After decades of teaching, writing, and reading about ecology and evolution I thought I had heard everything. But just recently I ran across an example of mutualism that surprised me.
Most vertebrates have lots and lots of parasites inside and outside of our bodies. Birds are no exception. They have it worse than humans because, unlike us, they cannot bathe away the oils, dirt, and parasites that accumulate on their skin, well protected as it is by feathers that are really good at repelling water.
But there are some things that birds do to reduce their parasite loads, besides preening with their beaks. They can expose themselves to environmental chemicals which may kill the parasites.
One example of an environmental chemical is smoke. Many naturalists have observed that birds will gather near a fire and extend their wings, allowing the smoke to get into their feathers. The smoke contains some chemicals that might help to reduce the growth of the parasites.
Another example, which I just read about, is that birds (in this case, a Eurasian jay) can stand near the entrance to an ant’s nest. They extend their wings, thus scaring the soldier ants that guard the entrance. The soldier ants squirt streams of formic acid at the birds, which does not harm them, but may reduce their parasite load.
Notice that this particular ecological interaction does not require that either the birds or the ants know what is happening. Natural and cultural selection reinforce the behavior, not the understanding.
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