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Article

Mercury Concentration and Distribution in Remiges, Rectrices, and Contour Feathers of the Barn Swallow Hirundo rustica

by
Luca Canova
1,*,
Federica Maraschi
1,
Roberto Ambrosini
2,
Alessandra Costanzo
2,
Marco Parolini
2,
Antonella Profumo
1,
Andrea Romano
2,
Diego Rubolini
2 and
Michela Sturini
1
1
Department of Chemistry, University of Pavia, 27100 Pavia, Italy
2
Department of Environmental Science and Policy, University of Milan, 20133 Milan, Italy
*
Author to whom correspondence should be addressed.
Environments 2025, 12(7), 249; https://doi.org/10.3390/environments12070249
Submission received: 22 May 2025 / Revised: 9 July 2025 / Accepted: 15 July 2025 / Published: 18 July 2025
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Abstract

Feathers are commonly used to monitor trace elements in birds, including heavy metals. Typically, a single feather is analyzed to avoid harming living birds, assuming it reflects the organism’s overall contamination. To verify this assumption, we analyzed mercury concentrations in 12 flight and contour feathers from 25 barn swallows Hirundo rustica (16 adults and nine juveniles) that had died accidentally in a colony of the Po Plain (northern Italy). The median concentration in all feathers examined was 1.03 µg g−1 in adults (range 0.76 µg g−1–1.30 µg g−1) and 0.39 µg g−1 in juveniles (range 0.28 µg g−1–0.71 µg g−1), which is consistent with the results of similar research carried out on other world regions. No significant differences were observed between sexes, whereas marked differences were observed between adults and juveniles. In adults, mercury concentration was similar across remiges, rectrices, and contour feathers while in juveniles it was higher in contour feathers than in flight feathers. Mercury accumulation was highest in primary remiges and contour feathers, accounting for 67.6% of total mercury in adults and 77.5% in juveniles. However, primary remiges cannot be collected from live adults due to their importance in flight. In juveniles, contour feathers carry about 50% of total mercury, suggesting ventral and dorsal plumage may be useful for assessing mercury burden. Our findings are consistent with the hypothesis that mercury accumulation in feathers aids detoxification, with early-molted feathers (primary remiges and contour feathers) containing higher mercury levels than those replaced later (rectrices and secondary remiges).

1. Introduction

Mercury (Hg) is a non-essential trace element that is mobilized from geological deposits by both natural and anthropogenic processes [1,2]. Natural sources of mercury include volcanoes and geological deposits of cinnabar, and deposition and transport is largely atmospheric, explaining its widespread distribution. Industrial activities such as mining, energy production, and electroplating contribute to the spread of the metal in the environment [3,4].
Once released into the environment, inorganic mercury (iHg) is methylated to methylmercury (MeHg), a neurotoxin that bioaccumulates in organisms and biomagnifies in trophic networks [5,6]. The bioaccumulation of MeHg has adverse effects on wildlife, which are more severe in predators such as marine mammals, bats, and insectivorous and fish-eating birds ([7,8] for a review). The main adverse effects of MeHg in wildlife concern the alteration of coordination and behavior or reduction in reproductive success, which can contribute to population declines [5,8,9]. The exposure to and the bioaccumulation of MeHg occurs almost exclusively through the diet and absorption takes place in the gastrointestinal tract [10]. Once ingested, it is progressively accumulated and the excretion process is slow, which can prolong its harmful effects in the body. The potential risk posed to homeothermic vertebrates by mercury, which we consider as a proxy of MeHg, explains why it is the subject of extensive research [11]. Another reason is that, unlike other trace elements, its routes of uptake and accumulation in organisms are well understood. Consequently, in biomonitoring studies of MeHg, researchers can exclude secondary contamination processes (such as external deposition or deposition via gland secretions), thereby reducing uncertainty about the metal’s origin and facilitating the understanding of its excretion processes [12,13].
In birds, mercury bioaccumulates in various tissues and appendages [7]. In this group of vertebrates, contamination by mercury occurs through feeding [12,13] and the process is better known than for other metals (i.e., Pb, Cd, Fe, and others), for which it occurs by various processes that can include external contamination, uptake via respiration, and trace elements redistribution on the body by uropygial secretions [12]. Birds can remove mercury from the blood and internal tissues by storing it in the inert structure of their feathers [14,15]. The feather remains receptive to mercury until it is connected to the bloodstream during the molt: mercury has a high affinity for proteins such as keratin, a sulfhydryl-rich protein that makes up developing feathers [16]. During this phase, mercury is transferred into the growing feathers as the body load gradually decreases. At the end of the molt, the feather disconnects from blood circulation and retains stable levels of the metal. In contrast, mercury bioaccumulates again in the internal tissues because the excretion pathway has been blocked. It is estimated that 50 to 93% of the total body burden of mercury is excreted through the molt [17,18,19]. For this reason, feathers have become the most widely used tissue for monitoring mercury and other heavy metal contamination in birds. In addition, they are easy to collect and store, and their removal has little or no impact on the welfare of the animals. Finally, the protocols for the collection and analysis of mercury from this tissue are rather effective [20].
The use of feathers as a probe of mercury contamination assumes that the concentrations observed in this tissue are related to those in internal organs. This has been confirmed in some bird species [21,22]. However, other studies have shown that mercury in the feathers does not accurately reflect recent exposure [23,24,25,26,27] and that there may be a discrepancy between its concentrations in the feather and other body compartments. The presence of such relevant differences can be related to a variety of factors, such as the type of feather used, its structural complexity, and the size, age, sex, and taxonomy of the birds sampled [12,23,28,29]. However, one of the most important factors is the different timing of feather growth during the molt, because mercury concentrations should decrease as this metal is excreted in previously molted feathers. Indeed, studies conducted so far have confirmed that the concentration of the metal in the primary remiges of some species tends to decrease from the innermost—i.e., closest to the bird’s body—remige to the outermost [30,31,32]. This makes the pattern of mercury excretion interpretable according to feather molt order, but it also increases the variability of concentrations in different feathers and may ultimately affect the predictive value of mercury in feathers relative to that in the body [2].
Consequently, the question of which feathers or groups of feathers are the most reliable indicators of mercury contamination remains unanswered, and an analysis of the available literature suggests that no single feather—which would represent what [29] has brilliantly called the ‘Holy Grail’ of ecotoxicologists—can accurately predict the level of contamination of a bird.
Spatial and temporal variability of metal levels and bioavailability add further complexity to the interpretation of contamination patterns in feathers. For instance, raptors, as predators, are good mercury bioaccumulators, but studies on this taxon have often relied on feathers taken from opportunistically harvested carcasses, which are collected over very large areas and long periods of time [31,33]. In other cases, the molting pattern of a species is poorly understood, leaving the problem of decoupling between mercury stored in feathers and that present in the body open [34,35]. In other cases, migration may affect the contamination patterns, as in the case of barn swallows Hirundo rustica: in a previous study [36] we demonstrated that feathers collected from adults are evidence of contamination that occurred in the wintering grounds, whereas current and contingent contamination can only be analyzed using nestling feathers. Finally, an accurate study of excretion patterns requires two different protocols: either sampling feathers from a large number of individuals, which increases the sources of variability as many individuals have different and unknown life histories and ecologies, or sampling several feathers from a few well-monitored individuals, which can hardly be performed on wild birds [37,38].
This study aims at analyzing the concentration and amount of mercury in the feathers of the barn swallow, a small insectivorous and migratory bird breeding in a large part of the temperate European region. The barn swallow is a sinanthropic species widely distributed across Europe, where it breeds semi-colonially in human buildings, often on old farms. This charismatic species is considered a symbol of low-impact agriculture and can be considered a flagship species of rural habitats [39]. In addition, the nests of this species are easy to locate and access, and the nestlings are very resistant to the stress of manipulation, as testified by the extensive studies conducted on this species [40]. These feathers make the barn swallow an ideal species for environmental monitoring. It is also a long-distance migrant: post-reproductive migration occurs from August through September, with European birds heading to sub-Saharan Africa. Reproductive migration starts in February, depending on weather and food availability. Once they arrive to their breeding grounds, barn swallows spread across Europe and begin to reproduce [41]. Local populations can show strong declines; however, the species is widespread and is considered of Least Concern by the IUCN.
We collected the feathers of 25 individuals (16 adults and nine juveniles) who had died accidentally in a colony in the Po Valley (NW Italy) and checked whether the mercury levels in a particular feather of a group of feathers (primary and secondary remiges, rectrices, and contour feathers) better represent the total amount of the metal in the plumage. This information would be valuable to identify how to sample alive individuals to properly assess their contamination levels without causing unnecessary harm to individuals.

2. Materials and Methods

2.1. Area and Study Species

During a long-term monitoring of barn swallow breeding populations [39], 25 barn swallows (16 adults and 9 juveniles) were found dead in July 2022 in a colony in the province of Cremona (NW Italy; 45°08′53.82′′ N; 9°52′47.58′′ E). The climate in the area is mild-continental with mean monthly temperatures during the barn swallow breeding season (April to August) spanning 13.5–24.9 °C and mean monthly precipitations of 47–88 mm. The investigated colony is on a farm surrounded by extensive maize fields (51% within 400 m from the farm, corresponding to the foraging area of barn swallows during the breeding period [39]), hayfields (26%), and other cereal fields (10%). All sampled birds had entered an old, abandoned house near to their nesting site and probably were unable to get out, dying of starvation. The house was then closed to avoid the death of other individuals. Overall, 19 pairs nested on the farm in that year. Dead individuals were collected, stored in plastic bags, and transported to the laboratory. Before this, the bodies of the juveniles were carefully examined to ensure that their feathers were fully developed, meaning they were no longer connected to the bloodstream. This was performed to assure that the cause of death did not affect the contamination levels of the feathers. In the lab, we collected 9 flight feather and 3 contour feather samples from the body of each dead swallow. Three primary remiges P9, P5, and P1, three secondary remiges S1, S4, and S7 (counting inwards), and the three rectrices R1, R4, and R6 (counting outwards) were plucked, together with three samples of dorsal (PD), ventral (PV), and throat feathers (PG); contour feather samples were analyzed at a weight equal to the average weight (0.012 ± 0.001 g) of the other feather types in order to make the results of the analyses comparable. Overall, 279 flight or contour feathers were removed and stored for chemical analyses.

2.2. Chemical Analysis

The sample treatment and analysis were carried out as detailed in [36]. Feathers were washed with deionized water, placed in an ultrasonic bath for 5 min with 1 M acetone solution, then rinsed in ultrapure water, and finally air-dried. Each feather was accurately weighed and microwave-digested (1600 W, 15 min, 200 °C, Mars 5, CEM s.r.l., Cologno al Serio, Italy) with a mixture of 5 mL of Trace-SELECT® Ultra ultrapure HNO3 (65% w/w) and 2 mL of H2O2 (30% w/w). After cooling, the digested samples were evaporated by an XpressVapTM accessory (CEM s.r.l., Cologno al Serio, Italy) to a small volume (approximately 0.5 mL) and diluted to 5 mL with ultrapure water in calibrated polypropylene tubes before analysis. Elements measurement was performed by an inductively coupled plasma quadrupole mass spectrometer (ICP-MS) (Elan DRC-e, PerkinElmer, Shelton, CT, USA) equipped with a standard ICP torch, crossflow nebulizer, nickel sampler, skimmer cones, and dynamic reaction cell™ (DRC). External calibration curves were generated by diluting a commercial multi-element standard solution (Merck VI for ICP-MS, Sigma Aldrich, Milan, Italy). Method detection and quantification limits (MDLs and MQLs, respectively) were obtained from the instrumental detection and quantification limits (IDLs and IQLs, respectively) calculated using the residual standard deviation (Sy/x) of the linear regression parameters as (3.3 × Sy/x)/slope and (10 × Sy/x)/slope, respectively, and are referred to in the overall procedure. Blanks, consisting of reagents only, and a certified reference material (BCR-397, trace elements in human hair, recoveries 87–100% RSD; 10% for Cd, Hg, Pb, Se) were submitted to the digestion procedure described above.

2.3. Statistical Analyses

Differences in mercury concentration and amount in feathers were evaluated by linear mixed models where the individual identity was entered as a random grouping factor. Models included age, sex, and the type of feather as factorial fixed effects and were followed by Tukey post hoc comparisons among feathers. Further analyses with mixed models with the same random structure were run separately for adults and juveniles to investigate differences in feather types (primary remiges, secondary remiges, rectrices, and contour feathers). To assess repeatability in mercury concentrations among feathers of the same individual, we relied on both ANOVA-based repeatability [42] and the intraclass correlation coefficient (ICC) of a mixed model that included only the individual identity as a random grouping factor. ICC values were then interpreted according to [43]: ICC < 0.40: poor repeatability; 0.40 ≤ ICC ≤ 0.59: fair repeatability; 0.60 ≤ ICC ≤ 0.74: good repeatability; ICC ≥ 0.75: excellent repeatability. Analyses were performed using the R statistical software version 3.6.2, SPSS version 26, and Past [44].

3. Results

Median concentrations of mercury in the 12 feathers belonging to four main categories of feathers (primary and secondary remiges, rectrices, and contour feathers) are the first outcome of our study; overall, we observed a low intra-individual variability of concentration in the 16 adults and nine juveniles analyzed (Table 1). Indeed, the measures on different feathers of the same individual were repeatable (F24,253 = 32.94, p < 0.001), and the ICC was 0.749, indicating excellent repeatability [41]. The median concentration in all feathers examined was 1.03 µg g−1 in adults (range 0.76 µg g−1–1.30 µg g−1) and 0.39 µg g−1 in juveniles (range 0.28 µg g−1–0.71 µg g−1). In adults, the highest median values were observed in the ventral down, primary remige 9 (P9), and rectrix 4 (T4), whereas in juveniles the highest median values were observed in the ventral and subgular down, rectrix 1 (T1), and remige 9 (P9). A mixed model including as fixed effects the age, sex, and a factor indicating the feather considered showed that mercury concentration did not differ significantly between males and females (F1,21,955 = 0.149, p = 0.703), while adults showed higher values than juveniles (F1,21.964 = 6.399, p = 0.019), and there were significant differences between feathers (F11,242,056 = 1.868, p = 0.044). Post hoc tests showed that the only significant differences were between PV and P5 (t242 = 3.433, p = 0.033) and between PV and T1 (t242 = 3.829, p = 0.009), while all the other pairwise differences were not significant (|t|242 ≤ 2.992, p ≥ 0.117). In adults, mercury concentration varied randomly, except for a tendency to increase between S1 and S7, while a slight decrease was observed in the secondary remiges of juveniles (Table 1). In the contour feathers of adults and juveniles, there was a tendency for mercury concentration to increase between the dorsal and ventral downs, but data in Table 1 generally suggest a random distribution not associated with any discernible sequence of feathers.
The analyses on the four feather groups with different functions and positions in the swallow’s body (primary PI and secondary remiges SII, rectrices, and down) shows that concentration did not significantly differ in adults (Figure 1; F3,161.13 = 0.158, p = 0.420), whereas they did in juveniles (F3,86.134 = 13.900, p < 0.001), with contour feathers consistently showing significantly higher values than the other three groups of feathers (t86.3 ≥ 4.058, p < 0.001; |t|86.0 ≤ 2.594, p ≥ 0.053) in all the other cases. The data suggest that in adults, mercury is evenly distributed between the flight and contour feathers, whereas in juveniles, there appears to be a slight but significant tendency for more metal accumulation in the contour feathers (Figure 1).
We observed an inter-individual variability of an order of magnitude in mercury concentrations in each of the 12 feathers in adults, whereas in juveniles, the inter-individual variability was much smaller (Table 1; Figure 2). Since concentration values can be time-dependent or affected by characteristics such as density and structure of feathers [29], we converted the metal concentrations in each feather into an absolute amount, considering their weight (Table 1); then, we coupled the analyses of concentrations with an evaluation of quantities of metal in the feathers. Moreover, the determination of the amount of mercury in each feather is useful to define the extent of total excretion and to verify the contribution of each of the 12 feathers to the total amount stored in all feathers. For this reason, we converted the metal concentrations in each feather into an absolute amount, considering their weight (Table 1). The median amount of mercury in the feathers was 0.012 µg g−1 (0.003 µg g−1–0.18 µg g−1) for adults and 0.003 µg g−1 (0 µg g−1–0.03 µg g−1) for juveniles.
The percentage of the total amount of mercury (µg) in the feathers represented by the four categories of feathers with different functions and positions in the swallow’s body is reported in Figure 3. In adults, primary remiges and contour feathers accounted for 36.0% and 31.6% of the total amount of mercury in feathers, respectively, while secondary remiges and rectrices accounted for 14.2% and 18.2%. In juveniles, contour feathers and primary remiges accounted for 49.9% and 27.6% respectively, while rectrices and secondary remiges accounted for 16.0% and 8.3%. This first level of analysis suggests that when we analyzed the amount of mercury, contour feathers and primary remiges, especially in juveniles, were better related to the total amount of mercury in the feathers, although the maximums observed were always less than 50%. When we analyzed the contribution of each of the 12 feathers in the adults (Figure 4), none of the detected quantity was proportionally high, and only P9, dorsal down, and ventral down accounted, respectively, for 21.6%, 16.6%, and 16.8% of the total amount of mercury. Among others, only rectrices T6 exceeded 10% of the total amount of mercury in the feathers; in the other cases this percentage was always <10%. In juveniles, the primary remige P9, dorsal down, and ventral down accounted, respectively, for 14.0%, 15.4%, and 26.4% of the total amount of mercury in the feathers, confirming the low predictive value of individual feathers in representing the total mercury in the plumage.

4. Discussion

4.1. Mercury Contents in Barn Swallow Feathers

Analyses of metal concentrations in bird feathers are often limited by sample size; multiple feather samples from a single bird are only possible from dead birds or chicks, which can be sampled repeatedly over time [37]. Consequently, the choice of the feather (s) to sample is often limited, arbitrary, and determined by the conditions under which the researchers are working, for instance, rectrices [45,46], primary and secondary remiges [17,33], or both contour feathers and remiges [13,15,47]. Small passerines pose additional problems due to their small size and fragility, which make feather plucking from adults or juveniles risky and sometimes ethically unacceptable. In our case, the presence of 25 dead individuals from the same colony made it possible to work on a relevant sample of their feathers to assess the pattern of metal deposition in the different types of feathers.
Most studies of metal and mercury contamination report high variability in concentrations both between feather types and among individuals, but valid comparative data are abundant only for birds of prey [15,31,33,48] and seabirds [48,49]. In these taxa, mercury is highly biomagnified due to the trophic level of the predator and is easily analyzed due to the size of the feathers. Similar data on small passerine birds are much rarer, and comparative studies are limited, although they are becoming more common environmental indicators due to their abundance, wide distribution, and position in the food chain [50].
The mercury concentrations reported in this study (Table 1) were highly variable both between and within individuals, ranging in our samples from 0.07 to 7.43 μg g−1 in adults and from 0 to 1.35 μg g−1 in juveniles. These values were similar to those observed by [51] in a mixed sample of adults and juveniles (i.e., from 0.30 to 4.34 μg g−1) of eastern populations of barn swallows, while [52] found concentrations of 1.33 μg g−1, 1.19 μg g−1, and 1.30 μg g−1 in adults from three Canadian populations and 0.64 μg g−1 and 0.87 μg g−1 in juveniles. These data were also similar to those found in adults from other populations sampled in northern Italy (1.24 μg g−1 [36]; 1.03 μg g−1, Table 1) and in juveniles (0.45 μg g−1 [53]; 0.39 μg g−1, Table 1). On a local scale, these data confirm the suitability of the barn swallow as a biomonitor of metal contamination in agricultural environments [36,53]; on a wider scale, they show a relative invariance of mercury concentrations in geographically very distant populations of barn swallows, suggesting that the excretion of mercury in feathers is, at least partly, physiologically controlled.
No significant differences in mercury were observed between adult males and females. The barn swallows of our population undergo a complete annual molt in the African wintering grounds and a partial molt of the body feathers at the end of the breeding season in Europe [54,55]. Molting is a process that accelerates the swallow’s metabolism [56,57] and induces the mobilization of proteins and lipids [58], increasing the release of bioaccumulated mercury into the bloodstream and its subsequent storage in the feathers. The lack of significant differences in mercury concentrations in the feathers of males and females confirms that the frequency of molting prior to spring migration does not differ significantly between males and females, as shown by [59].
In contrast, we found significantly higher mercury concentrations in adults than in juveniles, consistent with reports in other bird species [8,60,61]. In larger species, this difference is explained by an accelerated molting rate in juveniles compared to adults [62]. Nestlings and juveniles usually have lower mercury concentrations because they have undergone low bioaccumulation due to their young age and a more effective metabolic excretion [63,64]. In addition, rapid and continuous growth during the juvenile phase dilutes the mercury burden, resulting in lower feather concentrations when compared to adults [28]. In some marine species, this difference is also explained by the selection of different foraging areas where prey that have a lower mercury load are exploited by adults to provide food for nestlings [65]. The barn swallow is an insectivorous bird that selects prey primarily by size, and their inclusion in the diet depends more on the absolute abundance of large rather than small prey [66]. The diet of chicks consists of Coleoptera and Hymenoptera followed by Diptera [67,68], with no apparent difference between the diet of juveniles and adults. In addition, there is no evidence that adults feed in areas other than where they collect food for the chicks [66]. The only differences could be in the size of prey ingested by the adult or brought to the chicks, which is unlikely to explain the differences in mercury concentrations observed in adult and juvenile feathers. This difference is most likely due to physiological processes related to metabolism and the progressive increase in mass of the chick, which dilutes the body burden of mercury, although alternative explanations exist. For example, ref. [69] observed that several passerine species that overwinter and molt in Africa, including the barn swallow, have much higher concentrations of various metals in their feathers than resident species [70]. This pattern has been explained by the greater availability of trace elements in central Africa compared to Europe [71].

4.2. Prediction of Individual Mercury Exposure by Specific Feathers and Feather Categories

The usefulness of feathers as biomonitors of mercury contamination has recently been questioned on several grounds, ranging from the high variability in concentration between feather types of the same individual, the discrepancy between mercury concentrations in feathers and blood or other tissues, to the possibility that part of the mercury is of exogenous origin and differentially contaminates the feathers most exposed to the atmosphere. However, these objections are countered by others defending the use of feathers as biomonitors. First, several studies attest to a markedly endogenous origin of the mercury found in the feathers, which is ingested and subsequently excreted through the blood that nourishes the feather [15,28,72,73,74,75]. Moreover, variability in mercury concentrations in primary feathers may follow the molting pattern and, therefore, a process of individual detoxification, which ultimately supports the value of feathers as biomonitoring tools. Recently, ref. [51] found mercury concentrations of 0.42 µg g−1 in barn swallow bones, 0.64 µg g−1 in the liver, 0.48 µg g−1 in muscles, and 2.17 µg g−1 in feathers. This confirms that feathers are the main deposit of the metal, accumulating on average 64.4% of the total body burden of mercury in swallows, thus confirming that feathers can be used to monitor mercury contamination at least in this species.
The results of our study show that the concentration of mercury is repeatable in the feathers of barn swallows, with only contour feathers showing a slight tendency for higher mercury values (Figure 1 and Figure 2). Adults and juveniles also describe two different contamination events, since the feathers carried by adults in the European breeding grounds are largely mutated in the African wintering grounds and therefore testify the contamination experienced by the swallow in the foraging areas of Africa [36]. The mercury in the feathers of juveniles, on the other hand, represents a great proportion of the metal accumulation experienced by the chick during the weaning period after hatching; although evidence of maternal transfer of mercury into eggs has been confirmed, the efficiency of transfer and the amount in passerines are generally low [76,77].
Quantifying the mercury contained in groups of feathers or in the 12 feathers allowed us to analyze the dynamics of mercury release in feathers (Figure 3 and Figure 4). Adults store most of the mercury in the primary remiges (36% of the total amount in feathers) and in the contour feathers (31.6%): hypothetically, the amount of mercury contained in these feathers could represent 75% of the total mercury load in the individual and could be considered as a predictor of total amount of mercury in the plumage. From a practical point of view, however, remiges do not represent an option for sampling live birds because their removal can seriously compromise flight efficiency. Contour feathers taken from the ventral, dorsal, and subgular parts can be a better option, as they can represent a relevant part of the total amount of mercury in the feathers and can be collected without harming the individual.
We also analyzed the distribution of mercury concentrations and amounts in the 12 feathers to test the hypothesis that storing the metal in the feathers represents a detoxification process for the swallows. If the storage of mercury is a way of eliminating mercury from the body, it would be expected that the first feathers to molt would be those accumulating the highest concentrations and amounts of metal, and that these values would tend to decrease as the molt progressed. Indeed, the higher levels of mercury found in primary and contour feathers appear to be consistent with the description of molting given by [54,78]. In the barn swallow molting proceeds in stages, starting with contour feathers and primary remiges and then moving on to rectrices [66]. As the primary remiges molt from the inner to the outer, the pattern shown in Figure 3 and Figure 4 is consistent with a massive release in the innermost primary remige (P9) and in the contour feathers of the dorsal and ventral plumage. These data support the idea that there is a tendency to remove metals by storing them in the first feathers to molt.

4.3. The Issue of External Contamination

The possibility that feather categories such as contour feathers and remiges may be representative of the level of individual contamination is suggestive and finds some support in our data. However, some authors have attributed the high levels of trace elements in contour feathers and associated down to the structure of this feather category, which is able to retain more exogenous metallic particles and therefore show greater amounts of metals [15,16,73,79]. This criticism requires further investigation and experimental verification, but, in our opinion, it is not entirely convincing, as it implies an insufficient cleaning of the sample and therefore a methodological error [80,81]. Second, if the structure of contour feathers and down favors external contamination, we should observe a difference between the concentration of metals in the ventral and dorsal plumage of juveniles since the former is always in contact with the nest floor, which is made of mud containing a high quantity of trace elements, and with fecal contamination, whereas the latter is only exposed to air. From the data in Figure 2 and Figure 4, we did not observe such differences in concentration, and only in juveniles the ventral plumage seems to carry more mercury than the dorsal plumage. We recognize that external contamination can be a real problem for some metals, but it may be a secondary issue for mercury because of both trophic uptake and endogenous excretion routes; moreover, in other vertebrates such as small rodents, the amount of mercury in mammalian hair does not vary with the intensity of hair washing during pretreatment [82]. Another study found that a preliminary washing with diluted nitric acid, the solvent we used in this study, was efficient in removing external contamination from waterbird feathers [83]. Waterbirds typically transfer preen oil from the uropygial gland to the body feathers; if nitric acid is effective at removing most trace elements that cause external contamination, such as lead and arsenic, in their feathers, it should be even more so for birds such as swallows, which use less preen oil than waterbirds, and for metals that accumulate internally, such as mercury [84]. Therefore, we consider that external metal contamination cannot explain a higher concentration of mercury in contour plumage; in our opinion, since contour plumage is one of the first to molt, it is likely to receive greater amounts of the mercury that the organism demobilizes during molting.

5. Conclusions

To the best of our knowledge, this is one of the few studies investigating the pathways of mercury deposition in specific parts of the feathers of small insectivorous birds. The results show that mercury accumulation in both adult and juvenile swallows remains below threshold levels in bird tissues (0.05 μg g−1; [7]), and therefore, there is no apparent risk of adverse physiological effects in our population. Comparison with the little information available in the literature shows a remarkable similarity between our data and those collected in China, Canada, and the USA. There may be a strong physiological control on barn swallow excretion levels and perhaps the potential for large-scale comparisons of mercury contamination in this species.
Adults had much higher mercury loads than juveniles. This is probably due to the juveniles’ metabolism and accelerated growth, which constantly dilutes and reduces the concentration and amount of mercury transferred to their feathers.
The mercury concentration was similar in the adults’ four groups of flight, tail, and contour feathers, while in juveniles, it was significantly higher in contour feathers. The amount of mercury, expressed as a percentage of the total mercury load in the feathers of each individual, was high in the primary remiges and contour feathers of both adults and juveniles, and cumulatively represents 67.6% of the total mercury load in adults and 77.5% in juveniles. Compared to the innermost remiges, P9 is the single feather that stores most mercury in the plumage (22% in adults and 14.5% in juveniles); its removal and generally the removal of any remiges or sections of them is not an option for mercury studies on this species because these flight feathers are of outmost importance for flight efficiency in this long-distance migrator.
Contour feathers may be the best target for mercury contamination studies, particularly in juveniles, accounting for about 50% of the total mercury load in the plumage. The distribution of mercury in the 12 feathers seems consistent with the hypothesis that the accumulation of mercury in the feathers is a detoxification process and that the amount of mercury deposited in the feathers is density-dependent, with the first feathers receiving the highest metal load.
Further studies should analyze the feather contamination by other essential and non-essential metals. Although their excretion processes are more variable and the routes of entry into the bird body are multiple [84] such a study could provide a clearer picture of the metal contamination risks to barn swallows in our study area.

Author Contributions

L.C., M.P., R.A. and M.S.: conceptualization; M.S., F.M., D.R., L.C. and R.A.: methodology; F.M., M.P., A.C., A.R. and M.S.: investigation; A.P. and L.C.: resources; L.C., M.S., R.A. and M.P. writing—original draft preparation; all authors: writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This study was partially supported by the Fondazione Comunitaria Provincia di Lodi through the project “Proteggere la rondine, specie bandiera dell’ambiente agricolo lodigiano” (ExtraBand 2018/01). Luca Canova, Michela Sturini, Federica Maraschi and Antonella Profumo acknowledge MUR and the University of Pavia through the program “Dipartimenti di Eccellenza 2023–2027”.

Data Availability Statement

The data are available from the corresponding authors.

Acknowledgments

We are grateful to the owners of Cascina Benpensata that allowed us to conduct this study in their house and to Chiara Bensai for her contribution to preliminary analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
[Hg]mercury concentration

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Environments 12 00249 g001
Figure 1. Box-and-whisker plot of the mercury median concentrations (µg g−1) for primary remiges, secondary remiges, rectrices, and contour feathers of adult (n = 16) and juvenile barn swallows (n = 9).
Figure 1. Box-and-whisker plot of the mercury median concentrations (µg g−1) for primary remiges, secondary remiges, rectrices, and contour feathers of adult (n = 16) and juvenile barn swallows (n = 9).
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Figure 2. Box-and-whisker plot of the mercury median concentrations (µg g−1) in each feather for all the 12 feathers of each adult (n = 16) and juvenile (n = 9). P1, P5, and P9 are primary remiges; S1, S4, and S7 are secondary remiges; T1, T4, and T6 are rectrices; PD, PG, and PV are dorsal, subgular, and ventral contour feathers (details in Section 2.1).
Figure 2. Box-and-whisker plot of the mercury median concentrations (µg g−1) in each feather for all the 12 feathers of each adult (n = 16) and juvenile (n = 9). P1, P5, and P9 are primary remiges; S1, S4, and S7 are secondary remiges; T1, T4, and T6 are rectrices; PD, PG, and PV are dorsal, subgular, and ventral contour feathers (details in Section 2.1).
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Figure 3. Bar chart (±SE) of the percentage amount of mercury for each group of feathers (primary remiges, secondary remiges, rectrices, and contour feathers) in adult (n = 16) and juvenile (n = 9) barn swallows.
Figure 3. Bar chart (±SE) of the percentage amount of mercury for each group of feathers (primary remiges, secondary remiges, rectrices, and contour feathers) in adult (n = 16) and juvenile (n = 9) barn swallows.
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Figure 4. Bar chart (±SE) of the percentage amount of mercury for each feather in relation to the total amount present in each type of feather in adults (n = 16, (left)) and juveniles (n = 9, (right)). P1, P5, and P9 are primary remiges; S1, S4, and S7 are secondary remiges; T1, T4, and T6 are rectrices; PD, PG, and PV are dorsal, subgular, and ventral contour feathers (details in Section 2.1).
Figure 4. Bar chart (±SE) of the percentage amount of mercury for each feather in relation to the total amount present in each type of feather in adults (n = 16, (left)) and juveniles (n = 9, (right)). P1, P5, and P9 are primary remiges; S1, S4, and S7 are secondary remiges; T1, T4, and T6 are rectrices; PD, PG, and PV are dorsal, subgular, and ventral contour feathers (details in Section 2.1).
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Table 1. Median, minimum, and maximum of mercury concentrations (µg g−1) and mercury excreta (µg) in the 12 adult and juvenile feathers of barn swallows (P1, P5, P9 = primary remiges; PD, PG, PV = dorsal, subgular, and ventral contour feathers; S1, S4, S7 = secondary remiges; T1, T4, T6 = rectrices; more details in Section 2).
Table 1. Median, minimum, and maximum of mercury concentrations (µg g−1) and mercury excreta (µg) in the 12 adult and juvenile feathers of barn swallows (P1, P5, P9 = primary remiges; PD, PG, PV = dorsal, subgular, and ventral contour feathers; S1, S4, S7 = secondary remiges; T1, T4, T6 = rectrices; more details in Section 2).
AdultsJuveniles AdultsJuveniles
Concentration (µg g−1)Amount (µg)
HeaderMedianMinMaxMedianMin Max MedianMin Max MedianMin Max
P11.100.143.470.380.201.280.0100.0100.0300.0020.0020.010
P50.830.143.400.340.140.600.0120.0020.0630.0040.0020.012
P91.250.205.370.420.100.770.0420.0060.1800.0080.0020.024
PD0.760.193.790.360.341.080.0290.0070.1320.0210.0110.034
PG0.920.093.660.520.271.140.0130.0010.0420.0070.0040.016
PV1.300.436.580.710.351.350.0170.0120.1260.0170.0060.031
S10.850.193.740.300.120.570.0070.0010.0330.0020.0010.006
S41.140.097.530.290.060.610.0070.0010.1560.002<0.0010.004
S71.210.074.320.280.070.620.006<0.0010.0270.001<0.0010.003
T10.780.143.040.450.070.530.0050.0010.0280.002<0.0010.003
T41.240.186.850.340.031.140.0090.0010.0620.004<0.0010.011
T61.110.175.470.520.160.870.0140.0020.0650.0030.0010.010
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MDPI and ACS Style

Canova, L.; Maraschi, F.; Ambrosini, R.; Costanzo, A.; Parolini, M.; Profumo, A.; Romano, A.; Rubolini, D.; Sturini, M. Mercury Concentration and Distribution in Remiges, Rectrices, and Contour Feathers of the Barn Swallow Hirundo rustica. Environments 2025, 12, 249. https://doi.org/10.3390/environments12070249

AMA Style

Canova L, Maraschi F, Ambrosini R, Costanzo A, Parolini M, Profumo A, Romano A, Rubolini D, Sturini M. Mercury Concentration and Distribution in Remiges, Rectrices, and Contour Feathers of the Barn Swallow Hirundo rustica. Environments. 2025; 12(7):249. https://doi.org/10.3390/environments12070249

Chicago/Turabian Style

Canova, Luca, Federica Maraschi, Roberto Ambrosini, Alessandra Costanzo, Marco Parolini, Antonella Profumo, Andrea Romano, Diego Rubolini, and Michela Sturini. 2025. "Mercury Concentration and Distribution in Remiges, Rectrices, and Contour Feathers of the Barn Swallow Hirundo rustica" Environments 12, no. 7: 249. https://doi.org/10.3390/environments12070249

APA Style

Canova, L., Maraschi, F., Ambrosini, R., Costanzo, A., Parolini, M., Profumo, A., Romano, A., Rubolini, D., & Sturini, M. (2025). Mercury Concentration and Distribution in Remiges, Rectrices, and Contour Feathers of the Barn Swallow Hirundo rustica. Environments, 12(7), 249. https://doi.org/10.3390/environments12070249

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