Cuckoos are brood parasitic birds that lay their eggs in the nests of other bird species, relying on those host species to raise their young. This imposes huge costs on host species, as raising the cuckoo chick drains resources and often results in the death of the host’s own chicks. As a result, many host species have evolved defenses against cuckoo parasitism over time. Host defenses include rejecting foreign eggs from their nests, mobbing adult cuckoos to drive them away, and attacking cuckoo chicks if they do hatch in the nest. The evolutionary arms race between cuckoos and their hosts has led to an astonishing diversity of adaptations and counter-adaptations in both parties.
How do birds recognize foreign eggs?
The first line of defense used by many host species is simply recognizing and rejecting foreign cuckoo eggs. But how do birds know their own eggs from those of a cuckoo? There are several cues birds use:
Coloration and pattern
Birds have excellent color vision and many species have evolved distinctive egg appearances that differ from common cuckoo eggs. The color, spots, speckles and other markings on an egg are cues to its identity. Birds rejecting odd-looking eggs from their clutch is one of the most widespread anti-cuckoo adaptations.
Egg shape
The shape of eggs can also be used to identify foreign eggs. Birds recognize the oval-shaped curve of their own eggs. Cuckoo eggs that are too round or elongated in comparison will be rejected. Some cuckoos have evolved egg mimicry to match the size and shape of host eggs.
Egg size
Cuckoos may lay eggs that differ subtly in size compared to the host species’ eggs. Hosts may measure eggs with their beaks upon returning to the nest to detect these small differences. Size is less useful than coloration though, as cuckoo eggs also evolve to match host egg size.
Timing
Finally, the timing of egg laying can reveal an egg as foreign. Host birds know when they laid their own clutch, so an egg suddenly appearing during incubation stages is suspicious. Cuckoos often remove a host egg when they parasitize a nest, which can confirm an intruder has been there.
How do birds mob and attack cuckoos?
In addition to egg rejection, host birds have evolved harassment behaviors that act directly against adult cuckoos to deter parasitism. These include:
Mobbing
Birds of many species will mob predators and other intruders near their nest sites by gathering into groups and aggressively diving at the intruder. This mobbing behavior is also used against cuckoos, with multiple birds harassing the cuckoo to drive it away from the area.
Alarm calls
Special alarm or scolding calls are given by some species in response to the sight of a cuckoo near the nest. This alerts and recruits other birds to join the mobbing effort and chase away the cuckoo.
Attacks
Birds may go beyond mobbing to directly attack cuckoos. The attacks can include pecking, clawing and wing-beating the cuckoo to inflict pain. In some cases, birds may even kill adult cuckoos that persist near their nests.
How do birds discriminate against cuckoo chicks?
Despite the defenses against cuckoo eggs and adults, some parasites do inevitably succeed in hatching young in host nests. So hosts have a third line of defense – rejecting the foreign chicks after they hatch:
Differences in mouth coloration
When cuckoo chicks beg for food, their distinctive mouth markings can give them away. Host birds feed only chicks with the familiar colored gape pattern. This drives the evolution of chick mouth mimicry in cuckoos.
Differences in chick sounds
Subtle differences in the begging calls of cuckoo chicks may allow host birds to recognize them. Host parents may refuse to feed chicks making unfamiliar sounds.
Size differences
Cuckoo chicks often grow much larger than host chicks, becoming bullying, super-sized beggars. Host parents may selectively starve outsized chicks they perceive as alien. Size mimicry has evolved in some cuckoo species to reduce this discrimination.
Violent rejection
Host birds may go beyond discrimination and react violently against confirmed cuckoo chicks. Rejection behaviors include pecking, clawing and forcibly ejecting the imposter chick from the nest. This prevents wasting precious resources on unrelated young.
Have hosts evolved egg “signatures”?
An amazing discovery in recent years is that some host species may actually have evolved individualized egg “signatures” that allow fine-scale recognition of foreign eggs. Studies on Australian superb fairywrens have found that females have unique individual egg patterning allowing precise identification of their own eggs. A cuckoo egg lacking the signature pattern will quickly be rejected. This suggests egg signature as a possible arms race adaptation against mimicry by cuckoos.
How do arms races drive evolutionary one-upmanship?
The back-and-forth adaptations between cuckoos and their hosts provide one of the classic examples of an evolutionary arms race. A new defense in the host exerts natural selection on the parasite to counter it, which cycles endlessly:
Host Defense | Cuckoo Counter-adaptation |
---|---|
Egg rejection by color | Egg mimicry of coloration |
Egg rejection by size | Egg mimicry of size |
Mobbing of adult cuckoos | Secretive laying; distraction displays |
Chick rejection by mouth color | Chick mouth mimicry |
Neither side can “win” this endless cycle. The constant evolutionary adjustments simply escalate, driven by the high stakes of this natural battle.
Conclusion
Birds have evolved a diverse array of physical and behavioral adaptations to counter exploitation by brood parasitic cuckoos. These anti-cuckoo defenses span rejection of foreign eggs, mobbing and attacking adult cuckoos, discriminating against cuckoo chicks, and precisely identifying their own eggs. Cuckoo tricks like mimicry spark ever-sharper host defenses like signature patterns. This coevolutionary struggle provides one of the most vibrant examples of natural selection in action through evolutionary arms races. After eons of adaptation and counter-adaptation, hosts keep their enemies in check but the evolutionary contest continues.