Tuesday, March 26, 2019

Avian Drop Foraging; a precursor to powered flight?

In recent years, increasing numbers of tourists have ventured to the New World tropics in search of exotic birds. Many, if not most of these people expect to find symbolic species such as trogons, tanagers, toucans and parrots.

Others, like myself, travel to the tropics with the hopes of adding knowledge about bird ecology and evolution, in this the richest of environments.

Of course, seeing cotinga’s and puffbirds for the first time is always an exhilarating and memorable experience. Some memories however are more vivid to me than others, including one that I experienced many years ago in a lush and humid Panamanian rainforest.

It was on an early morning walk down a beautiful forest trail, where I happened upon a rarely seen Great Tinamou (Tinamus major), searching for fallen fruits and seeds. Clearly surprised by our encounter, the bird quickly scurried off into the underbrush and out of sight.

As I continued my search for the tinamou in the dense undergrowth, my eyes were suddenly drawn to a Northern Barred Woodcreeper (Dendrocolaptes sanctithomae) perched motionless on a rotting log. Located directly next to the woodcreeper was an Ocellated Antbird (Phaenostictus mcleannani), also motionless. I thought to myself; what has drawn the attention of these two species?

Curious, I began to scan the surrounding area very carefully, until I came upon an active army ant colony. Initially, it was assumed that the woodcreeper and antbird were simply eating the ants. But as I continued to watch their behavior, I began to realize that that the birds were not feeding on the ants at all, but rather on the insects flushed by the swarming army ants. As a newcomer to tropical birding, their unusual behavior was beginning to make more sense.

Ant followers, as the woodcreeper and antbird are typically known, are birds that feed by following swarms of army ants (Eciton burchellii) and take prey flushed by those ants. The best-known ant-followers are 18 species of antbird in the family Thamnophilidae, but other families of birds may follow ants including thrushes, chats, ant-tanagers, cuckoos, and woodcreepers (source: Wikipedia).

Of particular interest to me was the exotically colorful antbird. In addition to its unique plumage and large area of blue facial skin, the Ocellated Antbird also takes part in an unusual method of hunting.   

Often referred to as drop-foraging, Ocellated Antbirds normally perch on stems and small tree trunks within one meter of the ground, sallying down to capture potential prey. Ocellated Antbirds are particularly adept at clinging to thin, vertical stems, with strong legs and toes. 

Ocellated Antbird (Phaenostictus mcleanni)
Image Property of: neotropical.birds.cornell.edu; Donald Kirker
Drop-foraging offers birds numerous benefits. In the case of the antbird, it allows for close approach and close inspection of army ant colonies, giving it a leg up (literally!) on the competition. It also helps the birds avoid becoming overrun, or stung by the swarming ants.

Closer to home, I see the same type of behavior in our neighborhood Eastern Bluebirds (Sialia sialis).

Research shows that drop-foraging constitutes the method of almost 100% of foraging attempts during the early breeding season. During the height of breeding season, 80% of bluebird foraging attempts are perch-to-ground movements. Hovering is uncommon. Younger fledglings hop along the ground to forage before acquiring adult drop-foraging habits (source: Bird of North America online).

Eastern Bluebird (Sialia sialis) Drop Foraging
Image Credit: Evan Hambrick

The benefits of this technique are similar to the antbirds, allowing the bluebird to scan a wider area for insects, while remaining at a reasonable striking distance. This is in direct contrast to other local species like the American Robin (Turdus migratorius) and Common Grackle (Quiscalus quiscula), who expend a great deal more energy on foot during hunting forays.

Clinging low to elevated stems and tree trunks also permits the birds to search their surroundings for potential predators.

So, how did this type of hunting method evolve? Is this behavior more of a recent development? Or are there any clues in the fossil record that may increase our understanding of this behavior, helping us to possibly answer questions about the origin of flight.

In the 2015 book-The Rise of Birds; 225 Million Years of Evolution, author Sankar Chatterjee goes to great lengths discussing the three competing models for the origin of avian flight.

Two theories have dominated most of the debates, the cursorial ("from the ground up") theory proposes that birds evolved from small, fast predators that ran on the ground; the arboreal ("from the trees down") theory proposes that powered flight evolved from unpowered gliding by arboreal (tree-climbing) animals.

A more recent theory, "wing-assisted incline running" (WAIR), is a variant of the cursorial theory and proposes that wings developed their aerodynamic functions as a result of the need to run quickly up very steep slopes such as trees, which would help small feathered dinosaurs escape from predators (source: Wikipedia, Origin of Birds).

Origin of Flight; From Rise of Birds, Chatterjee 2015
In short, Chatterjee clearly favor a “trees down” scenario, where the evolution of flight (and advanced feather development) happens solely in the trees, in a sequence of progression from climbing to jumping to parachuting to biplane gliding to monoplane gliding, soaring, flapping to complex maneuvering flight.

He downplays the cursorial theory by stating that it fails to explain fully why the primary vanes of Archaeopteryx are so asymmetrical and complex, a condition seen only in modern volant birds.

Chatterjee continues; If Archaeopteryx were a ground-dwelling bird, as John Ostrom depicted, it would have had hair-like feathers, like those of ostriches and rheas. The cursorial theory works against gravity and is energetically more expensive. The effects of gravity would create additional stress on proto-birds during takeoff. To overcome the added stress, the supracoracoideus pulley system would be required during takeoff; its lack in Archaeopteryx indicates that it took off from trees to become airborne, not from the ground.

The cursorial theory does not address the necessary transitional form between the preflight stage and the active, flapping flight stage; flight would thus have evolved rapidly, from jumping to active flying, almost by saltation without any intermediate gliding stage. This theory does not explain adequately the origin of feathers, endothermy, or brain enlargement and three-dimensional perceptual control (source: The Rise of Birds, Chatterjee, 2015).

But what if we factor in the evolutionary behavior of drop-foraging? Could this be the go-between behavior that the “ground up” theory is looking for?

Let us envision for a moment a small feathered coelurosaur, hunting in dense undergrowth. A distant crashing sound sends the dinosaur running to the nearest conifer (cycadeoids, etc.), where it uses its powerful legs to leap and grasp onto the tree’s rough trunk. It nervously scans the surroundings, at the same time periodically flapping one of its feathered arms to keep balanced, similar to a medium sized bird on a small tube-feeder.

Blue Jay (Cyanocitta cristata) on tube feeder
Photo Credit: Paul Cianfaglione
As the threat subsides, the hungry animal continues to search for prey. Once spotted, the dinosaur forcefully pushes off the trunk and into the air, flapping and gliding a short distance to the forest floor where it renews its pursuit.

For a ground-dwelling dinosaur in the earliest moments of avian evolution, there are a number of good reasons why it might have wanted to leap into the air, without feeling the need to stay there.

I see Archaeopteryx as a predominantly terrestrial predator, like today's Greater Roadrunner (Geococcyx californianus) who uses its feathered forelimbs and strong legs for leaping and rudimentary take-offs from ground level, to better access food and escaping predators.

Drop-foraging among small feathered coelurosaurs would involve vertical leaps from the ground, cling-perching on low stems and trees, balance-flapping, leaping again, controlled flapping and gliding descent; effectively explaining the origin of advanced feathers, brain enlargement and three-dimensional perceptual control, without the need of parachuting from great heights.

I could also imagine the early forms of enantiornithine and pygostylians birds utilizing this same foraging behavior, since all were most likely still tied to nesting and feeding upon the forest floor.  

Mesozoic Bird Drop Foraging
Image Property of: leaubellon.tumblr.com
I often wondered if we could see any anatomical differences in the foot bones of antbirds, bluebirds or any other bark specialists, as a result of these unique actions. If so, how about in the fossil record.

White-breasted Nuthatch (Sitta carolinensis)
Image Property of: Stanislav Harvancik
As luck would have it, I remember years ago photographing a Chinese enantiornithine fossil with just that sort of anatomical anomaly.

Though it may have simply been an artifact of the preservation in the fossil, the ancient bird’s feet (foot) are a true eye-opener for the first-time observer!

Reminiscent of an aye-aye’s (Daubentonia madagascariensis) special thin middle finger, or Australian Striped Possum (Dactylopsila trivirgata) front forelimb, the fossil foot, at the slightest, may signal the start of a selected adaptation to dominant stem/trunk foot support. 

Enantiornithine Fossil Feet
Photo Credit: Paul Cianfaglione
Enantiornithine Fossil Foot
Photo Credit: Paul Cianfaglione

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