When army ants move out, a new study found that, instead of chasing each other away, birds work together to follow the column and hunt the insects that marching ants scare out of hiding.
| Ruddy woodcreeper, |
a bivouac-checking species
- Finding that birds largely work together to forage at army ant raids seems to demonstrate that cooperation is a better survival strategy than trying to keep food from the raids for their own species.
- In watching for the raids and the flocks that "attend" them, a key to avian cooperation may be what are termed "bivouac-checking" birds.
- These are birds that perch near the sites where army ants make their nests (bivouacs) and watch to see where and when the ants move.
- So when bivouac-checking birds see the movement of the columns and take off, other birds take the cue. They either know birds like the ocellated antbird follow ant columns or recognize vocalizations the specialized birds make when chasing the colonies.
- Birds may use each other as a way of finding army ant raids, which are very hard to locate in the forest because they are widely spaced and the ants are mobile.
Army ants scare up a lot of food when they're on the move, which makes following them valuable for predator birds. But instead of competing and chasing each other off from the ant "raids," as scientists had thought, birds actually give each other a heads up when the ants are marching, according to a new Drexel University study.
For more than a decade -- from 2005 until 2016 -- Sean O'Donnell, PhD, a professor in Drexel's College of Arts and Sciences, observed army ant "raids" and the birds that follow them. He hoped to find out whether birds really were aggressive toward each other during the ant marches or whether they actually cooperated to access the food (other insects and bugs) that ants rustle out of hiding.
After observing 74 swarms in Costa Rica, it seems birds are much more likely to play nice with each other.
"Overall, the results strongly supported facilitation -- species help each other to exploit shared resources," O'Donnell said of his study that was recently published in Biotropica.
In watching for the raids and the flocks that "attend" them, a key to avian cooperation may be what are termed "bivouac-checking" birds.
These are birds that perch near the sites where army ants make their nests (bivouacs) and watch to see where and when the ants move. Birds that fall into that category include the ocellated antbird and the blue-diademed motmot.
The prevailing thought has been that these specialized birds liked to keep the ant colonies they watched to themselves, not allowing other species to horn in on their finds.
But a frequent high diversity of species in flocks following the ant columns showed O'Donnell that birds that didn't specialize in tracking army ants (like the migrant species Kentucky warbler) were allowed to join and hunt.
So when bivouac-checking birds see the movement of the columns and take off, other birds take the cue. They either know birds like the ocellated antbird follow ant columns or recognize vocalizations the specialized birds make when chasing the colonies.
"Birds may use each other as a way of finding army ant raids, which are very hard to locate in the forest because they are widely spaced and the ants are mobile," O'Donnell said. "Observations suggest some birds are attracted to other birds at raids, and birds may even follow each other when moving among raids of different ant colonies."
However, there did seem to be some bullies.
O'Donnell noticed some pairs of species were almost never found in flocks together despite, independently, being ant-chasers. That indicated that these bird species might chase each other off as competition, or just avoid each other entirely. Pairs that seemed to be unable to be around each other included the blue-throated toucanet and the brown jay, as well as the wood thrush and the white-eared ground sparrow.
"These antagonistic pairs were often species of very similar body size or feeding behavior," O'Donnell explained. "Perhaps these species do compete very strongly at army ant raids."
All in all, finding that birds largely work together to forage at army ant raids seems to demonstrate that cooperation is a better survival strategy than trying to keep food from the raids for their own species.
"Having other birds around may be an advantage because there are more eyes and ears to detect predators," O'Donnell said. "If the raid is hard to monopolize, and food is very abundant there, then the costs of allowing other birds to attend may be low, further favoring positive species interactions."
Sean O'Donnell. Evidence for facilitation among avian army-ant attendants: specialization and species associations across elevations. Biotropica, 2017; DOI: 10.1111/btp.12452
This article appeared on Science Daily
African Dorylus raid
The name army ant (or legionary ant or marabunta) is applied to over 200 ant species, in different lineages, due to their aggressive predatory foraging groups, known as "raids", in which huge numbers of ants forage simultaneously over a certain area.
Another shared feature is that, unlike most ant species, army ants do not construct permanent nests: an army ant colony moves almost incessantly over the time it exists.
All species are members of the true ant family, Formicidae, but several groups have independently evolved the same basic behavioral and ecological syndrome. This syndrome is often referred to as "legionary behavior", and may be an example of convergent evolution.
Picture and text
More on army ants from National Geographic
- Army ants are nature's Mongol hordes: always on the move and ravaging everyone in their path.
- You can only run from them.
- And don't waste time...because when an ant colony is on the move they rule the road.
- Anything that stands in their way will be overwhelmed.
- Army ants have large, scissor-like jaws called mandibles.
- They use these weapons to slice their prey apart.
- They don't waste time eating and then digesting.
- While they are dicing up their prey, they spread a dissolving acid.
- Once flesh, muscles and tendons melt down to a liquidized matter, they eat it and keep going.
- The only defense is not moving a muscle. The solider army ants are nearly blind. About all they can distinguish is light and darkness. So they detect prey by movement.
- Patrols head out to forage.
- If the scouts find food, they'll relay the news via scent- back to the nest, so the colony can mobilize.
- Even a beetle six times the size of one ant is no match for an army.
- Some prey isn't eaten on the spot, but carved into pieces and toted back to the colony. It's like slaves building the pyramids: get enough of them, and they can haul almost anything.
- They're so numerous, the total weight of the world's ants roughly equals the weight of humanity.
- Their numbers make them a natural insecticide...able to exterminate an entire patch of ground...at least till they move on.