kidney, heart, muscle and blood. 2. Elimination factors for individual PCBs were calculated. Values of greater than 1 were obtained for PCBs with meta-para-unsubstituted carbon atoms in at least one ring, indicating that elimination exceeded accumulation in all or most tissues. By contrast, ortho-meta unsubstituted PCBs had elimination factors less than 1, thus indicating their impaired removal. 3. Tissues with high microsomal monooxygenase activity had the highest elimination factors for individual PCBs (i.e. liver greater than kidney greater than muscle greater than heart). 4. Distribution of individual PCBs was independent of sex and of ortho-chlorine substitution and showed that 90% of total PCBs in cadavers was present in adipose tissue, 2% in kidneys, 1% each in brain, muscle and heart and less than 0.1% in blood. 5. The distribution of the highly toxic non-ortho and mono-ortho substituted PCBs did not differ amongst all tissues analysed. 6. The present studies indicate that elimination of PCBs in vivo is favoured by the molecular feature of unsubstituted meta-para carbon atoms in the biphenyl moiety.     

Distribution and elimination in vivo of polychlorinated biphenyl (PCB) isomers and congeners in the pigeon

1. Pigeons were injected with a single dose of commerical PCB mixtures (Aroclor 1248 plus Aroclor 1260), killed 120 h later and the abundance of individual PCBs was determined in adipose tissue, gonads, liver, brain, kidney, heart, muscle and blood.

2. Elimination factors for individual PCBs were calculated. Values of >1 were obtained for PCBs with meta-para-unsubstituted carbon atoms in at least one ring, indicating that elimination exceeded accumulation in all or most tissues. By contrast, ortho-meta unsubstituted PCBs had elimination factors <1, thus indicating their impaired removal.

3. Tissues with high microsomal monooxygenase activity had the highest elimination factors for individual PCBs (i.e. liver> kidney > muscle > heart).

4. Distribution of individual PCBs was independent of sex and of ortho-chlorine substitution and showed that 90% of total PCBs in cadavers was present in adipose tissue, 2% in kidneys, 1% each in brain, muscle and heart and <0.1% in blood.

5. The distribution of the highly toxic non-ortho and mono-ortho substituted PCBs did not differ amongst all tissues analysed.

6. The present studies indicate that elimination of PCBs in vivo is favoured by the molecular feature of unsubstituted meta-para carbon atoms in the biphenyl moiety.     

Polychlorinated biphenyl (PCB) exposure in mothers and time to pregnancy in daughters

Developmental exposure to polychlorinated biphenyls (PCBs) disrupts reproduction in animals. Human data are lacking. We measured PCBs in preserved mothers' serum samples collected during 1960-1963, 1-3 days after their daughters' birth. We recorded time to pregnancy (TTP) in 289 daughters 28-31 years later. PCB congeners 187, 156, and 99 in mother's serum were associated with longer TTP in their daughters while PCB congeners 105, 138 and 183 were associated shorter TTP. Probability of pregnancy fell by 38% (95% CI 17-53%) and infertility was higher (30% not pregnant after 13 cycles versus 11% not pregnant after 13 cycles) among women whose mothers had a higher proportion of PCB congeners associated with longer TTP (75th percentile versus 25th percentile). This study demonstrates, for the first time, that developmental exposure to PCBs may disrupt pregnancy in humans.     

Apoptosis-mediated neurotoxic potential of a planar (PCB 77) and a nonplanar (PCB 153) polychlorinated biphenyl congeners in neuronal cell cultures

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants, some of which may be neurotoxic depending on the chemical structure of the congeners. This study investigated the neurotoxic potential of 3,3',4,4'-tetrachlorobiphenyl (TCB) (PCB 77, a non-ortho-substituted planar congener) and 2,2',4,4',5,5'-hexachlorobiphenyl (HCB) (PCB 153, a di-ortho-substituted nonplanar congener) by assessing cell viability and apoptotic cell death in neuronal cell cultures. We have combined morphological and biochemical techniques to establish the relevance of apoptosis in neuronal cell death induced by the two selected PCB congeners. Treatment with both planar and nonplanar congeners caused the loss of cell viability and accelerated apoptosis in a time- and concentration-dependent manner. However, the extent of apoptosis generated was greater for the non-ortho-substituted planar congener (PCB 77) than for the di-ortho-substituted nonplanar congener (PCB 153). This was correlated with the loss of cell viability since the planar congener was more cytotoxic. Based on our findings, the apoptosis induced by PCBs involves the increase of caspase-3 activity in neuronal cell cultures. It is reasonable to assume that PCB-induced apoptosis may be linked to the neurotoxic effect of these toxicants and that different molecular mechanisms may operate in the induction of apoptosis depending on the planarity or nonplanarity of the PCB congeners.     

Dioxin, dibenzofuran, and polychlorinated biphenyl (PCB) levels in food from Agent Orange-sprayed and nonsprayed areas of Laos

During the Vietnam War, a phenoxy-herbicide mixture called Agent Orange, which was contaminated with 2,3,7,8-tetrachlorodibenzodioxin, or TCDD, was used primarily for destruction of forest and other foliage in order to prevent enemy troop movement and protect American and allied troops and military bases in the south of Vietnam. Smaller areas of Laos and Cambodia were also sprayed with Agent Orange between 1962 and 1971 from fixed-wing aircraft. In 2001, 28 food samples consisting of meat, fish, and dairy products were collected in sprayed and nonsprayed areas of Laos and analyzed for dioxins, dioxinlike dibenzofurans, and selected polychlorinated biphenyl (PCB) congeners by high-resolution gas chromatography/high-resolution mass spectrometry at laboratories in Germany and Canada. Low levels of dioxin and other dioxinlike substances were found in Laotian food, similar to levels present in recent exported Vietnamese food to United States. Vietnam is geographically adjacent to Laos. Total dioxin toxic equivalent (TEQ) levels were similar in samples from sprayed and non-sprayed areas, ranging from 0.009 to 0.851 pg/g or parts per trillion (ppt) in sprayed Sepone, and from 0.022 to 0.537 pg/g or ppt wet weight in non sprayed Vientiane. However, the Laotian fish samples from the Agent Orange-sprayed area had, on average, a higher proportion of total TEQ from TCDD (31.7% vs. 9.4%) compared to the nonsprayed area. Some other food items, duck eggs and pork liver from Sepone, also had severalfold higher TCDD levels than similar food samples from Vientiane, 0.029 vs. 0.011 pg/g and 0.070 vs. 0.004 pg/g wet weight, respectively. There were no substantial differences in levels of dibenzofuran and PCB congeners. In general, elevated TCDD levels were not found in Laotian food samples. It is possible that dioxin-contaminated areas, or "hot spots," exist in Laos as they do in Vietnam, although they have not yet been identified.     

Effects of gestational and lactational exposure to coplanar polychlorinated biphenyl (PCB) congeners or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on thyroid hormone concentrations in weanling rats

Perinatal exposure to polychlorinated biphenyl (PCB) mixtures or to certain ortho-substituted PCB congeners dramatically reduces circulating thyroxine (T4) concentrations. It is not clear whether perinatal exposure to coplanar PCBs or2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has a similar effect. In this study, time-mated Sprague-Dawley rats were dosed with 2 or 8 mg/kg/day PCB 77 (3,3′,4,4′-tetrachlorobiphenyl), 0.25 or 1.00 μ/kg/day PCB 126 (3,3′,4,4′,5-pentachlorobiphenyl), 0.025 or 0.10 μg/kg/day TCDD, or corn oil vehicle orally on gestation days 10–16. At weaning, plasma total T4 concentrations in PCB 77 and TCDD high-dose female pups were significantly depressed, but the changes were modest (84.4 and 79.6% of control, respectively). T4 concentrations in PCB 126 high-dose females and all high-dose males were also depressed slightly, but the changes were not statistically significant. UDP-Glucuronosyl transferase (UDP-GT) activity towards 4-nitrophenol was increased in all high-dose groups. Thus, the modest decreases in T4 could be due in part to increased T4 glucuronidation by UDP-GT. Triiodothyronine (T3) and thyroid stimulating hormone (TSH) concentrations were unchanged in all groups. In contrast to the minor changes in thyroid hormone status, liver microsomal ethoxyresorufin-O-deethylase (EROD) was markedly induced in all exposure groups and thymus weights were depressed in the high-dose groups. Because doses of coplanar PCBs or TCDD that caused marked induction of EROD activity had only minor effects on T4, we conclude that changes in thyroid hormone status at weaning are not among the more sensitive effects of perinatal exposure to these compounds.     

<Superscript>1</Superscript>H NMR metabolomics of earthworm responses to polychlorinated biphenyl (PCB) exposure in soil

¹H NMR-based metabolomics was used to examine the metabolic profile of D₂O-buffer extracted tissues of Eisenia fetida earthworms exposed for 2 days to an artificial soil spiked with sub-lethal concentrations of polychlorinated biphenyls (PCBs) (0, 0.5, 1, 5, 10, or 25 mg/kg Aroclor 1254). Univariate statistical analysis of the identified metabolites revealed a significant increase in ATP concentration in earthworms exposed to the highest soil PCB concentration, but detected no significant changes in other metabolites. However, a multivariate approach which considers alterations in multiple metabolites simultaneously, identified a significant linear relationship between earthworm metabolic profiles and PCB concentration (cross-validated PLS-regression with 7 components, R²X = 0.99, R²Y = 0.77, Q²Y = 0.45, P < 0.001). Significant changes in pair-wise metabolic correlations were also detected as PCB concentration increased. For example, lysine and ATP concentrations showed no apparent correlation in control earthworms (r = 0.22, P = 0.54), but were positively correlated in earthworms from the 25 mg/kg treatment (r = 0.87, P = 0.001). Overall, the observed metabolic responses suggest that PCBs disrupted both carbohydrate (energy) metabolism and membrane (osmolytic) function in E. fetida. The ability of ¹H NMR-based metabolomics to detect these responses suggests that this method offers significant potential for direct assessment of sub-lethal PCB toxicity in soil.     

Comparison of polychlorinated biphenyl (PCB) induced effects on innate immune functions in harbour and grey seals

Polychlorinated biphenyls (PCBs) are known to have detrimental effects on the innate immune system of several mammalian species. Top predators such as marine mammals may be badly affected as PCBs can bioaccumulate in their blubber to high concentrations and previous studies have suggested that harbour seals may be particularly vulnerable to the immunotoxic effects of such contaminants. To investigate the effects of PCBs on innate immune functions in phocid seals, blood samples were collected from harbour and grey seals and exposed in vitro to a mixture of Aroclors. Separated mononuclear (PBMCs) and polymorphonuclear (PMNCs) leukocytes from each species were incubated with Aroclors (at 3 and 30 ngml(-1)) for 3 and 24 h incubation periods, after which phagocytosis, respiratory burst and cytotoxic activity were measured. The phagocytic activity of harbour seal PMNCs was decreased at both incubation times and at both Aroclor concentrations tested, but there was no effect on the grey seals. Similarly, the respiratory burst activity of harbour seals was decreased at both incubation times, but only at the higher concentration used. There were no differences in the cytotoxic activity of the PBMCs with respect to incubation times or concentrations in either species. However, differences were observed in the level of cytotoxic activity against YAC-1 target cells, with the grey seal PBMCs showing higher levels of activity. The observed differences in phagocytosis, respiratory burst and cytotoxic activity of the leukocytes following incubation with PCBs may have implications for the previously recorded differences in disease susceptibility between grey and harbour seals.     

Health of tree swallow (Tachycineta bicolor) nestlings exposed to chlorinated hydrocarbons in the St Lawrence River basin. Part I. Renal and hepatic vitamin A concentrations

Sixteen-day-old tree swallows (Tachycineta bicolor), near fledging, were collected in 1999 and 2000 from nine sites representing a gradient of dioxin concentrations, within the vicinity of the St. Lawrence River in Canada and the United States, to determine if organochlorine contaminants correlated with vitamin A levels measured as retinol and retinyl palmitate. Mean concentrations of hepatic retinol ranged from 3 mg /kg to 13 mg /kg, and from 0.35 mg /kg to 1.5 mg /kg for renal retinol. Mean concentrations of hepatic retinyl palmitate ranged from 18 mg /kg to 146 mg /kg, and 1mg /kg to 6 mg/kg for renal retinyl palmitate. In 1999, molar ratio of renal retinol: retinyl palmitate was significantly and positively correlated with total polychlorinated dibenzodioxin (PCDD) concentration. Among sites, total PCDDs ranged from 5.4 ng /kg wet weight to 79.5 ng /kg wet weight in tree swallows. These results suggest that current levels of organochlorine contaminants in the St. Lawrence River and surrounding tributaries may be interacting with the vitamin A pathway. Lower circulating levels and higher tissue concentrations of retinoids may result in compromised immune function and reduced reproductive success in adult birds.