Tuesday, January 13, 2015

Nocturnal Creatures (Part II: Bats)

My home state of New Jersey (and its neighboring states) proudly boasts nine species of bats; six of these can be found within its borders year-round, while the other three are only with us during the warmer months of the year.

Costa Rica on the other hand has an astounding diversity of bats; to date, the list consists of 111 species!  This is more than all other Costa Rican mammal species combined, and is more than twice the number of bat species found in all of North America -- not bad for a country with less area than Lake Michigan!

Lesser Sac-winged Bats (Saccopteryx leptura) in a tree cavity
Our efforts to study neotropical owls (see Nocturnal Creatures Part I) have provided us with the exciting opportunity to get to know some of the many bat species on the eastern Nicoya Peninsula.  Whether we like it or not, setting up nets at night in the tropics inevitably results in encounters with bats.  Luckily, we like that just fine.  Some of the bat species we have captured alongside the owls we are studying have been truly spectacular; indeed, some of them are nearly as large as the owls themselves!

The Greater Spear-nosed Bat (Phyllostomus hastatus),
 the second-largest bat in Costa Rica
When we were on the first net check of our very first night of owl-netting, we approached a net containing one owl and two Greater Spear-nosed Bats (Phyllostomus hastatus, pictured above and below).  These bats are so large that, in the dim light cast by our headlamps on our approach, we thought they were owls!  The two bats had wingspans of sixteen and seventeen inches, just a few inches shy of the wingspan of the Pacific Screech-Owl that had been captured alongside them.

Greater Spear-nosed Bat (Phyllostomus hastatus)
On the opposite end of the size spectrum we have the Black Myotis (Myotis nigricans), which is one of several equally tiny Costa Rican bats in the genera Myotis and Rhogeesa.  The M. nigricans we encountered just a few days before Christmas weighed just 3.5 grams, about the same weight as a Ruby-throated Hummingbird!

The tiny Black Myotis (Myotis nigricans)
With many outwardly similar-looking genera and species, identification of tropical bats can be a meticulous process.  Distinctions between species are often as subtle as differences in the basal coloration of individual hairs or in the arrangement of teeth.  Since this is the first year we have set up nets after dark, it has been a fun challenge to keep up with the sudden influx of unfamiliar bats that need identifying.
The characteristic "gap" (actually filled by two tiny premolars)
between the molars and canines of species in the genus Myotis
Other notable mentions include the Jamaican Fruit Bat (Artibeus jameicensis), the most commonly encountered bat at Finca Pura Vida; the Common Long-tongued Bat (Glossophaga soricina), which we believe to be at least one of the species that visits the hummingbird feeders at night to sip nectar; and the Pale Spear-nosed Bat (Phyllostomus discolor), the second-largest bat we've captured at the Finca.
The Jamaican Fruit Bat (Artibeus jameicensis)
The handsome (?) face of a Pale Spear-nosed Bat (Phyllostomus discolor)
The nine bat species we have netted so far at Finca Pura Vida are only a fraction of those that occur in the area: the nearby Reserva Natural Absoluta Cabo Blanco -- where one of our bird banding stations is located -- has a list of 41 bat species that have been observed there, and that list continues to grow.  Affiliates of the Cabo Blanco Reserve recently conducted a bat survey, identifying bats both in-hand (using mist nets to capture them) and by recordings of their sonar (which, not unlike bird songs, can be diagnostic of particular species).  During this survey they captured a Silver-tipped Myotis (Myotis albescens) -- a species never before recorded in the reserve -- in addition to twenty other species which were already on the Reserve's official mammal list.

We are thrilled to have the opportunity to become more familiar with the nocturnal fauna of the neotropics, most of which are seldom seen by diurnal animals like Homo sapiens.  The demands placed upon these flying nocturnal creatures have driven the evolution of adaptations that render them utterly bizarre and endlessly fascinating, and the chance to see them up-close is just plain cool.

Saturday, January 3, 2015

Following Birds that Follow Ants

Army ants (Eciton burchellii and related species) mean serious business in tropical forests: each day the members of a colony form a large swarm that raids the surrounding forest, stinging to death and dismembering anything small or slow enough to fail to escape.  A passing swarm (which can easily be thirty feet wide and consist of several hundred thousand individuals) creates a panic among the invertebrates of the forest floor, forcing them to come out of hiding and flee.

A swarm is an event of such magnitude that it is clearly audible to human ears.  It consists of the clicks and taps of grasshoppers and crickets attempting to stay ahead of the front, and the ominous hissing of millions of ant legs passing over the leaves of the forest floor.  It is rare that ant-scale events draw the attention of human-sized observers, but when they do, they fascinate us; the mass emergence of periodical (17-year) cicadas and the migrations of monarchs and dragonflies are two prominent examples that regularly make headlines.

As the swarms move through the forest, they remain connected to their stationary above-ground colony by a single column.  The column consists of ants traveling in both directions: those returning from the front of the swarm carrying captured food items, and those leaving the colony having dropped off their booty to return to the swarm.  The video below shows ants traveling along their column.  Ants traveling toward the camera are returning to their colony with food; those traveling away are leaving to rejoin the swarm.

video

These events attract the attention of certain bird species, which join the front of the swarm to opportunistically capture the invertebrates and other small animals that the ants flush out of hiding. Ant-following birds from the surrounding forest temporarily ignore territorial boundaries and congregate in large numbers to take advantage of the sudden availability of prey.

Northern Barred-Woodcreeper following a trail army ants up a tree trunk
These raids provide unique opportunities to see many birds concentrated at ground-level.  One particularly large army ant swarm occurred nearby the bird banding station at Cabo Blanco late last month, attracting large numbers of Ruddy Woodcreepers, Gray-headed Tanagers, Red-crowned Ant-Tanagers, and Northern Barred-Woodcreepers.  In addition to being an enjoyable spectacle on its own, it was doubly exciting to see that most of the birds present were sporting our aluminum leg bands:

Ruddy Woodcreeper

Gray-headed Tanager
I watched this group of birds for quite a while as the ant swarm moved through the forest.  I decided to fall back when the swarm started down the slope of a steep ravine, and when my tolerance for army ant stings reached its critical threshold.