Bird Guts Are a Dead End for Stick Insect Eggs, Digestion Experiments Reveal

Stick and leaf insects are among the most accomplished masters of disguise in the animal kingdom, but their most remarkable imitation may be the one nobody watches happen. The eggs of many phasmid species look uncannily like plant seeds, complete with hard shells, textured surfaces and even fatty appendages that mimic the elaiosomes ants find irresistible on real seeds. That resemblance has fueled a long-standing scientific debate: if these eggs so convincingly masquerade as seeds, could seed-eating birds accidentally swallow them and then disperse them across the landscape, much as they do with genuine seeds? A new study from researchers at Kiel University delivers the most rigorous answer yet, and it is largely bad news for the seed-mimicry hypothesis.

The research, published in The Science of Nature, took a two-pronged approach that combined living feeding trials with laboratory simulations of avian digestion. Led by Judith Burack, Eberhard Haase, Stanislav N. Gorb and Thies H. Büscher, the team selected eggs from seven phasmid species chosen specifically to represent the extraordinary morphological diversity of the order. Among them were the small capitulate eggs of Carausius morosus, the large capitulate eggs of Tirachoidea jianfenglingensis, lentil-shaped eggs of Dajaca napolovi, pinnate eggs of Phyllium mabantai, hairy eggs of Orestes draegeri, rough-surfaced eggs of Nuichua rabaeyae, and the eggs of Sungaya aeta, which females bury using a secondary ovipositor. By spanning this range of shapes, sizes and surface structures, the experimenters aimed to determine whether any egg morphology confers a survival advantage during a trip through a bird’s digestive tract.

The feeding experiments used domestic pigeons, Columba livia f. domestica, as the granivorous test subjects. For each trial, batches of 16 to 25 eggs from a single species were packed into a one-milliliter gelatin capsule that was placed deep into the bird’s throat to guarantee swallowing, sidestepping the possibility that pigeons might simply crush the eggs with their beaks. Each pigeon was then housed individually for 24 hours in a nest box fitted with a wire mesh grid above a paper sheet, allowing droppings to be collected and sieved for eggs and egg fragments. Every recovered intact egg was incubated on damp paper towels at room temperature and monitored daily for hatching. The stakes of this design were clear: any egg that survived the full digestive journey and later produced a nymph would constitute direct evidence of ornithochory, the bird-mediated dispersal conventionally associated with plants.

The results were unambiguous. Across all seven species, most eggs emerged from the pigeons ground into tiny fragments, and not a single egg that passed through the digestive system hatched afterward. The lone partial exception was Dajaca napolovi, the species with disc-shaped eggs: eight of the fifty eggs fed to pigeons were recovered structurally intact from the droppings, along with assorted eggshell pieces from other species. Interestingly, the fatty capitula of several species frequently emerged fully undamaged, a testament to the resilience of that lipid-rich structure, but capitula alone cannot hatch. The mechanical gizzard, with its grinding stones, had effectively destroyed the embryos of every species regardless of how convincingly their eggs resembled seeds. The finding echoes earlier work by Matan Shelomi, who in 2011 fed more than 900 phasmid eggs to quails and chickens and recovered only one unbroken egg.

Yet intactness alone does not guarantee viability, and this is where the study’s laboratory simulations proved decisive. The researchers dissected the digestive gauntlet into its three principal stressors: acidity, heat and mechanical abrasion. They subjected batches of 50 eggs each of Carausius morosus and Dajaca napolovi to distilled water at 38 to 42 degrees Celsius to mimic avian body temperature, to hydrochloric acid diluted to a pH of 1.9 to 2.2 matching the avian proventriculus, and to vigorous stirring with two-to-three-millimeter stones replicating gizzard action. A fourth treatment combined all three stressors, and untreated control groups allowed baseline hatching rates to be established. Eggs were then incubated for months, because phasmid embryos are famously patient, sometimes taking more than half a year to emerge.

Acidity emerged as the silent killer. In the control groups, 70 percent of Carausius morosus eggs and 72 percent of Dajaca napolovi eggs hatched, but after acid exposure the hatching rate for both species was zero. Scanning electron microscopy revealed why: the acid attacked the chorion, the outer eggshell, dissolving its mineralized layers and leaving holes, structural damage and crystalline precipitates that likely represent calcium salts leached from the shell. Phasmid eggshells are known to incorporate biomineralized material, including calcium oxalate, which lends rigidity against physical threats but turns out to be chemically vulnerable in strong acid. The findings align with prior research showing that acid exposure dissolves the calcium oxalate-rich middle layer of the chorion and degrades its mechanical properties, and with studies on mayflies and mosquitoes demonstrating that low pH disrupts embryonic development in other insects as well.

Heat and mechanical stress produced species-specific effects. For Carausius morosus, a species native to India where pre-monsoon temperatures routinely climb above 35 degrees Celsius, the heat treatment was surprisingly benign: 60 percent of heated eggs still hatched, albeit after a longer incubation of about 112 days compared to roughly 80 days in controls. Dajaca napolovi, which inhabits cooler mountainous regions of northern Vietnam, fared poorly under the same thermal challenge, with only two of 50 eggs hatching after incubation periods exceeding 200 days. Mechanical stirring with stones crushed most Carausius eggs, but a larger share of the slippery, disc-shaped Dajaca eggs survived, and ten of those hatched. Friction experiments on a motorized tilting stage explained the disparity: Dajaca napolovi eggs slid at angles as low as roughly 5 degrees on smooth substrates, compared with more than 22 degrees for Carausius morosus, meaning the Vietnamese species’ low-friction eggs may simply slip between gizzard stones rather than being crushed between them.

Synthesizing the feeding trials and the simulations, the authors conclude that successful bird-mediated dispersal of a single phasmid egg through a granivorous bird would require a trifecta of preadaptations: mechanical toughness or slipperiness to survive the gizzard, chemical resistance to withstand gastric acid, and thermal tolerance to endure body temperatures around 40 degrees Celsius. No species tested possessed all three. Even the eight structurally intact Dajaca eggs recovered from pigeon droppings failed to hatch, most likely because the acidic phase of digestion, not the mechanical phase, had already doomed their embryos. For the investigated phasmatodean species, long-distance dispersal via granivorous birds therefore appears highly unlikely, and the visual mimicry of seeds seems to serve other purposes, such as defense against egg parasites, rather than a ticket through a pigeon’s gut.

The story may not be over, however, because not all birds digest seeds the same way. Previous work by Kenji Suetsugu and colleagues showed that when eggs excised from gravid females of three phasmid species were fed to insectivorous brown-eared bulbuls, five to twenty percent remained intact depending on species, and two eggs of the flightless Japanese stick insect Ramulus mikado actually hatched. Phylogeographic analysis of Ramulus mikado has since provided evidence consistent with historical long-distance dispersal, and the facultative parthenogenesis of many phasmids means that viable eggs inside a gravid female could potentially survive her predation and be dispersed when the female herself is eaten. Because insectivorous birds lack the stone-filled gizzards of seed-eaters and process prey differently, the digestive conditions their eggs encounter may be far gentler. The Kiel team suggests that future research should examine the digestive physiology of frugivorous and insectivorous birds and the protective role of the maternal body, to determine whether rare dispersal events through predators, rather than through seed-mimicry, could explain how flightless stick insects colonize distant habitats. For now, the seed disguise remains a marvel of convergent evolution, but as a dispersal strategy aimed at granivorous birds, it appears to be a costume without a function.

Beyond the immediate question of bird digestion, the findings carry weight for a broader puzzle in phasmid biology: how flightless, slow-moving insects achieve the geographic isolation that drives speciation. With wings reduced or absent in many species and even functional wings rarely used, adult stick and leaf insects have few options for crossing inhospitable terrain. Passive transport of the egg stage has therefore long been considered the most plausible route to new habitats, making the fate of eggs inside potential animal vectors a matter of evolutionary consequence rather than mere curiosity.

The study also highlights how little is known about the chemical ecology of phasmid eggshells. The biomineralized layers that give the eggs their rigidity appear to be a double-edged adaptation, hardening the shell against physical hazards while creating a chemical vulnerability to the low pH of a bird’s proventriculus. Understanding how different species vary the composition and thickness of these layers could reveal whether any lineage has evolved genuine resistance to gastric conditions.

There is an ecological irony in the results as well. The capitulum, the lipid-rich appendage atop many phasmid eggs, chemically and visually mirrors the elaiosome of myrmecochorous seeds, and ants respond to it similarly, carrying eggs into their nests and gnawing off the capitulum without harming the embryo. Ant-mediated dispersal, unlike bird-mediated dispersal, involves no grinding gizzard and no strong acid, suggesting that the seed-mimicry of phasmid eggs may be aimed at a much smaller and far more forgiving partner than the birds the resemblance superficially invokes.

Subject of Research: Experimental testing of whether stick and leaf insect (Phasmatodea) eggs can survive digestion and dispersal by granivorous birds.

Article Title: Can granivorous birds disperse stick and leaf insect eggs? Integrating in vivo digestion trials and in vitro functional experiments

Article References: Burack, J., Haase, E., Gorb, S. N., & Büscher, T. H. (2026). Can granivorous birds disperse stick and leaf insect eggs? Integrating in vivo digestion trials and in vitro functional experiments. The Science of Nature, 113(5), Article 106. https://doi.org/10.1007/s00114-026-02157-5

Image Credits: AI Generated

DOI: 10.1007/s00114-026-02157-5

Keywords: Phasmatodea, stick insects, egg dispersal, ornithochory, granivorous birds, pigeons, seed mimicry, gizzard, gastric acid, eggshell biomineralization, friction, zoology