Double-crested Cormorants self medicate by swallowing stones

A just-published paper suggests that Double-crested Cormorants (Phalacrocorax auritus) in the Great Lakes may swallow stones to reduce the number of parasitic nematodes–and in so doing, are self-medicating. Many birds ingest stones to help with the digestion of physically hard objects, but fish, the primary meal of the DCC are soft-bodied and don’t need the kind of mechanical assistance. So why are DCCs found with ingested stones?

Researchers found that:

At a Lake Ontario site, females more often had small stones in their stomachs and were less parasitized by nematodes than were males, and males with small stones had fewer nematodes than males without small stones. We did not find similar patterns in small stone presence or parasitism at a Lake Erie site; however, Lake Erie birds had fewer parasites and lower proportions of birds with small stones.

Which, in turn lead to the conclusion that since the birds that had stones were less likely to have nematodes, then the stones are the bird-equivalent of some anti-worm medication. Smart birds.

Small rant:

Interestingly, the Lake Ontario site was located at Presqu’ile Provincial Park and the Lake Erie site was located at Middle Island, Ontario. These are two sites where DCC culls have taken place in the recent past, with websites reporting a May 2008 cull on Middle Island that matches with the date in the paper’s method section. I can’t find any mention in the paper to the “source” of the birds or how the birds were “collected”.

Some may say there is no space for politics in scientific research and regardless about your feelings about recent DCC culls, I think that because the article makes no specific mention of how and where the researchers got their DCC gastrointestinal tracts, it does nothing but reduce the transparancy and therefore, the quality of the work.