Birding the understory forests of the Osa

lizandabraham Birding the understory forests of the Osa

Liz Jones

Liz and husband Abraham Gallo own and operate Bosque del Rio Tigre, a lodge in Dos Brazos that specializes in birding.  You may reach her at liz@osaadventures.com
Black-cheeked Ant-Tanager by Cindy Beckman

Black-cheeked Ant-Tanager by Cindy Beckman

It’s been quiet in the forest this morning.  Only a few understory dwellers have passed our way. Then suddenly, there are birds everywhere and all of us are excited.  What happened?  The variety and motion is amazing.  There are birds at our feet, eye level and above our heads, maybe 8-10 different species.

Understory birds often form foraging flocks of different species, moving together through the forest, some staying with the flock and others joining in or dropping out as they move along.   These groups are commonly referred to as a mixed flock.

But one wonders why so many different species are foraging together, appearing to compete for the same resources.

llThere is an incredible amount of rarely observed activity going on in the  forest understory.  A wealth of insects, small reptiles and amphibians dwell under the leaf litter or tree bark, tucked in root masses of epiphytes or in the small aquatic reservoirs of bromeliads or tree holes, caterpillars feeding on leaves.  Many species hide in the curled still-attached dead leaves, or leaves caught by living foliage while falling.  Dead wood hosts a menagerie of boring beetles and larvae.  There are bats, frogs and insects that spend the daytime hours in the conical, unfurled leaf of heliconia and Marantaceae.  All these small forest dwellers provide a food source for the more easily viewed avian species

Each avian species, or group of species, has a unique forage method and a preference for a certain level of the forest, giving them the ability to forage together without interfering with one another.  Foraging together is safer and more efficient.

Woodcreepers work their way up tree trunks, checking in holes and under bark. Chestnut-backed and Bicolored Antbirds stay close to the forest floor searching under leaves and around dead wood. Eye-ringed Flatbill (a flycatcher) and Ochre-bellied and Ruddy-tailed Flycatchers sit about 5-15  feet from the forest floor and pick off insects that are either under branches and leaves or just passing through. Leaftossers throw leaves in the air to uncover the insects on the forest floor. Black-faced Antthrush picks around in the leaf litter and Marbled Woodquail behave like chickens scratching away the leaf litter and top soil.  Ant-Tanager, ant shrike, and foliage-gleaners prefer the middle level of the understory.  Rufous Mourner and Rufous Piha prefer the upper understory. Antwrens move between the middle and upper levels of forest understory.

Rufous Piha by Andrew Russell Black-hooded Antshrike by Michael Wickens Northern barred Woodcreeper side view
 Rufous Piha by Andrew Russell  Black-hooded Antshrike by Michael Wickens  Northern barred Woodcreeper side view

Imagine a Buff-throated Foliage-gleaner (yes, that is what it does, gleans insects from foliage) working solo through the mid-understory forest, checking for insects in the hanging dead leaves.  Now 15 minutes later a Black-hooded Antshrike passes through the same area, same level.  It will check many dead leaves as well, but there will be little to glean from them, because the foliage-gleaner got there first. Of course, there may have been many spots missed on the first round but there is no way the second forager would know which leaves were missed. These two birds moving together would have been more efficient, with more spots checked and no energy wasted.

While foraging, the foliage-gleaner’s head is tucked in the dried leaves and the antshrike is looking under green leaves, as well as in the dead ones.  Both are vulnerable, unable to detect the approach of a forest raptor looking for a meal.  But with other species moving together and more eyes, there is less chance of a surprise attack.

The vigilant and vocal Rufous Piha, referred to as the guardabosque, or forest guard, calls loudly when danger arrives.  He sits quietly, high in the understory and waits for the larger insects stirred up by the other species.  Although he is often found alone, his presence in the flock provides him with more abundant prey.

Clearly this mixed-flock strategy, developed through evolution, is beneficial to all involved.

Birders always walk in the forest with hopes of encountering a great flock and even better, an army ant swarm followed by a mixed flock.

Sin títuloAn army ant swarm will attract many individuals and species.  The birds attending a swarm are not there to eat the ants and do not seem to be bothered by them either. The army ants are great at routing out all the hidden insects, amphibians and small reptiles, providing a feast for all. The mix of avian species can be varied but normally in any given area there will be a basic group attending all swarms with an assortment of others joining in occasionally (see tables).  These flocks are distinctly different from the normal understory flocks, with a different mix and more individuals of each species and the various species behaving a bit differently.  Many of the foragers will work closer to the ground then they normally would and much closer together due to the abundance of prey, much of it,  near the ground.

When a birder does encounter a particularly active flock, it is important to check for the presence of ants. One clue will be the sounds of rustling and light tapping as the ants, fleeing insect prey and birds move around. There will also be soft calls from the various bird species.  If you do see ants, look for their trails and study the movement.  It is possible to stand between the ant trails in the middle of the swarm and watch for quite awhile. But one must be vigilant, since the trails change frequently.

Years ago, as I hiked with local friends from Dos Brazos to Carate, we ran into our first good flock near the end of the hike when we were extremely tired.   All four of us started watching the birds and forgot to check for ants.  We realized, too late—when the ants were already inside our boots—that we were in the middle of an ant swarm.  To our considerable disappointment we scared off all the birds as we hopped around to get out of our boots and brush away the ants.

Black-striped Woodcreeper Bicolored Antbird Buff-throated Foliage-gleaner by Andrew Russell
 Black-striped Woodcreeper  Bicolored Antbird  Buff-throated Foliage-gleaner by Andrew Russell

 

 

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