Osa Birder

 lizandabraham  

The Diversity of Tropical Rainforest Housing

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

   TURISMO

Boat-billed Herons and young on a platform nest constructed with twigs

Boat-billed Herons and young on a platform nest constructed with twigs

The primary goal of all species is to reproduce and continue to exist as a species.  For birds, site selection and nest construction is a major part of successful reproduction.  Over eons of evolution, birds have developed an amazing diversity of techniques.

As we enter the dry season , we will start to observe signs of nesting.  Birds collecting spider webs, cleaning out cavities, carrying bits of material including sticks, twigs, mosses, or dried leaves , and often behaving in a secretive manner.

The Black-faced Antthrush enters their stump cavity nest from the top.

The Black-faced Antthrush enters their stump cavity nest from the top.

Nesting behavior is fascinating to observe and there is always a temptation to get a closer look ,  although  close observation can endanger a nest.  Nest failure is very high in the tropics and our careless observation methods can increase this rate. The generally accepted failure rate is around 80%.    Major causes of failure are predation and lack of forage resources.  The latter is a growing problem due to the erratic climatic issues we are currently experiencing.  But something as simple as a branch or tree falling (a common event in the rainforest) can destroy a nest.

Band-tailed Barbthroat nest hanging from a marantaceae leaf

Band-tailed Barbthroat nest hanging from a marantaceae leaf

There are many nest predators that we often fail to see.  Snakes, lizards, raptors, toucans, mot-mots, tayras , (a large weasel), raccoons , squirrel and capuchin monkeys, all prey on nests.  When we, large animals, walk in the forest , we are watched by many of these species and by merely standing near a nest , we may inadvertently bring attention to it.  I cannot count how many times I have seen a nest destroyed within an hour of someone (including myself) taking pictures up close, or stopping to look into the nest.  The best way to observe a nest you have spotted is to continue walking and then observe it from a distance with binoculars or a scope.

A typical loose woven Oranage-collared Manakin nest

A typical loose woven Oranage-collared Manakin nest

Many birds are cavity nesters, using holes in dead or live trees, river and forest banks.  A dead tree could house several nests of several different species.  We had a tree last season with the nests of Fiery-billed Aracari, Black-crowned Tityra and Golden-naped Woodpecker, all at one time.  Often one can watch different species fighting over a hole or one throwing out the nest of another.  One of my favorite tree cavity nests has an ingenious design, making predation by toucans almost impossible.  In the back of the tree cavity, there is a vertical opening to a second story nesting platform.  I have seen this twice, after the tree had fallen, while Golden-naped Woodpeckers were in occupancy. It is hard to say who actually designed the nest since the woodpeckers may have moved in after another bird had excavated the hole. The Black-faced Antthrush uses a broken-off tree or stump and enters the nest from the top. The Blue-crowned Mot-mot burrows into forest banks, sometimes as much as a couple meters.  Slaty-tailed Trogon prefer to excavate a hole in a termite nest .

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An example of a multi-layered nest constructed by the Black-cheeked Ant-Tanager

Other birds build their own “cavities” out of local materials.  Many species, including some flycatchers and  wrens, Banana quits,  Scarlet-rumped Cacique , and Slaty Spinetail ,  create  a completely enclosed nest with a side entrance .  Designs range from a huge sloppy looking ball of forest debris to an intricately woven cylinder with a long tunnel entrance attached to the side.  The unruly looking nests are well camouflaged and look natural.  The finely woven nests, appearing less natural, are often decorated on the outside with lichens, mosses, and small seed pods.

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Black throated Trogon in a cavity nest

Spot- crowned Euphonia has a creative approach to the cavity nest.  They will use any small cavity available, natural or constructed, often a curled-up cecropia leaf stuck in the canopy or a hole in the root mass of a bromeliad clump. One year we had an old Scarlet -rumped Cacique nest hanging in the second story of our lodge and the Spot-crowned Euphonia remodeled it for their own use.

Completely exposed nest of the Long-billed Starthroat

Completely exposed nest of the Long-billed Starthroat

Most of us are familiar with the typical cup nest often seen in the gardens of the temperate regions. The variety of designs range from very small fine cups of the hummingbirds to larger bulky cups designed by some tanagers.  Pigeons and raptors build massive platform nests of sticks and twigs.

Some constructed nests are built with distinctive layers.   On the outside some sort of camouflage layer, often randomly-placedlichen, moss, sticks , twigs, or leaves.   In the middle, is the structural part, more finely woven, and in the center, a soft lining.     The hermit hummingbirds, and many other species, put a “tail” on the lower part of the nest giving the impression that the nest is simply a bit of debris that has fallen from the canopy.

The odd placement of a White-necked Jacobin nest

The odd placement of a White-necked Jacobin nest

Nest placement varies greatly, and is often surprising.  A White-necked Jacobin prefers to nest on top of a large understory leaf using spider webs to attach the structure.  The hermit hummingbirds take a torn heliconia, banana, marantaceae or palm leaf and attach their nest to the underside of the torn part with spider webs  to create a natural roof.  Some birds prefer to nest over moving water although the species may rarely be seen along streams. The Striped Owl nests  in overgrown fields among tall grasses.

Protection from strong rain and sunis often a factor, but not always.  The Long-billed Starthroat , a large species of hummingbird, builds a nest on an exposed , top branch of a cecropia.  The camouflaged nest appears to be a nodule left by a fallen branch but it is completely exposed to the elements.

A typical loose woven Oranage-collared Manakin nest

A typical loose woven Oranage-collared Manakin nest

Sometimes there are unfortunate errors.  First-year birds often make mistakes but maybe there is a poor “nest-building gene” as well?  We watched an Orange-collared Manakin finish her nest and knew the nest was doomed.  It was so loosely woven that the egg , once laid, would clearly fall through.  Two days later, a broken egg was found below.  Tropical Gnatcatchers  once built a well-hidden nest high in a Peine de Mico (Monkey brush tree),  which unfortunately lost its leaves just as the young were almost ready to fledge.  The Swallow-tailed Kites were quick to attack the exposed young.  One rainy July , along a stream, we found a nest of the Scaly-breasted Wren, a terrestrial wren species.  The nest was in a short tunnel less than one foot above the water level and the water could have risen at any time and flooded the cavity.

If you keep your eyes peeled and with a bit of luck, you will have the pleasure of observing the many different ways that birds approach housing construction challenges.  But remember  to be very careful in your observations so as to not tip off nearby predators and thereby increase the already high threat to the nests, eggs, and hatchlings.

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