Species Differences in the Sensitivity of Avian Embryos to Methylmercury
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We injected doses of methylmercury into the air cells of eggs of 26 species of birds and examined the dose–response curves of embryo survival. For 23 species we had adequate data to calculate the median lethal concentration (LC50). Based on the dose–response curves and LC50s, we ranked species according to their sensitivity to injected methylmercury. Although the previously published embryotoxic threshold of mercury in game farm mallards (Anas platyrhynchos) has been used as a default value to protect wild species of birds, we found that, relative to other species, mallard embryos are not very sensitive to injected methylmercury; their LC50 was 1.79 μg/g mercury on a wet-weight basis. Other species we categorized as also exhibiting relatively low sensitivity to injected methylmercury (their LC50s were 1 μg/g mercury or higher) were the hooded merganser (Lophodytes cucullatus), lesser scaup (Aythya affinis), Canada goose (Branta canadensis), double-crested cormorant (Phalacrocorax auritus), and laughing gull (Larus atricilla). Species we categorized as having medium sensitivity (their LC50s were greater than 0.25 μg/g mercury but less than 1 μg/g mercury) were the clapper rail (Rallus longirostris), sandhill crane (Grus canadensis), ring-necked pheasant (Phasianus colchicus), chicken (Gallus gallus), common grackle (Quiscalus quiscula), tree swallow (Tachycineta bicolor), herring gull (Larus argentatus), common tern (Sterna hirundo), royal tern (Sterna maxima), Caspian tern (Sterna caspia), great egret (Ardea alba), brown pelican (Pelecanus occidentalis), and anhinga (Anhinga anhinga). Species we categorized as exhibiting high sensitivity (their LC50s were less than 0.25 μg/g mercury) were the American kestrel (Falco sparverius), osprey (Pandion haliaetus), white ibis (Eudocimus albus), snowy egret (Egretta thula), and tri-colored heron (Egretta tricolor). For mallards, chickens, and ring-necked pheasants (all species for which we could compare the toxicity of our injected methylmercury with that of published reports where methylmercury was fed to breeding adults and was deposited into the egg by the mother), we found the injected mercury to be more toxic than the same amount of mercury deposited naturally by the mother. The rank order of sensitivity of these same three species to methylmercury was, however, the same whether the methylmercury was injected or maternally deposited in the egg (i.e., the ring-necked pheasant was more sensitive than the chicken, which was more sensitive than the mallard). It is important to note that the dose–response curves and LC50s derived from our egg injections are useful for ranking the sensitivities of various species but are not identical to the LC50s that would be observed if the mother bird had put the same concentrations of mercury into her eggs; the LC50s of maternally deposited methylmercury would be higher.
KeywordsMethylmercury Wild Bird Common Tern Great Egret Clapper Rail
This research was funded by the CALFED Bay-Delta Program’s Ecosystem Restoration Program (grant no. ERP–02D-C12) with additional support from the USGS Patuxent Wildlife Research Center. We thank Donna Podger and Carol Adkins of the California Bay-Delta Authority for help and project support. Steve Schwarzbach and Tom Suchanek provided much appreciated guidance and support during the development of the project, and Collin Eagles-Smith and Tom Maurer handled much of the work with reports and budget matters. We thank Kevin Brittingham, Michael Hammond, Michael Hoffman, and Dan Murray for their help in conducting the laboratory parts of the study and Julie Yee and Linda Green for statistical advice. We are very grateful to the following people who helped in one way or another in the collection and shipping of eggs to us: Ray Adams, Josh Ackerman, Terry Adelsbach, David Allen, Tom Augspurger, Wayne Bauer, Alicia Berlin, Sue Cameron, Melissa Duron, Collin Eagles-Smith, David Evers, Karen Gaines, Brian Heinz, Patricia Heinz, Michael Koterba, Jerry Longcore, Frank McGilvrey, Peter McGowan, Jason Miller, Jane Nicolich, Holliday Obrecht, Matthew Perry, Michael Rickard, Darren Rumbold, Lucas Savoy, and Paul Spitzer. Barnett Rattner and Natalie Karouna-Renier provided helpful reviews of the manuscript.
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