Cuckoos are brood parasitic birds, meaning they lay their eggs in the nests of other bird species and let those birds raise their young. When a female cuckoo lays her egg in another bird’s nest, she will often push one or more of the host eggs out of the nest to make room for her own egg. This ensures the host bird spends its efforts raising the cuckoo chick rather than its own offspring. But how do cuckoos know which eggs to push out?
Egg discrimination abilities
Cuckoos have evolved excellent egg discrimination abilities to identify which eggs in a nest belong to the host bird and which egg is their own. They use visual cues like egg size, shape, color, and pattern to recognize foreign eggs. Some key abilities cuckoos have include:
– Sensitive vision to detect small differences in egg appearance
– Color vision to see contrasting egg colors
– Spatial cognition to remember locations of their own egg
– Imprinting on their own egg characteristics
These skills allow a cuckoo to quickly identify which eggs in a clutch are hers versus the host bird’s. This informs which eggs she should push out.
When do cuckoos push out eggs?
Cuckoos tend to push eggs out of nests in two key situations:
When laying their own egg
Most often, a cuckoo will eject eggs from a nest immediately after laying her own egg in the nest. By quickly pushing out some existing eggs, she makes room for her own egg to take their spot. This prevents the host bird from realizing an extra egg has been added.
When returning to monitor the nest
Cuckoos will also sometimes push out more eggs on return visits to a nest after initially laying their egg. This replenishment keeps the cuckoo egg ratio higher in the clutch, increasing the chances the host bird will raise the cuckoo chick.
How cuckoos eject eggs from nests
Cuckoos have an innate, specialized behavior for pushing eggs out of foreign nests. Here is how they do it:
Step 1: Locate and grasp an egg
The cuckoo lowers its head into the nest and uses its beak to grasp an egg to eject. The bird often chooses eggs that look most different from its own egg based on visual cues.
Step 2: Carry egg in beak
The cuckoo lifts its head out of the nest with the egg secured gently but firmly in its beak.
Step 3: Toss egg out of nest
Finally, the cuckoo tosses its head forcefully to launch the egg out of the nest. Cuckoos are able to eject eggs up to 6 feet away from nests located high up in trees and bushes.
Step 4: Repeat
A cuckoo may repeat this sequence several times in quick succession to eject all the eggs it wants removed. It takes 10-20 seconds per egg on average.
How many eggs do cuckoos push out?
The number of eggs a cuckoo pushes out depends on factors like:
– The size of the host bird’s clutch
– How many existing eggs there are
– How many eggs the cuckoo lays herself
Host species | Typical # eggs pushed out |
---|---|
Song thrush | 1-2 eggs |
Reed warbler | 2-3 eggs |
Meadow pipit | 3-4 eggs |
In general, cuckoos tend to push out at least half the eggs in a given nest. They usually don’t eject the entire clutch, as that might make the host bird abandon the nest entirely.
How do cuckoos know which eggs to push out?
So how do cuckoos decide specifically which eggs in a nest to push out? Research shows they tend to eject eggs based on these criteria:
Eggs that look most different from their own
Cuckoos preferentially pick eggs that contrast most with their own egg in size, color, and patterning. This increases chances their egg will blend in.
Eggs on the nest periphery
Cuckoos often eject eggs on the outer part of the clutch as those are easiest to grasp and remove. Interior eggs are more protected.
Early-stage eggs
Newly laid eggs are also easier targets than developed eggs with embryos inside. Cuckoos avoid more advanced eggs.
Eggs laid on previous days
Cuckoos can tell roughly how old eggs are based on color, warmth, and developmental stage. They tend to pick older eggs to push out.
How do cuckoos learn this behavior?
Cuckoos possess this unique egg-rejection behavior instinctively. Some key evidence it is innate includes:
– Young cuckoos reared alone still demonstrate the behavior
– Hand-raised cuckoos immediately ejected foreign eggs from nests in experiments
– Related cuckoo species that don’t practice brood parasitism do not show egg-rejection behavior
This implies cuckoos are born genetically programmed to identify foreign eggs and know to push them out of nests automatically. It helps promote their reproductive strategy.
Why don’t host birds stop cuckoos?
An interesting question is why host birds don’t do more to stop female cuckoos from entering their nests and pushing out eggs in the first place. There are a few explanations:
Rapid speed of cuckoos
Cuckoos are very quick when replacing eggs. Host birds are often gone from the nest and return to find eggs already ejected.
Cuckoos resemble hawks
A cuckoo’s shape and flight pattern resembles a hawk, causing fear in small host species that prevents them from intervening.
Limited prevention options
Small birds have few options to keep larger cuckoos away from the nest in the open environment. So they make the best of a bad situation.
Lack of individual nest defense
Host birds don’t stay by or actively defend individual nest sites. Instead they rely on camouflage, so cuckoos can access nests unchallenged.
Do cuckoos ever make mistakes?
Cuckoos are very adept at identifying and selectively pushing out only foreign eggs from nests. However, they do occasionally make mistakes, including:
Ejecting their own egg
In rare cases, a cuckoo mistakenly tosses out its own egg, though visual cues usually prevent this.
Damaging their own egg
A cuckoo might crack or otherwise damage her own egg in the process of ejecting other eggs.
Pushing out too many eggs
Cuckoos may mistakenly eject too many eggs, causing the host bird to abandon the nest entirely.
Choosing the wrong nest
Young or inexperienced cuckoos may mistakenly target the wrong species’ nest that doesn’t match their egg type.
How do host birds fight back?
Host birds have evolved some interesting counteradaptations to reduce the impacts of cuckoo parasitism:
Rejecting foreign eggs
Some host species have evolved the ability to recognize non-mimetic cuckoo eggs and eject them from nests themselves.
Nest abandonment
Hosts may completely abandon nests with eggs that don’t match to cut their losses and start over elsewhere.
Buried eggs
Burying eggs deep in domed nests makes them hard for cuckoos to access and push out.
Helper birds
Species with cooperative breeding have extra helpers to guard and defend the nest from cuckoos.
Mobbing cuckoos
Host birds may mob or harass adult cuckoos to drive them away from nest areas.
Conclusion
In summary, cuckoos possess specialized skills and behaviors that enable them to precisely identify which eggs in a clutch are not theirs and selectively push those foreign eggs out of nests. This adaptation benefits cuckoos by ensuring host birds devote energy toward raising cuckoo young rather than their own. It is an ingenious reproductive strategy that relies on the cuckoo’s exceptional ability to discriminate egg types and toss out the ones that don’t belong. But hosts continue to evolve their own counter-defenses against this tricky parasitic bird.