Effects of Avian Eggshell Oiling With Diluted Bitumen Show Sublethal Embryonic Polycyclic Aromatic Compound Exposure

Breeding birds that become oiled may contaminate the shells of their eggs, and studies of conventional crude oil suggest that even small quantities can be absorbed through the eggshell and cause embryotoxicity. Unconventional crude oils remain untested, so we evaluated whether a major Canadian oil sands product, diluted bitumen (dilbit), would be absorbed and cause toxicity when applied to eggshells of two species, domestic chicken (Gallus gallus domesticus) and double‐crested cormorant (Nannopterum auritum). We artificially incubated eggs and applied lightly weathered dilbit (Cold Lake blend) to the eggshells (0.015–0.15 mg g⁻¹ egg in chicken; 0.1–0.4 mg g⁻¹ egg in cormorant) at various points during incubation before sampling prehatch embryos. Polycyclic aromatic compound (PAC) residue in cormorant embryos was elevated only at the highest dilbit application (0.4 mg g⁻¹ egg) closest (day 16) to sampling on day 22. In contrast, cormorant liver cytochrome P450 1a4 (Cyp1a4) mRNA expression (quantitative polymerase chain reaction assay) was elevated only in embryos treated with the earliest and lowest dilbit application (0.1 mg g⁻¹ egg on day 4). These results confirm that dilbit can cross through the eggshell and be absorbed by embryos, and they imply rapid biotransformation of PACs and a nonmonotonic Cyp1a4 response. Despite evidence of exposure in cormorant, we found no detectable effects on the frequency of survival, deformity, and gross lesions, nor did we find effects on physiological endpoints indicative of growth and cardiovascular function in either chicken or cormorant. In ovo dilbit exposure may be less toxic than well‐studied conventional crude oils. The effects of an oil spill scenario involving dilbit to bird embryos might be subtle, and PACs may be rapidly metabolized. Environ Toxicol Chem 2022;41:159–174. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.