The year is 1982. The US Fish and Wildlife Service (USFWS) permeated the dry, tropical forests of a particular Pacific island in search of the Mariana Crow. They walked along prepared transects and counted. Using the magic of statistics, they extrapolated. And they determined that there were nearly one thousand crows inhabiting the small vegetated limestone dot that is Rota.
In 1995, the US Fish and Wildlife Service repeated their survey. This time they estimated the Rota crow population to be around five hundred individuals.
Today, there are fewer than three hundred crows.
Islands are curious places. Before we humans figured out how to navigate the big blue ocean and sky, the only life forms to find their way to these isolated land masses did so very infrequently. They waited patiently on a raft of flotsam, or they yielded to the unpredictable forces and directions of the wind. One day, that raft knocked up against the shore of a distant island, slurping against the sea soaked rocks. On it, a small gecko, a cockroach, a flea, a tick, a budding Pandandus nut. They were cast upon the shore. A ballooning spider and wind-blown seeds swirled towards the island as well. A few seabirds built their nests on the rocky outcrops. A vagrant honeyeater... a lost flycatcher... a disoriented crow...
But not everyone remained. Without a mate it was impossible to procreate, harder still to begin a permanent population. But every once in a while, someone did manage to stick around. And they reproduced, and spread throughout the island. They changed and adapted, and became well-suited to island life. And it took a very, very long time.
Lichens gave way to moss, and mossy beds hosted cozy spaces for small vascular plants. They died and became soil, and the soil layer thickened, and larger plants showed up, took root, and grew. They permeated the island. Large, fruiting trees provided food for newcomers. Slowly, the Mariana Islands became home to various species of white-eye, flycatcher, dove, kingfisher, megapode, and rail. Geckos and skinks had claimed their positions. A few species of bat also made themselves at home. Not to mention the insects. Grasshoppers, crickets, katydids, beetles, leafhoppers, flies, mosquitos, and hemipterans took up residence among grasses, beneath leaves, within the soil, and they were an excellent source of food for everything else. New species arrived, died, stayed, anchored, other species went extinct. Wave after wave of life and death, colonization and extinction. And things went on this way for hundreds of thousands of years...
Plants and animals were not the only life forms to find their way to islands. Humans inevitably happened upon them as well. Four thousand years ago, southeast Asians navigated the Pacific in search of new worlds to colonize. The Mariana Islands eventually became home to these people. And they propagated, and changed, and became well-suited to island life. We know them as the Chamorros.
It is impossible to know what the island was like before the arrival of the Chamorros. History has a way of repeating itself, and if we've learned anything from previous human colonizations, it can be assumed that the Chamorros had an impact on the native wildlife of these islands, and a negative one at that. Brief glimpses into Rota's natural history in the form of cave excavations shed some light. Of twenty-two avian species native to Rota, 13 no longer exist. These include a large parrot, a parrotfinch, a flycatcher, a pigeon, a giant ground dove, a swiftlet, and a handful of marsh birds and waterfowl (Rota once had natural wetlands). That says nothing about the mammals, reptiles, insects, and plants, the freshwater fish (no one has looked into it yet). But we can't blame these extinctions on the Chamorros alone.
In the late sixteenth century, another group of humans showed up. This time they were Spanish. And they used brute force to make the Mariana Islands their own. They reduced the Chamorro population of approximately one hundred thousand, to a mere 3500. And no doubt the Chamorros were not the only ones to suffer. The Spanish were able to invade on a much larger scale than the Southeast Asians before them. They brought technology, religion, diseases, and domesticated animals. They also ate pigeons, doves, and fruit bats. And more and more of them arrived on ships. Ballast water containing a soup of new species was dumped on and around the islands. Forests were cleared and the land was worked in order to provide food for a growing population. Churches were built in order to worship, schools were erected. Rats pervaded the forests, cats became feral, pigs, goats, and cows trampled the land. And things went on this way for a few hundred years...
After the Spanish-American war in 1898, Guam was gifted to the United States, and the remaining islands were claimed by Germany. Then, in 1915, the Japanese decided they wanted to go to war with Germany. They were the next to invade, and a few years later, they were mandated the northern Mariana islands by the League of Nations (now the United Nations). The islands soon became covered in sugar cane fields. And with the sugar cane came more rats, and copious amounts of insects. DDT, unknown then of its toxic cascading effect, was sprayed to control insect pests. The Japanese also introduced the Black Drongo (Dicrurus macrocercus), a small black bird native to Asia and the Middle East, to aid in insect control. No doubt any remaining marshes were sucked dry. Wetlands disappeared, and with them, surely a few birds and fish. The drongo flourished. So did the rats, and the cats. And things went on like this for a few decades...
All the while, the Mariana Crow persisted within remaining patches of mature limestone forest.
It's difficult to even guess at how many crows inhabited Rota when the Chamorros arrived, or when the Spanish invaded, or when the Japanese took over. It's even more difficult to guess when the crow arrived, and from where. Some people speculate that its closest living relative is the Slender-billed Crow (Corvus enca) from Indonesia. But it could have come from any number of places.
We do know that it is disappearing, and quickly.
Since the 1960s the crow population has been on a steady decline. Or perhaps people just began to notice it then. In any case, within the last thirty years the population went from 1000 birds to fewer than 300. One study suggests that no crows will remain in seventy-five years (http://www.sciencedaily.com/releases/2010/12/101220091917.htm).
So, what's the deal?
Aside from the fact that these isolated Pacific islands are difficult to get to and take a long time to accumulate established breeding populations, they are also subject to the occasional typhoon. Whether they were a precursor to the plight of the crow, or just added insult to injury, the series of typhoons that hit the Marianas in the mid-1990s rocked the island of Rota. A great number of trees were defoliated and food sources were scarce. With little food and few places to build nests, the subsequent breeding seasons undoubtably yielded few young crows. Not to mention the number of crows and other birds that were swept away with the storms. Before the 1982 USFWS census no one had paid much attention to the Mariana Crow. No one had any idea how many there might be. No one will ever know the absolute cost of the typhoons.
We can also look to the south for some clues. The Mariana Crow used to persist on the larger island of Guam. But then the Brown Tree Snake (Boiga irregularis) was introduced (by humans) in the late 1940's. Young snakes thrived on small lizards and geckos, then subsisted on birds as they grew larger. They ate all the crows, and made little work of each and every other avian species.
Rota presently does not have an established population of this snake, but there are personal accounts of their presence. They very well could be here. But could they be affecting the crow population at such a low density? Consider other potential predators. What about rats, cats, and monitor lizards (oh yeah, they aren't exactly supposed to be here either but no one is sure who to blame for this...). Consider competing species, the Black Drongo for example. What about disease? Could all of these factors add up to answer the question of why the Mariana Crow has become so scarce?
Blood samples are periodically shipped to the University of Washington for processing, but no diseases have been singled out as the reason for the crow decline. There is, however, solid evidence that cats are eating crows. Besides the fact that biologists have found nearly a dozen crow carcasses displaying the tell-tale signs of cat predation, one cat in particular was caught red-handed. A stomach analysis revealed half-digested crow feathers.
Bingo.
But can we blame it all on cats? I suppose only time will tell. That cat with the crow in its gut was the key to getting proper funding for a feline eradication program on Rota. Assuming that cats are the main culprit, and if the project is successful, then adult mortality rates should drop, and crow numbers would increase. But this will take some time.
In the meantime, it wouldn't hurt to consider and explore a few more theories as to why the crows are disappearing.
There are species that occur on Rota that were introduced by humans for pest control, or accidentally. The Black Drongo, Black Rat (Rattus rattus), and Mangrove Monitor (Varanus indicus) are three such species. Black Drongos could certainly be competing with the crow, and rats and monitor lizards are both known to eat eggs. But nest predation has not been identified as a major concern... yet. The Black Drongo on the other hand, which has been introduced to a number of Pacific Islands, has a track record for causing native species to decline and even become extinct. Here on Rota I have personally witnessed a few of our radio-tagged crows fall victim the wrath of territorial drongos.
The Black Drongo has done very well for itself indeed. They're everywhere. Perched on telephone wires, on the branches of trees, swooping mid-air, devouring insects. They primarily prey on insects. Crows also enjoy insects, but they are omnivores and will eat anything from lizards and hermit crabs, to bird eggs, fruit, and seeds. Thus, competition for food probably isn't an issue. But Rota is a small island, and these two species could be competing for space. Unfortunately no studies have investigated the potential spacial war between the drongo and the crow, so there is no way to know.
Nest predation is another potential concern. Rats and monitor lizards like to eat bird eggs. They probably eat crow eggs. But again, there is no concrete evidence to pin anything on these guys. The University of Washington does conduct nest monitoring during the breeding season but without infrared cameras, it is impossible to capture nocturnal predators (such as the rat and monitor), and again, no way to know for sure.
So there it is. Well, sort of. A lot of maybes. A series of storms. Potential competitors for space, egg thieves, and predators. Which one presents the biggest threat to the survival of the Mariana Crow? How will we go about fixing it? Only time (and money) will tell.
Hopefully we figure it out within the next 75 years.
In 1995, the US Fish and Wildlife Service repeated their survey. This time they estimated the Rota crow population to be around five hundred individuals.
Today, there are fewer than three hundred crows.
 |
| Coral Atoll, Rota, CNMI |
Islands are curious places. Before we humans figured out how to navigate the big blue ocean and sky, the only life forms to find their way to these isolated land masses did so very infrequently. They waited patiently on a raft of flotsam, or they yielded to the unpredictable forces and directions of the wind. One day, that raft knocked up against the shore of a distant island, slurping against the sea soaked rocks. On it, a small gecko, a cockroach, a flea, a tick, a budding Pandandus nut. They were cast upon the shore. A ballooning spider and wind-blown seeds swirled towards the island as well. A few seabirds built their nests on the rocky outcrops. A vagrant honeyeater... a lost flycatcher... a disoriented crow...
But not everyone remained. Without a mate it was impossible to procreate, harder still to begin a permanent population. But every once in a while, someone did manage to stick around. And they reproduced, and spread throughout the island. They changed and adapted, and became well-suited to island life. And it took a very, very long time.
Lichens gave way to moss, and mossy beds hosted cozy spaces for small vascular plants. They died and became soil, and the soil layer thickened, and larger plants showed up, took root, and grew. They permeated the island. Large, fruiting trees provided food for newcomers. Slowly, the Mariana Islands became home to various species of white-eye, flycatcher, dove, kingfisher, megapode, and rail. Geckos and skinks had claimed their positions. A few species of bat also made themselves at home. Not to mention the insects. Grasshoppers, crickets, katydids, beetles, leafhoppers, flies, mosquitos, and hemipterans took up residence among grasses, beneath leaves, within the soil, and they were an excellent source of food for everything else. New species arrived, died, stayed, anchored, other species went extinct. Wave after wave of life and death, colonization and extinction. And things went on this way for hundreds of thousands of years...
Plants and animals were not the only life forms to find their way to islands. Humans inevitably happened upon them as well. Four thousand years ago, southeast Asians navigated the Pacific in search of new worlds to colonize. The Mariana Islands eventually became home to these people. And they propagated, and changed, and became well-suited to island life. We know them as the Chamorros.
It is impossible to know what the island was like before the arrival of the Chamorros. History has a way of repeating itself, and if we've learned anything from previous human colonizations, it can be assumed that the Chamorros had an impact on the native wildlife of these islands, and a negative one at that. Brief glimpses into Rota's natural history in the form of cave excavations shed some light. Of twenty-two avian species native to Rota, 13 no longer exist. These include a large parrot, a parrotfinch, a flycatcher, a pigeon, a giant ground dove, a swiftlet, and a handful of marsh birds and waterfowl (Rota once had natural wetlands). That says nothing about the mammals, reptiles, insects, and plants, the freshwater fish (no one has looked into it yet). But we can't blame these extinctions on the Chamorros alone.
In the late sixteenth century, another group of humans showed up. This time they were Spanish. And they used brute force to make the Mariana Islands their own. They reduced the Chamorro population of approximately one hundred thousand, to a mere 3500. And no doubt the Chamorros were not the only ones to suffer. The Spanish were able to invade on a much larger scale than the Southeast Asians before them. They brought technology, religion, diseases, and domesticated animals. They also ate pigeons, doves, and fruit bats. And more and more of them arrived on ships. Ballast water containing a soup of new species was dumped on and around the islands. Forests were cleared and the land was worked in order to provide food for a growing population. Churches were built in order to worship, schools were erected. Rats pervaded the forests, cats became feral, pigs, goats, and cows trampled the land. And things went on this way for a few hundred years...
After the Spanish-American war in 1898, Guam was gifted to the United States, and the remaining islands were claimed by Germany. Then, in 1915, the Japanese decided they wanted to go to war with Germany. They were the next to invade, and a few years later, they were mandated the northern Mariana islands by the League of Nations (now the United Nations). The islands soon became covered in sugar cane fields. And with the sugar cane came more rats, and copious amounts of insects. DDT, unknown then of its toxic cascading effect, was sprayed to control insect pests. The Japanese also introduced the Black Drongo (Dicrurus macrocercus), a small black bird native to Asia and the Middle East, to aid in insect control. No doubt any remaining marshes were sucked dry. Wetlands disappeared, and with them, surely a few birds and fish. The drongo flourished. So did the rats, and the cats. And things went on like this for a few decades...
All the while, the Mariana Crow persisted within remaining patches of mature limestone forest.
It's difficult to even guess at how many crows inhabited Rota when the Chamorros arrived, or when the Spanish invaded, or when the Japanese took over. It's even more difficult to guess when the crow arrived, and from where. Some people speculate that its closest living relative is the Slender-billed Crow (Corvus enca) from Indonesia. But it could have come from any number of places.
We do know that it is disappearing, and quickly.
Since the 1960s the crow population has been on a steady decline. Or perhaps people just began to notice it then. In any case, within the last thirty years the population went from 1000 birds to fewer than 300. One study suggests that no crows will remain in seventy-five years (http://www.sciencedaily.com/releases/2010/12/101220091917.htm).
So, what's the deal?
Aside from the fact that these isolated Pacific islands are difficult to get to and take a long time to accumulate established breeding populations, they are also subject to the occasional typhoon. Whether they were a precursor to the plight of the crow, or just added insult to injury, the series of typhoons that hit the Marianas in the mid-1990s rocked the island of Rota. A great number of trees were defoliated and food sources were scarce. With little food and few places to build nests, the subsequent breeding seasons undoubtably yielded few young crows. Not to mention the number of crows and other birds that were swept away with the storms. Before the 1982 USFWS census no one had paid much attention to the Mariana Crow. No one had any idea how many there might be. No one will ever know the absolute cost of the typhoons.
 |
| Adult Mariana Crow |
We can also look to the south for some clues. The Mariana Crow used to persist on the larger island of Guam. But then the Brown Tree Snake (Boiga irregularis) was introduced (by humans) in the late 1940's. Young snakes thrived on small lizards and geckos, then subsisted on birds as they grew larger. They ate all the crows, and made little work of each and every other avian species.
Rota presently does not have an established population of this snake, but there are personal accounts of their presence. They very well could be here. But could they be affecting the crow population at such a low density? Consider other potential predators. What about rats, cats, and monitor lizards (oh yeah, they aren't exactly supposed to be here either but no one is sure who to blame for this...). Consider competing species, the Black Drongo for example. What about disease? Could all of these factors add up to answer the question of why the Mariana Crow has become so scarce?
Blood samples are periodically shipped to the University of Washington for processing, but no diseases have been singled out as the reason for the crow decline. There is, however, solid evidence that cats are eating crows. Besides the fact that biologists have found nearly a dozen crow carcasses displaying the tell-tale signs of cat predation, one cat in particular was caught red-handed. A stomach analysis revealed half-digested crow feathers.
Bingo.
But can we blame it all on cats? I suppose only time will tell. That cat with the crow in its gut was the key to getting proper funding for a feline eradication program on Rota. Assuming that cats are the main culprit, and if the project is successful, then adult mortality rates should drop, and crow numbers would increase. But this will take some time.
In the meantime, it wouldn't hurt to consider and explore a few more theories as to why the crows are disappearing.
There are species that occur on Rota that were introduced by humans for pest control, or accidentally. The Black Drongo, Black Rat (Rattus rattus), and Mangrove Monitor (Varanus indicus) are three such species. Black Drongos could certainly be competing with the crow, and rats and monitor lizards are both known to eat eggs. But nest predation has not been identified as a major concern... yet. The Black Drongo on the other hand, which has been introduced to a number of Pacific Islands, has a track record for causing native species to decline and even become extinct. Here on Rota I have personally witnessed a few of our radio-tagged crows fall victim the wrath of territorial drongos.
The Black Drongo has done very well for itself indeed. They're everywhere. Perched on telephone wires, on the branches of trees, swooping mid-air, devouring insects. They primarily prey on insects. Crows also enjoy insects, but they are omnivores and will eat anything from lizards and hermit crabs, to bird eggs, fruit, and seeds. Thus, competition for food probably isn't an issue. But Rota is a small island, and these two species could be competing for space. Unfortunately no studies have investigated the potential spacial war between the drongo and the crow, so there is no way to know.
Nest predation is another potential concern. Rats and monitor lizards like to eat bird eggs. They probably eat crow eggs. But again, there is no concrete evidence to pin anything on these guys. The University of Washington does conduct nest monitoring during the breeding season but without infrared cameras, it is impossible to capture nocturnal predators (such as the rat and monitor), and again, no way to know for sure.
So there it is. Well, sort of. A lot of maybes. A series of storms. Potential competitors for space, egg thieves, and predators. Which one presents the biggest threat to the survival of the Mariana Crow? How will we go about fixing it? Only time (and money) will tell.
Hopefully we figure it out within the next 75 years.
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