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How do you tell a Fish Crow
from an American Crow?
Fish Crows are a rather small species endemic to the
Southeastern US. Typically they have been restricted to the coastline from southern New England to Texas, but in the last few
decades have been expanding their range, especially inland up large rivers.
Visually, Fish Crows are difficult to tell from American Crows.
Unless one has a great deal of experience in close observation of the species, identification is only safely done by voice.
The calls of Fish Crows and American Crows are readily told
apart. American Crows most frequently give the familiar "caw caw." Fish Crows have a much more nasal call that may be better
enumerated "awh" or "uhn." The most diagnostic call of the Fish Crow is the double noted "uh-uh." I always say that if you want to
tell the species of crow, ask it if it is an American Crow. Fish Crows will deny this by their emphatic "uh-uh!"
Fish Crow calls can be confused with the begging calls of
American Crows. It should be pointed out that these begging calls are given not just by dependent young crows, but also by adult
crows in certain situations. Most prominently, early in the breeding cycle of American Crows the females will give begging calls frequently.
Why do crows congregate in large numbers to sleep?
One of the great animal phenomena of the world is the congregation
of large numbers of birds into a single group to sleep together. Such communal sleeping groups are known as "roosts." Many species
roost in groups; such things as crows, robins, starlings, blackbirds, swallows, and herons. Most do this only outside of the breeding
season. Some species, like starlings, also forage together in great numbers. Others, such as herons, disperse out from these gathering
areas to forage singly. For crows, roosts are primarily a fall and winter thing. Numbers peak in winter and then decrease near the
beginning of the breeding season (usually in March). It appears that all crows will join winter roosts, even territorial breeding crows. Most
breeding crows sleep on their territories during the breeding season, but join the roosts afterward.
Just why birds congregate in such large groups is still largely a
matter of conjecture. A number of hypotheses have been constructed to explain it:
* One is that the birds simply are congregating in the most
favorable spot (protection from predators, protection from the elements, the only trees suitable for roosting, etc.), and they don't
mind doing it with a bunch of other birds. This idea is kind of analogous to a crowded hotel: everyone has the same needs being
met at the same place, but no one is really interacting with anyone else.
* Another idea is that the birds get some protection from
predators by being in a large group. This is the "wagontrain" analogy: safety in numbers. Crows are most afraid of large owls, and
sleeping with a bunch of other crows could afford some protection for an individual crow.
* Another idea is the information center hypothesis, where
information about profitable foraging areas is transmitted. The idea is that an individual that did poorly foraging for itself on one day can
watch for other individuals coming in to the roost that look fat and happy, that obviously found some rich source of food. Then the
hungry individual can either backtrack the happy ones' flight paths, or follow them out first thing in the morning to the good food source.
* Another food related idea is the patch-sitting hypothesis. This
theory is similar to the first one mentioned, in that roosts congregate around a large, non-defendable, reliable food source. So, first thing
and last thing in the day, food is available. It need not be the best food, but it is something to eat to get them going. The birds can
then disperse out and do whatever they need to do, having had some kind of breakfast first. Roosts, then, will form in suitable
roosting habitat near these large food sources. For crows, such abundant sources might be landfills, commercial composting facilities, or certain types of agricultural fields.
Crows have been congregating in large roosts in the fall and winter
for as long as there have been crows. Crow roosts can range from small scattered roosts of under one hundred individuals to the
spectacularly large roosts of hundreds of thousands, or even more than a million crows! A roost in Fort Cobb, Oklahoma was estimated
to hold over two million crows (Gerald Iams, 1972, State of Oklahoma Upland Game Inventory W-82-R-10). Most roosts are much smaller, but roosts of tens of thousands are common.
Before heading to roost, crows will congregate in some area away
from the final roosting site, usually an hour or two before complete darkness. Here the crows spend a lot of time calling, chasing, and
fighting. Right at dark the main body of the group will move toward the final roosting spot. Sometimes this final movement is relatively
quiet, but usually it is still quite noisy. I have seen crows coming together from several separate congregation areas, heading to one
final staging area where they all coalesce, then everyone heads to the final roost. The final roost can be a cohesive group in a single
woodlot, or it can be rather diffusely spread out over quite a wide area of suitable trees.
Many, perhaps most, people who witness large roosts or the flight
lines to them are reminded of Alfred Hitchcock's movie "The Birds." I think this association is unfortunate. It makes the allusion that
somehow what we are watching is sinister, unnatural, and threatening. In fact, it is none of the above, but one of the most
natural things in the world. I would prefer to replace this association with the idea that such roosts are something to be marveled at. To
me they always bring up the idea of Passenger Pigeons. When Europeans first came to North America, the Passenger Pigeon (Ectopistes migratorius) was the most abundant bird on earth.
Migrating flocks were said to darken the sky for hours as they passed. Despite their incredible abundance, they are completely
gone now, driven extinct by the early years of the 20th century. A combination of habitat destruction (the complete devastation of the
eastern hardwood forests) and hunting for sale as meat in commercial markets destroyed one of the greatest natural spectacles on earth. Not a single Passenger Pigeon remains on
earth today, nor do any people that remember seeing their massive flocks. I would like for people to look at the large congregations of
the similarly-sized American Crows going to roost and think that, despite how impressive they might be, they are but the slightest hint
of what the Passenger Pigeon flocks must have been like.
West Nile virus - The Crow Connection
Crows, both American and Fish, are being associated with the
disease outbreak because they are the obvious victims: they are dying in unusually large numbers.
I first received calls about oddly sick crows in the New York City
area the last week of August 1999. Several people called me about finding sick and unusually approachable crows, as well as an unusual number of dead crows. Finding dead birds (other than
roadkills or window strikes) is unusual, as most sick birds seem to find some quiet and hidden place to die. Finding one dead crow
doesn't mean much, but finding several crows within a short period of time is highly unusual and indicates that something odd is going on.
I first suspected a poisoning event, especially since someone had
been poisoning pigeons in Central Park just previous to the finding of dead crows. The state pathologist's lab, however, got some of the
dead crows and tested them for a number of things, but found no indication of pesticide poisoning. Instead, the symptoms were
consistent with a viral infection. When I spoke with them in mid September they had sent samples to a lab in Colorado, but had not yet received the results.
At the same time the outbreak of "St. Louis encephalitis" was being
reported in much the same areas where crows were being found dead. It seemed odd to me that crows would be dying from that
disease, as most birds that are exposed to it get mildly sick, but do not die from it. A new disease of some other source of crow mortality made more sense.
On 23 September, the State of New York Department of Health
released the information that several bird specimens from the NYC area had been diagnosed as being infected with a virus that most
closely resembled West Nile virus. These birds included a crow from Westchester County, as well as birds from the Bronx Zoo.
Exact connections between the birds and the human illnesses have
been difficult to establish (as is to be expected).
Are crows spreading the disease (or will they be)?
Probably not, for several reasons. First is that crows don't move all
that far in the first place. Crows are only partially migratory (see my discussion of this topic on the crow faq page). Some of the crows
breeding in the New York City area will probably move a bit farther south in the winter, but nearly all will stay put. They can make rather
long daily movements, but not long distance migration. By the time the Canadian vacationers come down to mingle with the New York
crows (late October/ November) most of the mosquitoes will be inactive and the transmission rate will be very low.
The second reason that I doubt crows will be major carriors of the
disease is that many of them are dying from it. It is hard to judge from such a long distance away, but the number of dying crows
reported is frighteningly large. It is far more likely that other species that get the disease but are not killed by it will be the most
important source of the virus. (See the 27 October Wildlife Health Alert for a listing of species known to have the virus.)
If crows start dying in more southern states, it is likely that they will
be local southern crows infected via other bird species.
Why are crows being hit so hard from this disease and not other birds?
Good question; no one knows. Every species is different, of course,
and different responses to the same stressors are to be expected. As stated by the CDC web site: "Birds usually do not show any
symptoms when infected with West Nile virus. However, natural disease due to the virus has been observed in a pigeon in Egypt (7),
and inoculation of certain avian species (e.g., pigeons, chickens, ducks, gulls, and corvids) causes occasional encephalitis and death
or long-term virus persistence (7,10,17,18). Chick embryos may be killed by the virus (8)."
Of course, this is a new disease never encountered by the birds in
North America before. Any time a population is exposed to a brand new form of a disease, epidemics can happen because they have no defenses.
Reports of dead American Crows, Fish Crows, and Blue Jays
indicate that the vulnerability is a family thing.
Where did the disease come from?
No one knows. It has been suggested that it came from a bird
smuggled into the US from Africa or Asia. Because NYC is such a huge port of entry into this country, this possibility seems
reasonable. Legally imported live birds must first undergo a month of quarantine, in theory to prevent just such an incursion of disease
into the country. Illegally imported birds obviously do not pass through the quarantine. A migrant bird from Africa, Asia, or Europe
is an even less likely possibility, but the probability if non-zero. Africa is pretty much out (or if not, the birders want to know where
that African bird is!), but Europe is not. A very few European migrants show up on the East Coast every year, but only shorebirds would be candidates for an August arrival.
The 17 Dec 1999 issue of Science had two articles on West Nile.
One [Lanciotti, R.S., et al. 1999. Origin of the West Nile Virus Responsible for an Outbreak of Encephalitis in the Northeastern
United States. Science 286 (5448): 2333-2337] described the strain found in North America as matching a strain isolated from the brain
of a dead "goose" in Israel in 1998 (no species given, nor any indication if it was a wild or domestic bird). The authors state "The
WN virus could have entered the Western Hemisphere through a number of mechanisms, including travel by infected humans,
imporation of illegal birds or other domestic pets, or unintentional introduction of virus-infected ticks or mosquitoes."
What should you do if you find a dead or dying crow (or other bird)?
Because the crows appear to be dying from this disease, they can
act as sentinels for its detection. Just like the canary in the mineshaft, crows may be the first to go and can alert us to the
presence of the virus. Similarly, because domestic bird die-offs may be related to infection with West Nile-like virus, reports of bird die off
may be an important surveillance mechanism for monitoring spread of the disease. If you find a dead bird you should contact your
county health department, and they can advise you on further action (check the government listing of your telephone book). Counties will
complete a report form for the State Health Department. All the health departments want very badly to track this virus, so our help will be appreciated.
Do not worry about the dead birds themselves. There is no evidence
of bird to human transmission of the virus; rather, the virus is spread by infected mosquitoes (which only bite LIVE birds). Dead birds can
be disposed of by burying (three feet deep) or by double-bagging them and disposing of them with regular trash. Gloves should be
worn when handling them, but masks or other respiratory protection is not needed.
If you are outside the known disease area and you find a dead bird,
DO NOT PANIC. Every bird dies at some time, and most discovered dead birds will not be infected. As I stated before, one dead bird doesn't mean much, but many dead ones might.
Article Source: Wikipedia.
  
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