Cuckoo birds are known for their unique and sometimes bizarre behaviors when it comes to breeding and raising young. Here are some of the strangest things cuckoo birds do:
They lay eggs in other birds’ nests
One of the most well-known behaviors of cuckoo birds is that they lay their eggs in the nests of other bird species, particularly small songbirds. They remove one of the host eggs from the nest when they lay their own. This is known as brood parasitism. Some species specialize in specific host species, while others will use the nests of many different kinds of birds.
Why do they do this?
There are a few key reasons why cuckoos practice brood parasitism:
- It allows them to avoid the energy costs of building a nest and incubating eggs.
- It reduces the chances of their eggs being predated.
- They can produce more offspring by laying eggs in multiple nests.
By foisting the responsibilities of raising young onto other birds, cuckoos can conserve their own energy for future breeding attempts.
How do they trick host birds?
Cuckoos have evolved several clever adaptations that allow them to trick their hosts:
- Their eggs often mimic the coloration and patterning of their host species’ eggs.
- They will remove one host egg when they lay their own, so the overall clutch size often remains the same.
- They time their egg laying closely with the host bird’s, so their egg doesn’t stand out as different.
- Their eggs hatch first, so the cuckoo chick gets a head start on growth.
What happens to the host bird’s chicks?
Sadly, most host birds end up raising the cuckoo chick instead of their own young. Being larger and louder, cuckoo chicks often outcompete host chicks for food and resources, eventually starving them or pushing them out of the nest. Some species of cuckoo chicks even have adaptations like a hollow in their back for holding host eggs or chicks to push them out.
They mimic the calls of predatory birds
Many cuckoo species have learned to mimic the calls of birds of prey, like hawks, falcons, and eagles. They use these mimicked cries to scare potential host parents away from the nest when they are laying their eggs. The hosts flee from what they think is a nearby raptor, leaving their nest unattended and allowing the cuckoo access to lay their egg without being detected. Some examples of cuckoos proficient at mimicry include:
| Cuckoo Species | Raptors Mimicked |
|---|---|
| Common Cuckoo | Eurasian Sparrowhawk |
| Horsfield’s Bronze Cuckoo | Oriental Honey Buzzard |
| Shining Bronze Cuckoo | White-bellied Sea Eagle |
How convincing are their mimics?
The cuckoos’ mimics are incredibly accurate and can fool even human observers. Analyses of the vocalizations show they match the predatory species in multiple aspects like frequency, amplitude, and tempo. Their ability likely evolved due to the strong selective pressure of tricking highly discerning host parents.
They have a short breeding season
Most cuckoo species have a relatively short, intense breeding season compared to many other birds. They need to squeeze all their mating and egg-laying into just a few weeks before making their long migrations. Exact timing varies by habitat, but here’s an overview of when select cuckoo species breed:
| Species | Location | Breeding Season |
|---|---|---|
| Common Cuckoo | Europe | May – July |
| Yellow-billed Cuckoo | Eastern North America | June – July |
| Asian Koel | Indian Subcontinent | March – August |
| Pallid Cuckoo | Australia | September – January |
Why the rushed breeding?
Cuckoos are long-distance migrants that travel from their wintering grounds to take advantage of seasonal food resources for breeding. They have a narrow window of optimal conditions to reproduce successfully before migrating again. The short breeding season prevents them from having multiple clutches and maximizes the chance that their precocial young will fledge before fall migration.
Cuckoo chicks grow incredibly fast
Since they need to push host eggs and chicks out to get the full resources of the nest, cuckoo chicks grow at a rapid rate. Their growth allows them to eliminate competition and fledge before the host parents realize something is wrong.
Just how fast is their growth?
The growth rates of cuckoo chicks is nothing short of astonishing:
- They can gain up to 3x their hatching weight in just one day.
- Many species fledge within 3 weeks of hatching.
- Larger cuckoo species like roadrunners can grow from 3 grams at hatching to 170 grams at fledging.
- Cuckoos have been recorded gaining 10% of their fledging weight within just one hour after hatching.
What adaptations help them grow so fast?
Cuckoo chicks have several special adaptations that enable their rapid growth:
- They have large, gaping mouths to receive as much food as possible.
- Their begging calls are very loud and frequent to stimulate feeding.
- They direct growth hormones to maximize size over development.
- Their digestive system is highly efficient at assimilating nutrients.
- As nestlings, their blood glucose levels spike dramatically after feedings.
They perform distraction displays to protect their young
If a predator approaches a cuckoo nest, the parent birds will put on a distraction display to draw attention away from the nestlings. These displays involve noisy alarm calls and injury-feigning behaviors.
Examples of cuckoo distraction displays
- Fluttering on the ground with dragging wings, as if injured.
- Feigning a broken wing by flying erratically and landing with splayed feathers.
- Fanning and exposing their tail feathers while calling loudly.
By convincing predators they are easy targets, the parent cuckoos can keep threats away from the nest and give their chicks time to flee to safety if necessary. Their distracting displays are a testament to the strong breeding instincts of cuckoos despite their hands-off parenting.
The young can kill their foster siblings
As mentioned earlier, cuckoo chicks often outcompete and kill host chicks sharing their nest. But remarkably, deadly sibling rivalry also occurs between cuckoo chicks of some species.
Multiple female cuckoos will sometimes lay eggs in the same host nest. When the eggs hatch, the cuckoo chicks will fight viciously, often with the largest killing its smaller foster siblings by ejecting them from the nest or starving them.
Why do they kill their own kind?
The killing of foster siblings by cuckoo chicks likely provides an evolutionary advantage by reducing competition for food. Only the fittest cuckoo chick will survive to fledge. This mortality of weaker chicks enables the species to maintain stronger gene lines across generations.
The ruthless competition between separate cuckoo broods highlights the do-whatever-it-takes approach cuckoos take to breeding success. For them, the ends justify the means when it comes to producing viable offspring.
Conclusion
From sneaky egg laying to murderous sibling rivalry, cuckoo birds engage in a variety of strange behaviors centered around reproducing successfully. While their practices may seem appalling, they are a result of evolutionary adaptations that increase breeding opportunities. Their unique traits prove the remarkable ability of natural selection to shape behaviors and anatomies specialized for reproduction, no matter the ethical costs.
