Following the army ant-following birds

By Corina Logan

It’s 4:00 am and I can’t believe I’m (just barely) awake. Not only that, but I have to go out there in the cold and rain. It’s so cold! I’m in the tropics – it’s not supposed to be cold in the tropics. I pull on my clothes (quickly, while still hiding under the covers), grab my gear, and head out into the darkness. I hurriedly walk up the muddy path; time is of the essence. I find the trail into the woods, which is marked with flags, and I hike across the hilly terrain through the dense tropical forest, arriving at my field site about 30 minutes before dawn – just in time. I go over to the army ant nest (called a bivouac because it is made from the interlocked bodies of the ants themselves) and look for activity, being careful not to step near any ants (I learned that lesson a couple of days ago when I decided that I could watch the ants while wearing trainers and not Wellington boots. Ouch. The soldiers have very strong mandibles and they leave a pheromone trail on you which attracts more soldiers by the masses). Just a few ants milling around outside of the hole. I walk about 5 meters away and sit down on a piece of plastic so I stay dry, then I open my umbrella above me. I hold as still as I can while searching the darkness around the army ant nest with my bare eyes and binoculars. My prize? Bivouac-checking birds.

I happened upon bivouac-checking birds when I agreed to be a field assistant for Sean O’Donnell, a professor at the University of Washington (now at Drexel University). We spent a month in a high-elevation Costa Rican cloud forest (which is why it was so cold) studying army ants and the migrating birds that come to the tropics over the winter and eat insects that flee from the thousands of army ants raiding through the forest. After we got to our field site, Sean told me about the bivouac-checking behaviour that is performed by some of the birds that attend army ant raids. After foraging at the front of the raid, some birds follow the column of army ants that connects the raid front to the bivouac (the column is a two-way highway: ants at the raid front bring prey to the bivouac and then return to the raid front to collect more prey) from the raid front to the bivouac and check the location of the bivouac. Then they fly away. The next morning when the ants start raiding again (after retreating to their bivouac for the night), usually just after dawn, these birds will come back to check the bivouac again: if the ants are already raiding, the birds will follow the ant column to the front of the raid for another meal, and if the ants are not yet raiding, then the bird flies to another army ant colony that it is tracking to check their raiding status.

For a biologist, this is a very interesting behaviour because it appears that some birds are able to track army ants in time and space which allows them to consistently encounter abundant food resources, which are patchily distributed throughout the forest making army ant raids difficult to encounter by chance. At this point I was a biologist but I was preparing to start a PhD in experimental psychology at the University of Cambridge under the supervision of animal cognition expert Professor Nicola Clayton. I had read some of Nicky’s papers on episodic-like memory (the ability to remember the what, when, and where of a personal experience) and future planning in western scrub-jays (a bird in the big-brained crow family) by the time I joined Sean in Costa Rica so I was starting to also think in terms of psychology. What struck me about bivouac-checking bird behaviour was that it looked like these birds might need to remember the past event of checking the bivouac location (episodic-like memory) to be able to return to the bivouac the next morning to see if the ants are raiding (planning for a future meal). This seemed like it could be a perfect system for merging my past in biology with my future in psychology. Had I not been exposed to both fields before I went to Costa Rica, I may not have made this connection. Now at the end of my PhD, I am further convinced of the importance of working across disciplines. I find psychology and field biology particularly complementary because experimental psychology brings to biological studies a methodological rigour and an expansion of thinking to include the cognitive aspects of subjects, while field biology studies the animal in the context of the surroundings in which it evolved, bringing the whole picture of the animal to psychology.

The ocellated antbird is one of the species that checks army ant bivouacs. Photo by Jose Luciani.

Corina Logan is a PhD Candidate and Gates Scholar at the University of Cambridge. With Nicola Clayton and Sean O’Donnell she has written the paper ‘A case of mental time travel in ant-following birds?’ for the journal Behavioral Ecology. You can read the paper in full and for free here.