Mercury toxicity risk and corticosterone levels across the breeding range of the Yellow-breasted Chat

Abstract

Mercury (Hg) is an environmental contaminant that can negatively impact human and wildlife health. For songbirds, Hg risk may be elevated near riparian habitats due to the transfer of methylmercury (MeHg) from aquatic to terrestrial food webs. We measured Hg levels in tail feathers sampled across the breeding range of the Yellow-breasted Chat (Icteria virens), a riparian songbird species of conservation concern. We assessed the risk of Hg toxicity based on published benchmarks. Simultaneously, we measured corticosterone, a hormone implicated in the stress response system, released via the hypothalamus-pituitary-adrenal axis. To better understand range-wide trends in Hg and corticosterone, we examined whether age, sex, subspecies, or range position were important predictors. Lastly, we examined whether Hg and corticosterone were correlated. Hg levels in chats were relatively low: 0.30 ± 0.02 µg/g dry weight. 148 out of 150 (98.6%) had Hg levels considered background, and 2 (1.6%) had levels considered low toxicity risk. Hg levels were similar between sexes and subspecies. Younger chats (<1 year) had higher Hg levels than older chats (>1 year). Hg levels were lowest in the northern and central portion of the eastern subspecies' range. Corticosterone concentrations in feathers averaged 3.68 ± 0.23 pg/mm. Corticosterone levels were similar between ages and sexes. Western chats had higher levels of corticosterone than eastern chats. Hg and corticosterone were not correlated, suggesting these low Hg burdens did not affect the activity of the hypothalamus-pituitary-adrenal axis. Altogether, the chat has low Hg toxicity risk across its breeding range, despite living in riparian habitats.

Keywords: Corticosterone; Mercury; Songbird; Yellow-breasted Chat.

Fig. 1

Study areas for Yellow-breasted Chats (Icteria virens). The breeding ranges of the eastern (I.v. virens) and western (I.v. auricollis) subspecies are shown. Star symbols represent birds sampled from study areas in the breeding season, the square represents birds sampled from a study area in the overwintering season, and triangles represent birds sampled from study areas during migration. Map made in ArcMap 10.7.1 (ESRI 2019) using a WGS 1984 Web Mercator projection and coordinate system

Fig. 2

Average mercury concentrations in Yellow-breasted Chats (Icteria virens) in their breeding range. Pentagons are sites where chats were sampled during the breeding season. Circles are breeding sites inferred from stable hydrogen isotope analyses; a single point was used to map these inferred groups that span larger areas based on assignment probabilities. Sample sizes are indicated beside each point. The breeding range of the Yellow-breasted Chat is shown in stippled grey in the background (IUCN 2016). Map made in ArcMap 10.7.1 (ESRI 2019) using a WGS 1984 Web Mercator projection and coordinate system. Note that while higher mercury concentrations are shown as darker shades of red, all levels are still considered low risk of mercury toxicity

Fig. 3

Mercury concentrations in Yellow-breasted Chat (Icteria virens) feathers. Untransformed data and means are shown. The right outer tail feather was used. Range position is divided by subspecies (Western, W and Eastern E) and position within range (North, N; Central, C; and Southern, S). Sample sizes are W.N = 22, W.C = 42, W.S = 14, E.N = 15, E.C = 26, E.S = 31. Age includes the first moult cycle (n = 97) and the definitive moult cycle (n = 53). Sexes are female (F, n = 38), male (M, n = 94), and unknown (U, n = 18). The total sample size is 150. Parameters identified the top model are denoted with asterisks

Fig. 4

Corticosterone concentrations in Yellow-breasted Chat (Icteria virens) feathers. Untransformed data and means are shown. The right outer tail feather was used. Range position is divided by subspecies (Western, W and Eastern E) and position within range (North, N; Central, C; and Southern, S). Sample sizes are W.N = 23, W.C. = 31, W.S. = 12, E.N. = 21, E.C = 64, E.S. = 45. Age includes the first moult cycle (n = 148) and the definitive moult cycle (n = 48). The total sample size is 196. Note that one outlier with corticosterone concentration of 35.53 pg/mm is not visible. The parameter identified in the top model is denoted with an asterisk

Fig. 5

Correlation between mercury and corticosterone in Yellow-breasted Chat (Icteria virens) feathers. Feathers represent breeding sites throughout North America (N = 127). Both variables were transformed to meet assumptions of normality. Pearson r = −0.054, df = 125, p = 0.54