Is the reappearance of large crow populations 15 years after the West Nile dieback a widespread occurence?
We used to have lots of crows in our area -- Northern Virginia, outside DC. Then West Nile virus struck roughly 15 years ago, and crows disappeared from our area.
According to the Centers for Disease Control and Prevention:
Since West Nile virus was discovered in the United States in 1999, the virus has been detected in over 300 species of dead birds. Although some infected birds, especially crows and jays, frequently die of infection, most birds survive.
Several years ago, the crows started to come back, and for the past two years, there has been a large flock at the local shopping center and we again routinely see crows on the trees adjacent to our patio.
Has this pattern been observed elsewhere -- disappearance of crows, and then resurgence?
Clarification: While anecdotal information from other geographical areas would be very welcome, I'm hoping someone has an answer based on a scientific survey or on information on whether the virus has mutated and/or whether resistant crow populations have emerged.
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This is not a complete answer, but there is evidence that crow populations are rebounding. From The Virginian-Pilot, July, 2017.
Crows? Who’s been seeing and hearing a lot of these noisy raucous critters this year?
I certainly have. The big black birds are raising Cain, up and down the North End.
This paper is based in Norfolk, about a four hour drive from my house. It is a far from a scientific article, so I won't quote more.
According to the St. Louis Post Dispatch, February 2013, crows are returning to the Midwest.
The Audubon Society’s Christmas Bird Count indicated that the bird [crow] population may be back to pre-virus levels in Illinois, and recovering part of its numbers in Missouri.
They’re almost back to where they were 10 years ago, said Robert Russell, a biologist at the U.S. Fish and Wildlife Service, referring to the Midwest. Our crows are pretty much back to normal.
Crows were especially hard hit by the West Nile Virus:
The death rate for infected crows was 99 percent when the disease first appeared in 1999.
There was, as of the publication of this article, no definitive explanation for the resurgence of the crow population:
No one knows why the crows seem to be returning now. West Nile is still with us, although at reduced rates. There were 21 human cases in Missouri last year, compared with 168 a decade earlier.
After the worst years for bird kills, scientists sampled birds to see if they were developing antibodies to the virus. “We didn’t find any,” said McGowan, the Cornell crow expert.
The answer might lie in a sort of high-speed evolution. The crows that survived have the most natural resistance to the disease, and they’re passing it on to their progeny, said Geoff LeBaron, the ornithologist in charge of the Audubon bird count.
In 2014, Bay Nature reported increasing crow populations in Berkeley, CA, possibly attributable to the birds' intelligence and adaptability. (There was not a dieback of crows in Berkeley.) According to the Cornell Lab of Ornithology, WNV became less lethal as it moved west and was less lethal to crows in diverse habitats. Going beyond crows to some other species of birds, the picture is not so happy, but this is beyond the scope of the question.
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