At the start of the nineteenth century, Passenger Pigeons were perhaps the most abundant birds on the planet, numbering literally in the billions. The flocks were so large and so dense that they blackened the skies, even blotting out the sun for days at a stretch. Yet by the end of the century, the most common bird in North America had vanished from the wild. In 1914, the last known representative of her species, Martha, died in a cage at the Cincinnati Zoo.

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