Toxic effects in adult birds appear to be quite variable, which may be a function of dosing scheme, PAH profile, biotransformation rate, lipid content, receptor affinity, or potency differences. For example, one study dosed adult mallards with a diet containing 0.4% PAHs (4000 mgg ^ for 7 months and found only minor effects. Even though the mixture of PAHs in this diet was composed mostly of LPAHs, these concentrations are extremely high, which may be a testament to the biotransformation ability for adults of this species. Another study with blackbirds and sparrows found LD50 values in the 100-200mgg~ range for various PAH compounds. It is unclear if the method of administration for this study (gavage) would produce a different result compared to that obtained for the mallard that was exposed via dietary ingestion.

PAHs appear to be very toxic to bird embryos, and the mallard has been shown to be one of the more sensitive species. Severe adverse effects have been observed when PAHs were added externally to a small area on mallard eggs. Increased mortality, decreased embryonic body weight, and increased abnormal survivors have been observed at several doses of chrysene, BaP, and 1,2-dimethylbenz[a]anthracene. Measured concentrations of PAHs in eggs indicate that effects were occurring at concentrations as low as 0.005-0.04 mgg1. Of course, measured concentrations should be adjusted for metabolism, especially if the tissue is sampled several days after the exposure. Bird eggs have been shown to be very efficient at metabolizing PAHs; one study showed that chicken eggs could metabolize more than 90% of the accumulated PAHs after 2 weeks. Once researchers determine the concentrations in eggs known to cause effects, comparison with the large database of measured values in field-collected animals will be very informative for determining risk for adverse effects.

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