Birds build intricate and carefully constructed nests to lay their eggs in and raise their young. But do they ever incorporate spikes or other sharp objects into the bottoms of their nests? This is an interesting question to consider from both a biological and evolutionary perspective.
Do any birds use spikes in their nests?
Some birds do in fact incorporate spikes and thorns into the bottoms of their nests. The most well-known example is the loggerhead shrike. This predatory songbird is found across North America and is nicknamed the “butcher bird” for its habit of impaling prey on thorns or barbed wire.
Loggerhead shrikes will build cup-shaped nests in the branches of trees and line the interior with grass, feathers, and fur. But they often weave sharp objects like twigs, thorns, and stiff plant stems into the exterior bottom of the nest. Researchers believe they do this to deter nest predators. The spikes poke up menacingly, making it harder for snakes, raccoons, and other animals to climb up the nest and eat the eggs or nestlings.
Other birds like European magpies have also been observed incorporating thorny twigs into their domed nests, likely for the same protective purpose. And some tropical weaver birds will make their suspended nests out of thorny vines or incorporate spikes. So while not common, spikes do show up in some bird nest architectures.
Why don’t more birds do this?
The vast majority of bird species do not use spikes in their nests. Most songbirds build cup-shaped nests out of soft materials like grass, moss, feathers, spider silk, and lichen. While these materials help cushion and insulate eggs, they provide little defense against predators.
However, most songbirds rely on other strategies to protect their nests:
- Careful placement – Nesting high up in trees or clifftops or hiding nests makes them hard to find.
- Camouflage – Using materials that blend into the surroundings helps conceal nests.
- Territorial defense – Actively chasing away or dive-bombing potential predators.
- Frequent monitoring – Checking on nests often to scare off encroaching animals.
Weaving durable yet soft nest materials may provide better insulation and structural integrity. Spiky materials may damage the delicate eggs or nestlings. The spikes may also hamper the adult birds entering and exiting the nest.
The risks of spikes likely outweigh the benefits for most species. But for certain birds like shrikes and magpies, the extra deterrence helps offset the risks and energy costs of incorporating them.
Do the spikes successfully deter predators?
There is some research showing that loggerhead shrikes experience higher nesting success when they build nests with more thorns and spikes woven into them:
- One study in Kansas found that 80% of loggerhead shrike nests with spikes in them successfully fledged young, compared to just 47% of nests without spikes.
- Another study in Ontario monitored 122 shrike nests and found those with spikes were predated at half the rate of those without.
So there does appear to be a protective benefit. However, nest predation is complex and depends on many factors. Spikes alone cannot guarantee success. The Canadian study still saw 45% of spiked nests get predated.
The effectiveness of the spikes may rely heavily on:
- The abundance of different nest predators in the habitat.
- The materials available for birds to build with.
- Proper incorporation into the nest structure.
- The wariness and density of the local predator population.
- The motivation and hunger levels of individuals looking for eggs or nestlings.
Clever and determined predators may still be able to bypass nest spikes if motivated enough. So while helpful, this strategy does not completely solve the problem of nest predation for shrikes and other birds.
Do Nest Spikes Harm Birds?
An important consideration is whether spikes integrated into nests pose any harm or risks to the adult birds and their offspring.
There are a few potential downsides:
Risk of injury
Sharp spikes could potentially injure adult birds as they incubate eggs or bring food to nestlings. The birds could cut themselves on the thorns or get spikes stuck in their feet or feathers.
However, birds that use this strategy likely evolve appropriate behaviors to avoid getting injured. For example, loggerhead shrikes will mainly perch on the rim of the nest and avoid extensive direct contact with the spiky bottom.
Discomfort or impaired insulation
Compared to soft nest materials, spikes may create an uncomfortable surface for birds to rest on during lengthy incubating sessions. The rigid spikes could also impair the nest’s ability to properly insulate eggs from temperature fluctuations.
Yet bird species like shrikes habitually use nest spikes, implying they can maintain adequate comfort and insulation. Careful construction with an interior layer of soft materials likely helps mitigate these issues.
Risk of damage to eggs or nestlings
Sharp spikes also pose a risk of accidentally piercing or scratching fragile eggs. Nestlings could similarly get poked or scraped as they move around and grow in the nest.
However, bird parents take care when settling onto the nest and will brood the eggs/young in such a way to avoid damage. No research indicates nest spikes increase accidental damage or mortality for the developing offspring.
Increased development costs
Finally, gathering thorny materials and expertly weaving them into the nest structure requires more time and energy. This takes away from other important activities like finding food and defending territory.
So there are costs for birds, but those that habitually use spikes seem to experience overall benefits that outweigh the risks and costs. Moderately spiky nests layered with soft materials appear to strike the right balance of deterrence and safety.
Examples of Birds That Use Spikes
Let’s take a more detailed look at a few bird species known to incorporate some form of spikes or thorns into their nest architecture:
Loggerhead Shrike
As mentioned, this predatory songbird is the classic example of purposefully placing spikes along the bottom of nests. Loggerhead shrikes are medium-sized birds (about the size of a robin) found across North America.
- Their nests are built from twigs, grass, feathers, and fur, with a lining of fine materials.
- Shrikes will then tightly weave in thorns, small branches, or stiff plant stems around the outer bottom rim of the nest cup.
- The spikes point outward and upward to deter snakes and climbing mammals.
- Nests are often built in small trees or large shrubs a few meters off the ground.
Research indicates that nests with more numerous and larger spikes experience enhanced protection. However, excessive quantities of long thorns may hinder the agile birds from entering and exiting easily. So moderation is key.
European Magpie
The Eurasian magpie is another corvid bird like crows and jays that occasionally incorporates protective spikes into their nests.
- They build large domed nests high up in trees constructed from thorny twigs and mud.
- While the main nest is made of soft materials, magpies may weave additional prickly twigs into the outer parts.
- Like shrikes, these spikes help deter climbing predators.
- Their nests also tend to be placed high up around the treetops for added safety.
Interestingly, scientists have observed magpies selectively collecting more thorny twigs after a predator has attacked their nest, adaptively boosting defenses for the future.
Grey Butcherbird
The grey butcherbird gets its menacing name from its tendency to impale prey on thorns and spikes, much like a shrike. Endemic to Australia, this songbird may also integrate sharp sticks into the rim of its nest occasionally.
- It constructs a cup-shaped nest of twigs lined with softer materials.
- Nests are often built moderately high up in trees.
- If available, butcherbirds may weave external spikes into the nest’s exterior for protection.
However, this behavior is not as well studied or confirmed as it is in loggerhead shrikes.
Yellow-billed Magpie
This corvid endemic to California bears similarities to the European magpie in sometimes building defensive nests.
- Their large, dome-shaped nests are constructed from mud, grass, roots, and feathers.
- They will occasionally incorporate thorny twigs on the outside of nests.
- The nests are placed high up near the tops of oak trees.
The thorny materials likely help deter common nest predators like squirrels, jays, and ravens.
House Sparrows
The ubiquitous house sparrow is better known for its soft, cozy nests of feathers and grass. But some populations have been observed using stiff plant stems and small spikes at times.
- House sparrows build domed nests in tree cavities, buildings, and other structures.
- While nest materials are generally soft, they may incorporate prickly sticks, straw, porcupine quills, or cigarette butts among the outer layers.
- This may help obstruct the small nest entrances from predators.
However, nest defense is not a strongly developed behavior in house sparrows as it is in shrikes. They rely more on nest site concealment and colony defense.
The Purpose and Advantages of Nest Spikes
Birds that incorporate spikes in their nests derive several potential benefits:
Deterring predators
As outlined earlier, the primary rationale for nest spikes is to deter potential predators. Malesacing prickly sticks, thorns, and stems help keep nest-raiding animals at bay.
While not a flawless strategy, research shows it can significantly reduce predation rates for some species like loggerhead shrikes. If used alongside other defensive tactics, spikes can be an effective component of nest protection.
Low energy investment
Gathering and weaving spikes into nests requires some extra effort up front. However, once constructed, the spiky deterrents are a largely passive and non-consumable defense.
Spikes don’t require active monitoring like many other anti-predator behaviors. And they last for multiple breeding attempts, paying dividends over time.
Minimizing risks
Spikes pose some inherent risks to adult birds and their eggs and nestlings through injury or impaired nest function.
However, evolutionarily adapted behaviors that minimize contact with spikes along with interior padding materials help ameliorate these risks.
The benefits of reduced predation seem to outweigh potential drawbacks for shrikes, magpies, and other species that use spikes.
Signaling aggression
The prominent display of thorns and spikes may also signal aggression and warning to predators.
Essentially, they advertise “this nest is dangerous and fiercely defended, so don’t get too close!”
Visually conspicuous nest spikes may play on the psychology of predators and intimidate or deter them before even making physical contact.
Disadvantages and Limits of Nest Spikes
While beneficial, there are some drawbacks and limitations to this anti-predator strategy:
Not effective against all predators
Birds generally adapt their defenses to deter the most common or dangerous predators in their environment.
However, nest spikes would likely be ineffective against predators that attack from the air. For example, spikes pose little impediment to aerial raptors like eagles, hawks, and falcons who strike from above. Nor do they stop arboreal snakes that can slither over the nest’s rim.
So spikes primarily target terrestrial predators like mammals. They are only one piece of the anti-predator toolkit for birds.
Materials may be scarce
Suitable spiky sticks, thorns, and plant stems needed to construct a fortified nest may be scarce or absent in certain habitats.
Birds can only use this strategy where thorny vegetation and woody debris is available nearby. In treeless environments or urban areas, they may have no access to usable spike materials.
Energy trade-offs
As mentioned, gathering and weaving together spikes takes significant time and effort. This diverts the birds’ energy budget away from other important activities like incubating eggs and finding food.
If proper food resources are lacking, birds may fail to build well-fortified nests or be unable to provision their offspring adequately. There are always trade-offs when investing extra in defenses.
Risk of over-fortification
There is likely an optimum level of spikes that balances predator deterrence and other costs and risks. Too few spikes may fail to deter predators, but too many could hinder the adult birds or damage eggs.
Research on shrikes found they experience optimal success with moderate fortification using thorns approximately 2-3 centimeters long. Excessive spikes did not improve outcomes.
So an appropriate degree of moderation and customization to the specific environment seems ideal when incorporating spikes.
Conclusion
While uncommon, the purposeful addition of spikes and thorns to nest exteriors is an intriguing anti-predator strategy employed by some birds like loggerhead shrikes. If done carefully, this thorny deterrent can reduce nest predation rates.
However, spikes come with inherent risks and availability limitations as well. They are far from universally used in the diverse world of bird nest architecture. Most species rely on alternative behavioral defenses like careful placement, camouflage, and active territorial defense.
Nest predation is a complex phenomenon that varies across species, environments, and individual contexts. A multifaceted approach balancing costs and benefits in each unique situation is key. Spikes can be one helpful tool in the toolkit for some species, even if the majority of birds build their homes free of prickly deterrents.
