Mangrove Swallows & Proboscis Bats in Rio Sarapiquí

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A group of mangrove swallows (Tachycineta albilinea) stretch and ruffle their feathers on a mossy branch over the waters of Rio Sarapiquí.  This species is distributed throughout coastal regions throughout Central America, inhabiting areas near mangroves and bodies of water in lowland tropical rainforest. They create nests in tree hollows and cavities a few meters above the water’s surface, usually not straying too far while foraging. During the breeding season they are monogamous and constantly patrol the area around their nests. After the young have fledged, swallows can be found in large flocks, this one composed of about 30 individuals.


Swallows are small-bodied and incredibly agile due to a streamlined body and triangular wings. In flight they cut through the air by making their wings into a characteristic V-shape, propelling themselves forward through momentary bursts of rapid wing beats. In fact, the genus name Tachycineta means “quickly-moving.” Swallows are some of the most efficient fliers, gracefully slicing through the air like a shuriken. They are adept predators of aerial insects with sharp vision for intercepting prey in mid-air. Some studies suggest convergence in eye morphology with raptors for increased visual acuity. Both groups have temporal foveae for frontal vision in addition to central foveae for lateral vision; the foveae are thought to aid in tracking of evasive aerial prey (e.g. dragonflies) by increasing the visual field. Swallows also have “long” eyes compared to other passerines, creating a longer focal length and increasing spatial resolution.

All animals in this post photographed in situ [1] from a boat

Rio Sarapiquí
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An elusive croc (Crocodylus acutus) emerges from the depths
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Green iguana (Iguana iguana)
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Proboscis bats (Rhynchonycteris naso)

Proboscis bats (Rhynchonycteris naso) are a curious group of bats for a number of reasons apart from their elongated nose. Like most bats they forage nocturnally, flying over waterways and echolocating to hunt midges, mosquitoes, and caddisflies. However, instead of looking for a dark refuge during the day, they choose to perch together on large flat vertical surfaces. These roosts can often be in direct sunlight, where the bats neatly arrange themselves into rows or curves. In the photo above you can see that someone is missing from the formation. Because the bats are exposed to sunlight, they regulate their body temperature by relocating roost sites according to ambient temperature. But the story gets more interesting. A territorial male will actively defend females at one site, but relinquish mating opportunities at another site— so a male’s reproductive fitness is influenced by temperature. Read more about my friend Luke Wilde’s cool study at La Selva here!

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Spectacled caiman (Caiman crocodilus)
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Green ibis (Mesembrinibis cayennensis)
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This second croc allowed for a better view than the first, sunbathing along the river banks. It’s always hard to comprehend the sheer size of these animals in person, truly an apex predator in any habitat of their choosing here in Costa Rica. Though equally robust, American crocodiles (Crocodylus acutus) tend to have narrower snouts than Australian salties (C. porosus), giving them more of a classic crocodile appearance.

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Two-toed sloth (Choloepus hoffmanni)

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