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

Spatial distribution plays a crucial role in speciation, affecting genetic divergence and reproductive isolation. Phasmatodea (stick and leaf insects) have limited adult mobility and may use their seed-like eggs for passive dispersal. The possibility of egg dispersal through birds is controversially discussed in current literature. Previous studies suggest that ornithochory of phasmid eggs is context-dependent and most eggs do not survive digestion by granivorous birds, while eggs consumed by insectivorous birds can potentially survive and hatch, presumably also when a gravid female is consumed. This study examined this question by testing how different phasmid egg morphologies withstand bird digestion, simulating the digestive system in laboratory conditions and evaluating egg viability post-treatment. Results showed that none of the eggs of the tested species hatched after being eaten by granivorous birds, although some eggs of the species Dajaca napolovi were found structurally intact. In the laboratory experiments, the low pH environment significantly affected survival, with none of the treated eggs surviving acid treatment. High temperatures and mechanical impact also influenced egg hatching depending on the species. The findings suggest that acid and temperature tolerance are prerequisites for bird-mediated dispersal of single eggs ingested by granivorous birds and mechanical impact by the gizzard renders long-distance dispersal via ornithochory by granivorous birds unlikely for the investigated phasmatodean species. To further elucidate the potential ornithochory of phasmid eggs by birds with different diets, studies examining the digestive conditions experienced by ingested eggs in other bird species are warranted, particularly regarding insectivorous birds and the role of potential protection of eggs within an adult female phasmid.