LAUSR

&NA; 1,2‐dibromo‐4‐(1,2‐dibromoethyl)cyclohexane (DBE‐DBCH ‐ formerly TBECH) is an emerging brominated flame retardant (BFR) pollutant with androgen potentiating ability and other endocrine disrupting effects in birds and fish. The objectives of this study were to determine the effects of exposure to environmentally‐relevant levels of DBE‐DBCH on circulating levels of thyroid and sex steroid hormones in American kestrels, and if hormonal concentrations were related to previously reported changes in reproductive success and courtship behaviors. Sixteen kestrel pairs were exposed to 0.239 ng &bgr;‐DBE‐DBCH/g kestrel/day by diet, based on concentrations in wild bird eggs, from 4 weeks before pairing until the chicks hatched (mean 82 d), and were compared with vehicle‐only‐exposed control pairs (n=15). As previously reported, DBE‐DBCH concentrations were not detected in tissue or eggs of these birds, nor were any potential metabolites, despite the low method limits of detection (≤0.4 ng/g wet weight), suggesting it may be rapidly metabolized and/or eliminated by the kestrels. Nevertheless, exposed kestrels demonstrated changes in reproduction and behavior, indicating an effect from exposure. During early breeding, males were sampled at multiple time points at pairing and during courtship and incubation; females were blood sampled at pairing only; both sexes were sampled at the end of the season. All comparisons are made to control males or control females, and the relative differences in hormone concentrations between treatment and control birds, calculated separately for each sex, are presented for each time point. Males exposed to &bgr;‐DBE‐DBCH demonstrated significantly (p=0.05) lower concentrations of total thyroxine (TT4) overall, that were 11–28% lower than those of control males at the individual sampling points, yet significantly higher (p=0.03) concentrations of free thyroxine (FT4), that were 5–13% higher than those of control males at the individual sampling points; females had similar concentrations of TT4 and FT4 at the time of pairing, and T4 was similar in both sexes at the end of the breeding season. Testosterone (T) concentrations in the treatment males were significantly higher during early (85%) and mid‐courtship (30%) (time*treatment p=0.001), whereas females demonstrated a reduction in T at the time of pairing (17%, p=0.05). In the treatment females, concentrations of 17&bgr;‐estradiol (E2) showed a non‐significant decrease (20%) and were positively correlated with T concentrations (p=0.03); E2 concentrations were below quantification limits in males. For males, some variation in T was also significantly associated with their sexual behavior (p<0.001) and FT4 concentrations (p=0.01). For females, there was no relationship between hormones measured at pairing and subsequent sexual behaviors or reproductive measures. This study demonstrates that exposure to &bgr;‐DBE‐DBCH at levels that are likely below those experienced by wild birds, affects the thyroid and sex steroid axes in birds and thus may be a contaminant of concern for wildlife warranting further research. Graphical abstract Figure. No caption available. HighlightsKestrel pairs were exposed to &bgr;‐DBE‐DBCH during breeding.Female kestrels had reduced testosterone and 17&bgr;‐estradiol at pairing.Male kestrels demonstrated reduced total thyroxine but increased free thyroxine throughout breeding.Male kestrels demonstrated increased testosterone during breeding.Sex and thyroid axes are affected by exposure to &bgr;‐DBE‐DBCH in birds.