Assertion: All birds except koel (cuckoo) build nests for retiring and taking rest during night time (day time for nocturnal)
Reason: Koel lays its eggs in the nests of the tailorbird.
A. If both the assertion and the reason are true and the reason is a correct explanation of the assertion.
B. If both the assertion and the reason are true but the reason is not a correct explanation of the assertion.
C. If the assertion is true but the reason is false.
D. If both the assertion and reason are false.

Hint: Birds belong to the class Aves, are the warm blooded vertebrates characterized by the presence of feathers, beak and are toothless. They lay a hard-shelled egg. They have 4 chambered hearts, high metabolic rate with strong and lightweight skeleton to support weight during flight.

Complete answer:
A bird nest is a place for laying eggs that require incubation and raises its young ones. The study of birds nest is Caliology. Most species conceal their nests to protect them from predators. Birds have evolved with sanitation methods to reduce the effects of parasites on the young ones.

Not all birds build their nests. Some species of birds directly lay their eggs on the ground. Some birds lay their eggs in the nest of other birds which are known as Brood parasites like Koel (Cuckoo) who lays eggs in the nest of crow. Now, these young ones are protected by the other birds known as foster parents.

Nests are primarily made for breeding but they can be used in the non-breeding season for roosting (means to sleep). So, most birds except Koel (Cuckoo) build a nest for retiring and taking rest during night time. So, the Assertion is true.

Koel (Cuckoo) is black in color and crow is also black in color and their eggs are also similar. So, Koel lays an egg in the nest of a crow, and the crow can't differentiate between the cuckoo’s eggs and her own eggs. Tailorbird is a green color bird, if a Cuckoo lays her eggs in this nest, then the tailor bird will easily distinguish the other eggs and will discard it from the nest. So, Reason is false.

So, the correct answer is Option (C) The assertion is true but the reason is false.

Note: Birds nests are also built by humans as a method to conserve certain birds whose population is declining, like Swallows. These nests are known as Artificial nests. These are built of plaster, wood, terracotta. It is an important tool for the conservation of many species.