The passenger pigeon became extinct mainly due to:

The correct answer is Option (2) → Over-exploitation by humans

Over-exploitation happens when human ‘need’ turns into ‘greed’ and a natural resource is taken faster than it can recover. NCERT gives the passenger pigeon and Steller’s sea cow as examples of species that were wiped out in the last 500 years because humans hunted them in huge numbers.

Option (1) Habitat loss and fragmentation — Incorrect. Habitat loss and fragmentation is the most important cause of extinction in general, but NCERT does not give the passenger pigeon as its example. Its examples are the shrinking tropical rain forests such as the Amazon.

Option (2) Over-exploitation by humans — Correct. The passenger pigeon was killed by humans on such a large scale that the species could not survive. NCERT names it among the extinctions due to overexploitation by humans.

Option (3) Alien species invasions — Incorrect. Alien species invasions harm native species after a foreign species is brought in, as with the Nile perch in Lake Victoria. No such invader caused the loss of the passenger pigeon.

Option (4) Co-extinctions — Incorrect. Co-extinction is the loss of a species because a partner it depends on in an obligatory way has died out. The passenger pigeon was lost because of hunting, not because a partner species vanished.

NCERT Extract:
Loss of Biodiversity – “… (ii) Over-exploitation: Humans have always depended on nature for food and shelter, but when ‘need’ turns to ‘greed’, it leads to over-exploitation of natural resources. Many species extinctions in the last 500 years (Steller’s sea cow, passenger pigeon) were due to overexploitation by humans. Presently many marine fish populations around the world are over harvested, endangering the continued existence of some commercially important species.”