Why do some birds tolerate cuckoo chicks in their nests?

It's actually not just "some" birds that tolerate cuckoo chicks in their nests, it's most birds.

Cuckoos have been labelled by humans as a parasitic bird, which lays its eggs in the nests of other birds. However cuckoos can also be beneficial to their host.
Sometimes birds will actually remove the cuckoo's egg from their nest. But most of the time they will actually accept it as one of their own.

There are two reasons why a bird will accept a cuckoo's egg as their own.
First of all, cuckoos are actually rather mean. If the cuckoo witnesses its egg being evicted from the nest, then the cuckoo will likely return to the nest later and crush the host bird's eggs. Most birds seem to have learnt that removing a cuckoo's egg from their nest will end badly, so they just accept it.

The second reason is because the cuckoo egg actually provides the nest with more defence from predators, especially with the spotted cuckoo.
Spotted cuckoo chicks for example secrete a terrible smell. This smell makes the nest seem unappetising to predators, such as cats and Birds of Prey. So the host bird's nest is benefiting from extra protection if they accept the responsibility of raising the cuckoo's chick.

In a way, the cuckoo bird works like the Mafia, running a protection racket. That is if you pay your dues (accept the cuckoo's eggs as your own), then you will get protection. But refuse and your nest gets smashed up.


Further reading:

• New Scientist Article: Cuckoo chicks protect their host from predators with smells