Recently, it was found that the mercury content in the feathers of seabird specimens preserved in the 1980s was only half of that of the current live birds of the same species. The accumulation of mercury in seabird feathers is caused by seabirds eating fish, which shows that the content of mercury in seafish is higher than that more than 100 years ago.
The hypothesis of the appeal argument is that after mercury from fish is absorbed by seabirds, the residual content in feathers will change with time.