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Are you outside? Look up, look around you, and if you don’t
spot one immediately, wait a few minutes. Your chances of seeing
a common raven (Corvus corax) are very good. These jet-black birds
with glossy, iridescent feathers, chisel-shaped black bills, and wingspans
of up to four feet are ubiquitous in the Mojave and Colorado deserts. |
But this wasn’t always the case. Ravens were uncommon
in the California deserts in the first half of the 20th century; wildlife
biologists now estimate that raven populations have increased here
by approximately 1000 percent in the last 35 years. How has this happened?
Although the question cannot be answered with any finality, much of
the raven’s success seems to hinge on its innate intelligence
and its ability to exploit conditions provided by human habitation.
Wherever people
are, ravens thrive. They are world-class opportunists. |
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It has been said that more has been written about the
raven than about any other bird. Even before recorded
history, Native Americans told many
stories about the raven, the majority of which characterized it as a mischievous
and clever animal. Perhaps ravens fascinate us because they have been observed
playing with sticks and passing stones to each other, and they have been
taught to count, mimic human speech, and to read the face of a clock. Ravens
reportedly mate for life (although this statement has been contested by
some observers) and they have an extraordinarily diverse vocabulary of
vocalizations. |
The social life of a raven is complex. These birds are very gregarious
and sometimes form groups composed of as many as one hundred individuals.
Their ability to produce a wide range of sounds may attest to a sophisticated
avian vocabulary that, although largely impenetrable to humans, allows
them to express themselves and communicate a vast amount of information
crucial to their survival. A begging nestling, for example, will eventually
produce a “yell call,” which is thought to bring the attention
of other juvenile ravens to a newly-discovered food supply. |
Other types of vocalizations include “vocal play,” a sort
of warm-up repertoire that prepares the young raven for more advanced sounds.
After being
fed, nestlings will produce “comfort sounds,” consisting of a soft
quiet medley. If a person disturbed a raven’s nest, or a bird of prey flew
overhead, the raven might issue a “predatory alarm,” a series of
short high-pitched keck-keck-keck calls. When in flight, a raven might produce
a “demonstrative call” to announce its presence within a particular
territory. If it is being pursued by another raven whose territory it has invaded,
it will utter “chase calls.” “Knocking calls” are made
only by females, either to establish their dominant status within a group, or
in displays where they are being courted by a male. Surprisingly, ravens in the
wild do not appear to mimic other sounds. The ability to mimic human speech has
been clearly observed only in ravens raised in isolation. |
Ravens may live as long as 50 years. They reach sexual maturity at about
two to three years old, and after breeding the female typically lays three
to seven eggs, out of which two to four young birds may successfully fledge.
It is the female who appears to do most of the nest-building, although
males assist in the process by delivering twigs and sticks to the female
as she constructs a nest first by stacking the larger pieces and then by
weaving a cup from the smaller branches and twigs. Once the eggs are laid
it takes about 18-20 days for them
to hatch. |
Over the course of the last two decades ravens have earned a villainous
reputation among naturalists in the California deserts—they have
been identified as one of the main predators of desert tortoise (Gopherus
agassizii) hatchlings, a species listed as “threatened” on
the federal Endangered Species List. Early efforts made by several agencies
to mitigate the raven’s impact on desert tortoises have had limited
success. Raven populations continue to increase while tortoise populations
remain in decline. Where tortoises usually suffer from loss of habitat
due to human encroachment, ravens often prosper: open landfills provide
acres of available food; roadways become slaughterhouses for wildlife subsequently
consumed by scavengers such as ravens; utility poles provide excellent
nesting platforms; and pet food left outdoors where it is accessible to
ravens, coyotes, and other scavengers tips the scales of survival—to
the detriment of other species. |
Whereas a certain number of ravens usually die off each year due to a
natural reduction in seasonal food sources, the availability of a steady
supply of pet food has reduced that number. Ravens are neither predators
nor scavengers—they are both. Some of their energy income is collected
through predation while another portion is obtained by scavenging, but
when people unwittingly provide them with fast food in the form of dog
chow or shredded fishheads, why should they bother to hunt for a gourmet
meal? They’re shrewd enough to grab what they can, when they can.
And grab they do. |
How is it that the human presence in a landscape can have such radically
disparate impacts on different species? The answer may lie in the innate
behavior of those species and how they are or are not able to adapt to
the human influences imposed on them. Desert tortoises need open land with
enough vegetation to sustain them and space in which to forage for that
vegetation. They also need plenty of earth in which to dig their burrows.
Ravens do not have those same requirements—they are able to find
what they need in a landscape shrunk by human development yet swollen with
the detritus often introduced by human civilization. |
In order to help restore the balance between ravens and tortoises,
and between ravens and the other species they affect, we can become aware
of some of our current practices and change them. These are some of the
things we might do to help control raven populations: (1) Keep a lid on
all garbage receptacles. (2) Cover and store food if you leave your campsite
during the day. (3) Do not feed any wildlife! (4) Observe the speed limit:
speeding increases the incidence of roadkill accidents. (5) Use drip irrigation
in your yard: pools of water attract ravens. (6) Bring pet food and water
bowls indoors when they are not being used
by your pets. |
Although we did not intentionally create the recent explosion in raven
populations, it seems as though we have inadvertently enabled ravens to
dominate the California desert landscape. If we behave responsibly today,
we may yet restore balance to the desert ecosystem. If not, certain species
may become a tale of nevermore. |
by Park Ranger Caryn Davidson |
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http://www.nps.gov/jotr/animals/birds/ravens/ravens.html
last modified: 06/03/04
web editor: Sandra kaye |