Author Posting. © The Author(s), 2016. This is the author's version of the work. It is posted here by permission of American Chemical Society for personal use, not for redistribution. The definitive version was published in Environmental Science & Technology 50 (2016): 10226–10235, doi:10.1021/acs.est.6b02108. ; Common and roseate terns are migratory piscivorous seabirds with major breeding colonies within feeding range of the PCB-contaminated New Bedford Harbor (NBH, MA, USA) Superfund site. Our longitudinal study shows that before PCB discharges into NBH ceased (late 1970s), tern eggs had very high but variable PCB concentrations. But egg concentrations of PCBs as well as DDE, the degradation product of the ubiquitous global contaminant DDT, have since declined. Rate constants for temporal decline of PCB congeners in tern eggs varied inversely with log10KOW (n-octanol-water partition coefficient), shifting egg congener patterns away from those characterizing NBH sediment. To estimate the toxic effects on tern eggs of PCB dioxin-like congener (DLC) exposures, we extrapolated published laboratory data on common terns to roseate terns by characterizing genetic and functional similarities in species aryl-hydrocarbon receptors (AHRs), which mediate DLC sensitivity. Our assessment of contaminant risks suggests that terns breeding near NBH were exposed historically to toxic levels of PCBs and DDE; however, acute effects on tern egg development have become less likely since the 1970s. Our approach demonstrates how comparative genetics at target loci can effectively increase the range of inference for chemical risk assessments from tested to untested and untestable species. ; This work was supported in part by NOAA grant number NA14OAR4170074 (Woods Hole Sea Grant Project R/P-80) and by National Institute of Environmental Health Sciences (NIEHS) grant P42ES007381 (Superfund Basic Research Program at Boston University). ; 2017-08-26