Monday, March 14, 2011



There are many species of birds that follow massive assemblies of nomadic army ants that share a commensalistic relationship. The bird species include the White-tailed Ant Thrush (Neocossyphus poensis), Brown-chested Alethe (Alethe poliocephala), and Red-tailed Bristlebill (Bleda syndactyla)[3]. There are also several species of army ants including Dorylus wilverthi, Dorylus molestus, Labidus praedator, and Eciton burchelli [1,3]. As the army ants swarm across the forest floor in search of insects and arthropods, the birds follow along behind and eat the flying bugs the army ants stir up that the ant can not eat themselves. The birds benefit from the ants foraging, but the ants have little to no harm done to them. Army ant species have been found worldwide on a variety of terrains, although they are most common in tropical and subtropical regions between 45 degrees south and 45 degrees north [6]. Scientists have observed that where army ants reside, there are species of ant-following birds close behind.


There are four main families and types of ant-following birds: walkers (Cuculidae), climbers (Dendrocolaptidae), clingers and hoppers (Formicariidae), and perchers (Thraupidae).The largest group of regular ant followers is in the family Formicariidae, but there are more than 50 species that follow ants regularly [1]. These birds species include White-tailed Ant Thrush (Neocossyphus poensis), Brown-chested Alethe (Alethe poliocephala), and Red-tailed Bristlebill (Bleda syndactyla).[3] There are also several species of army ants including Dorylus wilverthi, Dorylus molestus, Labidus praedator, and Eciton burchelli, that are followed by a variety of birds listed above [1,3]. The army ants most likely evolved slowly from individuals to form such massive groups, after recognizing the greater success in predation [6]. Over time, army ants such as E. burchelli and L. praedator have become critical links between bird species and forest floor arthropods, providing food resources that would otherwise be unavailable for many birds in the leaf litter [4]. However, the birds are usually unrelated, and congregate around ant swarms randomly [5].



In this commensalistic relationship, the birds have the greatest benefit of following the army ants, while inflicting little to no harm on the army ants. The birds derive a benefit from the ants by eating the insects and/or arthropods disturbed by the ant “marches” through the forest. There is no break in food availability because ants swarm in almost all seasons and weather. [1] However, there is some cost for both. Birds do not intentionally try to harm army ants, but occasionally the insects the birds eat that were flushed out by the army ants already have army ants on them. Therefore, the army ants are ingested along with the insects, decreasing the fitness of those ants. A large flock of birds also occasionally takes more insects and/or arthropods than they should in order to not harm the ants’ own diet, which would bend towards parasitism [2]. However, only a small fraction of the total number of birds in a species follows ants, so the amount of food taken from the ants is minimal, bending back towards commensalism [1]. In addition, the ants are somewhat unreliable at high elevations and extreme hot or cold weather [1]. The army ants benefit slightly from the birds eating the excess arthropods because without the birds, the forest floor may become overrun with prey species.


[1] Willis, Edwin O., and Yoshika Oniki. "Birds and Army Ants." Annual Review of Ecology & Systematics 9 (1978): 243-63. Web.

[2] Wrege, Peter H., et al. "Antbirds Parasitize Foraging Army Ants." Ecology 86.3 (2005): 555-9. Web.

[3] Peters, Marcell K., and Benjamin Okalo. "Severe Declines of Ant-Following Birds in African Rainforest Fragments are Facilitated by a Subtle Change in Army Ant Communities." Biological Conservation 142.10 (2009): 2050-8. Web.

[4] Roberts, Dina L., Robert J. Cooper, and Lisa J. Petit. "Use of Premontane Moist Forest and Shade Coffee Agroecosystems by Army Ants in Western Panama." Conservation Biology 14.1 (2000): 192-9. Web.

[5] CHAVES-CAMPOS, JOHEL, and J. ANDREW DeWOODY. "The Spatial Distribution of Avian Relatives: Do Obligate Army-Ant-Following Birds Roost and Feed Near Family Members?" Molecular ecology 17.12 (2008): 2963-74. Web.



  1. What is the history of this relationship? How did the birds figure out that this would be a good idea or evolve to better detect or follow the ants?

  2. are there any major costs to the birds? it seems like the only "cost" is that ants can be unreliable in certain situations, but that isn't nearly as costly as the fact that some ants are ingested by the birds.

  3. From what I have read, the history of this relationship is fairly random. Only a small amount of each bird species will follow ants, and the birds researchers have observed have no relationship with each other (ex. kin). The birds were most likely going after the same food as the ants, and over time figured out the ants were doing the same thing. Birds that did less work but still ate plenty were selected form, and the eating strategy repeated and adopted by further generations.

    There were no other costs to the birds that I could find. If the birds rely solely on the ants for food, and the ants do not swarm enough, the birds will lose a significant source of food. This decreases the fitness of the birds, which makes them more susceptible to another predator.