Antbirds, Antwrens, Antthrushes, Antpittas, and Antshrikes, oh my!

An army ant solider. (Eciton) You can see the reduced eyes quite well in this picture, army ants are effectively blind: they must smell and feel for prey.

As a kid, for whatever reason, nothing said the unexplored  jungle like army ants. I didn’t know all that much about them, just that they were these carnivorous ants that ravaged the forest floor. They were like quicksand, giant anacondas, piranha, or predatory cats; a near mythical denizen of the jungle that any would-be explorers would have to be mindful of, and avoid. (Think Indiana Jones and the Kingdom of the Crystal Skull)

Army ant Workers (Eciton) moving in a line. Their high organization makes these ants really interesting to watch.

The term army ant actually applies to many different genera of ants and  more than 200 species. They are linked by convergently evolving the trait of “raiding”, that is that they move in large numbers along the forest floor hunting for food. The most archetypal army ants is Eciton. Contrary to the visions of giant flesh-eating ants dancing around in my head as a kid, they are actually quite small when compared to something like the bullet ant, which also commanded legendary status in my mind as a child. (though saying that, army ant soldiers are as big as the largest temperate carpenter ants I have seen) Eciton are extremely polymorphic; there are  tiny workers only a fraction of the size attained by soldiers. Unlike a typical ant, they don’t make a nest in the ground; they clump together forming a nest out of workers, complete with passages and chambers, all  to protect the queen and the brood, in a structure known as a bivouac. They do this because when rearing brood they require a huge amount of food and have to move daily. Once the larvae pupate, they can settle down and stay in the same patch for a couple weeks. They also don’t breed like normal ants. To reproduce the colony breaks up with new queens taking a full complement of workers and soldiers with them when they leave, in a process called fission.

An army ant solider guarding a trail of workers.

While any person wondering the neotropics has little to fear from these ants, (I myself got stung by a couple and was surprised to find it fairly weak) arthropods in the leaf litter and understory shrubs do. They flee before the front, hoping to outpace the fast moving swarm. This is the really interesting part; army ants are such a force in neotropical forests that many other species from a wide range of taxonomic groups have become associated with these ants, taking advantage of these fleeing arthropods. Parasitoid wasps fly just ahead of the swarm, hovering like military helicopters, laying eggs is the very visible insects rushing away. Flies fill the air waiting to lay there eggs in anything substantial they do not eat, like vertebrates they kill by chance. Along with these arthropods, larger animals also take advantage of these ants.

Zeledon's Antbird (Hafferia zeledoni) at an ant swarm. This antbird can be found at mixed flocks, as well as by ants.

Many birds have learned that these ants stir up insects, and so follow these raids. These birds are kleptoparasites, stealing food that the ants would otherwise get. In my own experience various woodcreepers hang on tree trunks above the ants, puffbirds wait on the branches above, and gray-headed tanagers hang in the understory. Several groups of birds though are named for their association with ant swarm; antbirds often hop between pieces of wood, lianas, and shrubs while antthrush strut around like they own the place.

Two different antthrushes: the Black-headed Antthrush (Formicarius nigricapillus) and the Black-faced Antthrush (Formicarius analis). These birds move like little forest rails, and sort or strut around ant swarms

Birds with an ant prefix include: antbirds, antthrushes, ant-tanagers, and antpittas. Antbirds (family thamnophilidae) are further divided up into antbirds, antshrikes, antvireos, and antwrens. What can be a little confusing is a lot of the birds in these groups are not associated with ants at all, preferring to hunt for food without the aid of ants. Some of these birds join into mixed-flocks with other birds and they all forage together, while others hunt alone or in pairs.

Two birds named for ants that are not common at ant swarms: The Black-cheeked Ant-tanager (Habia atrimaxillaris)and the Barred antshrike (Thamnophilus doliatus). The Barred Antshrike sometimes will join ant swarms, while the ant-tanager, which is endemic to the Osa Peninsula, is not well understood.

    Costa Rica is home to a variety of both army ants and ant associated birds. One antbird though stands above the rest in terms of its beauty and difficulty to see, the Ocellated Antbird (Phaenostictus mcleannani). It is considered an obligate ant follower, rarely straying from the swarm. This is unlike many other antbirds like Zeledon’s Antbird (Hafferia zeledoni) which though often found attending raids, can also be found foraging with mixed flocks or by itself. Of Costa Rica’s some 900 bird species, the Ocellated stood out as one of the species I most wanted to see. My journey took me from Monteverde, over the continental divide, and down to San Gerardo Station in the Children’s Eternal Rainforest. It is at mid-elevation overlooking the highly photogenic Arenal Volcano, and stands out as one of the most beautiful places I have ever been.

Arenal from San Gerardo Biological Station. This was one of the most beutiful views I have had the pleasure of witnessing.

    I had been brought to Costa Rica on a study abroad program through the

Council on International Educational Exchange (CIEE) on a tropical biology and conservation program over the summer. This program, which began in the early 90s, has been exposing students to the tropics and allowing them to do independent research. I feel privileged to be able to be part of this. We were doing a four day field trip to the Station, to marvel at the Atlantic Slope, which was going to be the only time we ventured over to this side. This would be my shot, during free time, to try and find this king of the antbirds.

Bicolored antbird (Gymnopithys bicolor) in the Osa Pennisula. This little bird was at every ant swarm I have come across.

  My approach was to find the ants. If the Ocellated is an obligate follower, then if I find the right swarm, then I find the bird. It took me until the 2nd day to find the ants. They were moving into a pile of logs and other debris, forming their nightly bivouac. If they were camping out for the night, then that probably meant they were raising larvae, and so more attractive to antbirds. While I watched them move in, carrying their brood and some meat, (One large worker was dragging the head of a centipede that I certainly wouldn’t want to tangle with) a few birds hung around. My guess is they where some smaller antbirds, but I really didn’t get a good look at them. I did some macrophotography with the ants. I have been rapidly improving with my skills at macrophotography over the last year, and have already taken even better pictures of Zion Snails than those on the post from Spring 2018.

The next morning I headed to the swarm after breakfast. I arrived to see the raid in progress. Several lines of ants crossed the trail from the wood pile and into a patch of heliconia. In the thicket, I could hear an antthrush, and what could have been the Occelated. I wanted a visual though, since the Bicolored antbird sounds very similar. I began looking for either a way to look into the patch from the trail or a relatively open patch of forest to get up to the swarm. I quickly realized that there was no easy access into the patch. I thought about diving right into the heliconia recklessly, but I began thinking about all the potential venomous snakes and spiders that might be on the foliage. I then reasoned that the army ants would have chased off or killed anything I didn’t want to be bitten by, so I decided to follow there raid off trail. I entered into the heliconia, and began following alongside their line, I can understand why people use machetes. I followed them out into a more open area, where the center of the swarm was located. Everything was covered in ants, from the leaf litter, fallen branches and even the heliconia. The air was thick with mosquitos and flies. (my elbows are covered in mosquito bites as I write this) I quickly spotted a Zeledon’s antbird in the clearing. Ants began to get into my boots and up my pants, and I got stung. (but now I can say I have had army ants in my pants)

The Ocellated Antbird. (Phaenostictus mcleannani) This was the best of the 3 pictures I got of the largest and most beutiful antbird in Costa Rica.

    Then I saw it, an antbird with dark scalloping. It perched low on a branch in thick foliage. Like I had heard, it stayed right in the center of the swarm. Like many understory birds I found the Ocellated Antbird quite difficult to photograph. In the understory it is dark, and this particular bird also stayed in fairly thick foliage. I also spied a Bicolored antbird in the swarm, and many Zeledon’s antbirds. One was quite young, still having a gape. (a feature of young birds) The swarm began to move on, and the ocellated moved out of the clearing. The ants were heading down a slope that I certainly didn’t want to try to go down. I also had to get back to the station, so I began to work my way out of the thicket and back to the trail.

Sources

Franks, Nigel R.; Hölldobler, Bert (1987). "Sexual competition during colony reproduction in army ants". Biological Journal of the Linnean Society. 30 (3): 229–43. doi:10.1111/j.1095-8312.1987.tb00298.x.

Garrigues, Richard and Dean, Robert (2014) “The Birds of Costa Rica second edition” Zona Tropical Publication ISBN 978-0-9816028-8-2