Migration and Parasites, Revisted

Last year, I blogged about a neat meta-analysis showing that migratory bird species have more nematodes than non-migratory bird species. Tommy and Janet have recently extended that work, and you can learn more about that here. But today, I want to talk about how migration affects parasite assemblages within a single bird species, instead of between species.

Double-crested cormorants are funky-looking birds with funky geographic distributions. There are several subspecies with unique geographic locations (e.g., California, Southeastern-ish places), and some of the southern populations migrate, while others don’t. The non-migratory populations are sometimes protected, while the migrating populations are sometimes controlled via culling to reduce potentially detrimental impacts of cormorants on fisheries. So, if you were a manager, how could you figure out whether the cormorants hanging out in a given water body were migratory or not?

Having them all recite some Shakespeare and then guessing based on their regional dialects probably wouldn’t work. But don’t lose hope! There’s another way! After sampling migratory and non-migratory birds’ parasites from many sites, Sheehan et al. (2016) found that discriminatory analysis based on parasite data alone could predict whether a bird was migratory or not with 78% accuracy. That’s pretty good! (Also, check out the paper for some sweet graphics!)


This is a good reminder that we can learn a lot about host ecology by looking at hosts’ parasites. We can even figure out what some of the world’s most elusive sea creatures eat – and what eats them – just by looking at their parasites. Yeah. Parasites are cool.


Sheehan, K., D. Tonkyn, G. Yarrow, and R. Johnson. 2016. Parasite assemblages of Double-crested Cormorants as indicators of host populations and migration behavior. Ecological Indicators 67: 497-503.

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