Why do cuckoos lay their eggs in other birds' nests?
If a host bird throws out a cuckoo's egg, the cuckoo may come back and wreck the whole nest. Some scientists call this the mafia hypothesis.
▶ Start the storyCuckoos lay their eggs in other birds' nests because it lets someone else do the hard work: this strategy, called brood parasitism, relieves the parasitic parents of the investment of raising their young. The parasite manipulates a host into raising its chick as if it were its own, usually with eggs that resemble the host's. A cuckoo egg hatches earlier than the host's eggs, the chick grows faster, and in most cases it then evicts the host's eggs or young from the nest, on pure instinct, since no parent stays around to teach it.
The puzzle is why most hosts go along with it, since the parasite's chick usually looks very different and probably reduces the host's own breeding success. One answer is the mafia hypothesis: if a host rejects the parasite's egg, the parasite destroys the nest and injures or kills the nestlings, and that threat may keep hosts compliant. The great spotted cuckoo, which lays most of its eggs in magpie nests, keeps revisiting them, and in experiments, nests whose cuckoo egg had been removed were destroyed by the cuckoo. Another explanation is that the destruction pushes the magpie to build a new nest, giving the cuckoo another chance. With cowbirds, hosts that ejected the parasite's egg ended up with 60 percent fewer young than hosts that accepted it.
Step 1: Cuckoo lays a mimicked egg
Dropped into a host nest, often resembling the host's own eggs
Step 2: Host spots and rejects it
Some hosts can recognize and eject eggs that don't match
Step 3: Parasite retaliates
The cuckoo may destroy the nest and harm the nestlings
Step 4: Hosts stay compliant
The threat of retaliation may explain why many hosts simply accept the egg
Hosts aren't defenseless, though. Many recognize and eject foreign eggs or abandon a parasitized nest and start over, which sets off an evolutionary arms race as host and parasite coevolve. Cuckoo eggshells also tend to be thicker than their hosts', which may help them resist a host's attempts to puncture them. Similar tricks appear within a single species: coots sometimes slip eggs into other coots' nests, and female eiders prefer nests that already hold another female's eggs, since a lone first egg is the most vulnerable to predators.
Not every outcome is bad for the host. A 16-year study of carrion crows in northern Spain found that nests parasitized by the great spotted cuckoo were more likely to fledge at least one crow chick; the researchers attributed this to a strong-smelling, predator-repelling substance that cuckoo chicks secrete when attacked.
Quiz me
0/3
Recap
Laying in another bird's nest spares the cuckoo the cost of raising its young, and the threat of nest destruction may be one reason so many hosts put up with it.
Surprising fact · Some crow nests are more likely to fledge a chick when parasitized by a cuckoo, possibly because the cuckoo chick's strong-smelling defense repels predators.
Sources (2)
No source, no claim. Every fact in this lesson (16 claims) cites at least one of these.