Some spiders do prey on birds. This phenomenon is rare, typically involving specific types of spiders in particular circumstances. It does not represent a significant threat to global bird populations.
Spiders Known to Prey on Birds
A few spider species are known to prey on birds, mainly large spiders or those that build strong webs. The Goliath Birdeater tarantula (Theraphosa blondi), found in northern South America, is the largest spider by mass and body length, capable of a leg span up to 11 inches (28 cm) and weighing up to 6.2 ounces (175 g). Despite its name, this tarantula rarely preys on birds. Its diet consists mostly of large arthropods, worms, amphibians, and small terrestrial vertebrates like mice or lizards.
Large orb-weaver spiders, such as the Golden Orb Weaver, are also documented to occasionally catch small birds in their strong, expansive webs. Their robust webs can span a meter or more. The silk produced by some orb-weavers is remarkably strong, enabling them to ensnare creatures as large as finches or even bats. Huntsman spiders, known for their size and speed, primarily hunt insects and other invertebrates, but larger species have been reported to occasionally prey on small birds, particularly nestlings.
Hunting and Consumption Strategies
Spiders employ various methods to capture and consume birds, depending on their hunting style. Web-building spiders, like orb-weavers, rely on their strong silk to ensnare birds. When a small bird flies into the web, the sticky and durable silk traps it, and the spider quickly moves to subdue the struggling prey. The spider then wraps the bird in more webbing to ensure it is completely immobilized.
Active hunting spiders, such as large tarantulas and huntsman spiders, ambush or stalk their prey. These spiders typically target vulnerable birds, like nestlings, fledglings, or injured individuals, that might be on the ground or within their reach. Once the bird is captured, the spider delivers a bite to inject venom, which works to subdue and paralyze the prey.
Following immobilization, spiders begin the process of external digestion. They inject digestive enzymes into the bird’s body, which liquefy its internal tissues. The spider then sucks up the liquefied remains, leaving behind only the indigestible outer structures like bones and feathers. This process allows the spider to consume the nutrients from a prey much larger than itself.
Prevalence and Ecological Role
Bird predation by spiders is rare, often observed as an opportunistic feeding behavior rather than a primary food source. Most documented cases involve small, vulnerable birds such as nestlings or fledglings, or birds caught in unusual circumstances. While spiders are common predators of arthropods, birds are also common predators of spiders. Birds regularly consume spiders as part of their diet, including venomous species, because the venom needs to be injected to be effective and is neutralized by digestion.
Ecologically, the impact of spiders preying on birds is minimal and does not pose a significant threat to bird populations. Such events are more commonly documented in tropical and subtropical regions where large spider species reside. The occasional consumption of birds by spiders highlights the diverse and complex food webs found in nature, where even small creatures can occasionally prey on larger ones. The relationship between birds and spiders is often one of predation by birds on spiders, influencing spider behaviors and population dynamics.