Mammoth or Pigeon? De-extinction’s Choices

Imagine that you are standing on the streets of Toronto or Hamilton on a spring day sometime in the 19th century. The skies begin to darken as a massive flock of birds passes overhead, blotting the sun. The beating of billions of wings rumbles like thunder, stirring up a wind that chills you to the bone. The flock might take days to pass. The birds were passenger pigeons, once found across North America east of the Rockies. About the size of our pigeons today, the males had a coppery breast, the females were somewhat plainer, and they travelled in gargantuan numbers. A single flock could stretch over 150 kilometres, and number in the billions: in the spring of 1860, a flock estimated at over 3.7 billion birds flew over Ontario. Gorging themselves on tree nuts, such as acorns, they could clear entire forests; the weight of roosting pigeons would cause whole limbs to drop from the trees. And we think pigeons today are a pest.

Passenger pigeons were once the most...