Agricultural crops would have provided the birds with more food, compensating for the loss of oak cover. Li's team think European immigrants to North America could have reversed the downward trend temporarily, causing the population surge that resulted in flocks of passenger pigeons blotting out the sun. The decline in oak cover would have caused a corresponding decline in the pigeon population.
A study of fossil pollen records showed that oak cover has been decreasing since about 9,000 years ago. The team also looked at the availability of acorns, the birds' main food, over the past 21,000 years. The population shrunk during the last Ice Age and rebounded after the glaciers retreated. When the researchers reconstructed the birds' genetic history, they found that population changes corresponded with the Earth's climate history. Instead, it fluctuated, rising rapidly during population outbreaks and then declining again. This showed the population had not always been in the billions. They found that the passenger pigeon's effective population size, the number of individuals needed to produce the amount of genetic variation found in the species, was only about 330,000. To determine past population sizes, the researchers mapped the passenger pigeon genome, using DNA from three museum specimens collected across the birds' feeding range, and compared it with the genome of the modern domestic pigeon. However, Li and his team thought changes in the environment, independent of human activity, could have caused the population to fluctuate. What caused the birds to disappear so suddenly? Previous researchers blamed human hunting and deforestation. Within a human lifespan, the species was extinct.
Population estimates for the beginning and middle of the 19th century ranged from three to five billion. James Audubon, the artist and ornithologist, once described a flock of passenger pigeons blocking the sun for three consecutive days. In the 1800s, observers reported flocks of passenger pigeons blanketing the sky.
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