What Do the Snakes on Snake Island Eat?

The snakes on Ilha da Queimada Grande, commonly called Snake Island, eat mostly birds. The island’s sole snake species is the golden lancehead pit viper (Bothrops insularis), and migrant passerine birds make up over 80% of the adult diet, making this snake the only bird specialist in its entire genus.¹ That dietary narrowness is remarkable for a pit viper, and it shapes nearly everything about the snake’s body, behavior, venom, and yearly rhythm of activity.

The Island and Its Only Snake

Snake Island sits about 33 kilometers off the coast of São Paulo state in southeastern Brazil. It is a small, rocky, forested island with no permanent human residents, and the Brazilian navy restricts access to authorized researchers. The golden lancehead is endemic here, meaning it exists nowhere else on Earth. Estimates of the population have varied over the decades, but the snake is classified as critically endangered. Its close relatives on the Brazilian mainland, such as the common lancehead Bothrops jararaca, feed primarily on rodents and other small mammals. The golden lancehead’s switch to a bird-dominated diet is a direct consequence of island life: the island has no native terrestrial mammals that could serve as regular prey. Instead, the snakes have built their entire survival strategy around the dozens of bird species that pass through the island’s forest canopy during migration seasons.

Adults Eat Migratory Birds, Mostly Two Species

Roughly 41 species of migratory birds visit Queimada Grande at various points during the year, but the golden lancehead relies heavily on just two of them. Gut-content studies show that the Chilean elaenia (Elaenia chilensis), a small tyrant flycatcher, is the most common prey item found inside adult snakes.² This flycatcher arrives along the southeastern Brazilian coast toward the end of the wet season, during the austral summer. A second migratory passerine makes up most of the remaining bird prey, though the golden lancehead will opportunistically take other small birds that perch within striking range.

The concentration on just two species out of 41 visitors highlights how precarious this diet actually is. The snakes do not go hunting through the canopy. They are sit-and-wait ambush predators, coiling on branches and waiting for a bird to land close enough. If the two key prey species have a bad migration year, perhaps because of weather disruptions along their flyway or habitat loss on their breeding grounds in southern South America, the snakes have very few alternatives.

What Juveniles Eat Before the Switch to Birds

Young golden lanceheads do not start out as bird specialists. Juveniles feed on small lizards and frogs, prey items found closer to the ground that are easier for a small-bodied snake to overpower and swallow.³ This ontogenetic diet shift, from ectothermic prey on the ground to endothermic prey in the trees, is not unusual among pit vipers as a general pattern. Many mainland Bothrops species start with frogs and lizards and transition to rodents as they grow. What makes the golden lancehead unusual is that instead of moving to mammals, they move to birds, because the island simply has no suitable mammalian prey.

The shift also appears to coincide with changes in body size and climbing ability. As the snakes grow, they spend more time in shrubs and trees, positioning themselves along branches where perching birds are likely to land. Juveniles tend to stay lower, where lizards bask on rocks and leaf litter and small frogs are accessible.

How They Catch Birds

Golden lanceheads use a hunting technique called tail luring, and they appear to use it at every life stage. The snake coils on a branch or rock with its tail tip conspicuously exposed near its head. The tail tip is lighter in color and visually distinct from the rest of the body. When moved subtly, it mimics a small invertebrate, like a grub or caterpillar. Insectivorous birds investigating the lure come close enough for the snake to strike.

Evidence for this behavior goes beyond field observations. Researchers compared the frequency of tail-tip injuries in golden lanceheads to those of their mainland relative Bothrops jararaca, which rarely preys on birds. About 20.9% of golden lanceheads had visible damage to their tail tips, compared with only 7.3% of the mainland species.³ That threefold difference strongly suggests that birds are pecking at the lure, sometimes damaging the tail before the snake can strike. The injury rate is essentially a scar record of thousands of failed and successful hunting attempts over the snake’s lifetime.

The characteristic ambush posture, coiled with the tail near the head, is commonly observed in the field. It works both for juveniles luring lizards on the ground and for adults luring birds in the canopy. The strategy demands patience rather than pursuit: a golden lancehead may sit motionless for hours or even days, waiting for the right opportunity.

The Feast-and-Famine Cycle

One of the most striking aspects of the golden lancehead’s life is how long it goes without eating. The snake’s food supply is not available year-round. Migratory birds pass through Queimada Grande during defined seasonal windows, and outside those windows, there is little for an adult snake to eat. Genetic research has described the species as experiencing prolonged hibernation and aestivation each year, with dormancy potentially lasting up to eight months.⁴ During these months, the snakes drastically reduce their metabolic rate and live off stored energy reserves.

The survival peaks match the prey availability. Population studies found that the golden lancehead’s highest survival estimates coincide with the arrival of the Chilean elaenia at the end of the wet season.² When birds are around, the snakes feed, build up fat reserves, and reproduce. When birds leave, the snakes essentially shut down. This extreme cycle of gorging and fasting is more dramatic than what most mainland pit vipers experience, where rodent prey tends to be available more continuously throughout the year.

Weather variability adds another layer of risk. If an unusual storm system disrupts bird migration routes or a particularly cold winter reduces the number of birds that survive their journey north, the snakes have no backup food source large enough to sustain the adult population. Researchers have flagged this dependency as a key vulnerability for the species, on top of the usual threats that face critically endangered island endemics, like habitat degradation and the ever-present risk of invasive species introduction.

Venom Tailored for Bird Prey

The golden lancehead’s venom appears to have evolved under selective pressure related to its bird-heavy diet. Most mainland lanceheads that feed on mammals have venom profiles optimized for quickly immobilizing warm-blooded rodents. The golden lancehead also targets warm-blooded prey, but birds present a unique challenge: a bitten bird that flies even a short distance from the strike site may be lost forever in dense forest canopy. The venom therefore needs to work fast.

Genomic and functional analyses of golden lancehead venom show that it produces potent hemorrhagic, anticoagulant, myotoxic, and blood-pressure-lowering effects. The venom is particularly rich in three classes of toxins: snake venom metalloproteinases (SVMPs), snake venom serine proteinases (SVSPs), and C-type lectins (CTLs).¹ In plain terms, the venom causes rapid internal bleeding, prevents blood from clotting, damages muscle tissue, and drops blood pressure, a combination that can incapacitate a small bird within seconds.

The venom also contains phospholipase A2 toxins that cause muscle destruction. One of these, a type called PLA2-A, induces muscle damage in both mice and chickens in laboratory assays, while another contributes to clotting disruption.⁵ Interestingly, comparing the golden lancehead’s toxin gene region with that of the mainland Bothrops jararaca reveals that some toxin genes are conserved between the two species while others differ, suggesting that the island species’ unique diet has driven selective shifts in venom composition since the two species diverged.

This is a case where the evolutionary pressures are unusually clear. A mainland lancehead that bites a rat on the ground can follow a scent trail and recover its meal minutes later. A golden lancehead that bites a bird on a branch gets one chance. If the bird flies away and dies elsewhere, the snake invested venom for nothing. The premium on fast-acting venom is high, and the genetic evidence indicates the golden lancehead’s toxin arsenal has responded to that pressure.

Why No Mammals and Very Few Alternatives

People sometimes ask why the golden lanceheads do not eat rats, given that the island reportedly has some introduced rodent populations. The answer is partly about availability and partly about specialization. Any rodents on the island likely arrived via boats in relatively recent history and do not represent a large or reliable prey base the way migratory birds do. More importantly, the golden lancehead has become morphologically and behaviorally specialized for arboreal ambush hunting over thousands of years of isolation. Its climbing behavior, slender build relative to some mainland relatives, and the tail-luring strategy are all tuned for catching birds in trees, not chasing rodents on the ground.

That said, diet studies sometimes turn up non-bird prey in adult stomachs, including the occasional lizard. The 80% figure for bird prey means the remaining fraction is a mix of lizards, frogs, and possibly centipedes or other invertebrates taken opportunistically.¹ These non-bird items are more common in the diets of juveniles and subadults. For a fully grown adult golden lancehead, birds are overwhelmingly the main course.

How the Diet Compares to Mainland Relatives

The golden lancehead belongs to the Bothrops jararaca species complex, a group of closely related pit vipers found across southeastern South America. Among all members of this complex, the golden lancehead is the only one classified as a bird specialist.¹ The mainland species in the group feed predominantly on small mammals, supplemented by frogs, lizards, and occasionally birds. The dietary divergence is one of the clearest examples of how island isolation reshapes a predator’s ecology. When the island was cut off from the mainland by rising sea levels roughly 11,000 years ago, the ancestral lancehead population lost access to the mammalian prey base its relatives still enjoy on the continent. Over millennia, the population adapted to the most abundant and energy-rich prey available: the migratory birds that use the island’s forest as a stopover.

The comparison also shows up in the venom. Mainland Bothrops jararaca venom is potent in its own right and causes serious envenomation in humans, but its toxin profile is geared toward mammalian physiology. The golden lancehead’s venom, while sharing some toxin families, has diverged in ways consistent with targeting avian prey. Researchers have found differences in the relative abundance of specific toxin types and in whether certain toxin genes are present or absent between the two species, indicating that natural selection has pulled the two venoms in different directions as their diets diverged.

Cannibalism and Other Rarely Documented Behaviors

On an island with one dominant predator and limited food, the question of whether golden lanceheads eat each other comes up naturally. Cannibalism has been observed or suspected in the species, though it does not appear to be a routine part of the diet. It fits a broader pattern seen in other island snake populations worldwide, where competition for scarce food occasionally leads to conspecific predation, especially when size differences between individuals are large enough for one snake to physically overpower and swallow another.

Golden lanceheads also interact with invertebrates on the island in ways that are not well documented. Some researchers have noted the potential for the snakes to consume large arthropods, but these items are nutritionally trivial for an adult snake and are unlikely to make a meaningful contribution to energy budgets. The bottom line is that the adult golden lancehead’s caloric lifeline runs almost entirely through migratory birds, and everything else is a rare supplement at best.

What Happens If the Birds Stop Coming

The golden lancehead’s extreme dietary specialization is both an evolutionary success story and a conservation liability. The species has thrived on Queimada Grande for millennia precisely because its strategy of ambushing migratory birds works so well in this particular habitat. But it works only as long as the birds keep arriving on schedule and in sufficient numbers.

Climate change is already shifting the timing and routes of bird migrations across South America. Even modest changes in when the Chilean elaenia arrives along the São Paulo coast could create mismatches with the golden lancehead’s activity period. Researchers studying the snake’s population dynamics have noted that food shortages may depend on weather stochasticity and the dynamics of migratory bird movement, and that prey availability plays a special role in survival and population trends.² An island endemic that depends on two bird species, which themselves depend on climatic conditions across an entire continent, is vulnerable at every link in the chain.

Invasive species pose another threat. If rats or cats were to establish a large breeding population on the island, they could compete with the snakes for food, prey on juvenile snakes, or disrupt the delicate ecological balance that has allowed the golden lancehead to persist. For now, restricted access to the island helps protect it from accidental introductions, but the risk is never zero for an island this close to a major metropolitan coastline.

1
PubMed Central. The Golden Lancehead Genome Reveals Distinct Selective Processes Acting on Venom Genes of an Island Endemic Snake
2
PubMed Central. Population Dynamics of the Critically Endangered Golden Lancehead Pitviper, Bothrops insularis: Stability or Decline?
3
South American Journal of Herpetology. Tail Luring by the Golden Lancehead (Bothrops insularis), an Island Endemic Snake from South-Eastern Brazil
4
PubMed Central. Genetic Adaptations of an Island Pit-Viper to a Unique Sedentary Life with Extreme Seasonal Food Availability
5
Genome Biology and Evolution. The Golden Lancehead Genome Reveals Distinct Selective Processes Acting on Venom Genes of an Island Endemic Snake – Section: Results