A novel function of egg burial: Burying material prevents eggs rolling out of wind-swayed nests

Egg burial behaviour, that is, when parents bury the eggs with a layer of nest material during the egg-laying stage, has been described in various egg-laying animals. Several functions of egg burial have been described in animals with different life histories and breeding traits, but rarely reveal distinctive functions between sister species. In the polygamous Eurasian penduline tit, Remiz pendulinus, sexual conflict over care has been proposed to drive egg burial since, during egg laying, females hide eggs from males to prevent them abandoning the nest. Females then have the option to desert the clutch themselves and leave parenting to the male. However, in a congeneric species, the Chinese penduline tit, Remiz consobrinus, males have been seen with females in the nest at night, which indicates males know of the eggs’ existence. In this study, we investigated egg burial function in Chinese penduline tits and experimentally tested four hypothesized functions of egg-burying behaviour. (1) We found that egg burial is unlikely to play a role in sexual conflict resolution, as both males and females appeared to bury eggs during egg laying and both freely entered and roosted in the nests with eggs exposed at night. (2) Egg burial does not prevent nest parasitism, as no egg rejection or clutch abandonment was observed in clutches with model parasitic eggs. (3) Our results do not support the temperature regulation hypothesis since the temperature difference between buried and experimentally unburied eggs did not affect hatching success. (4) Notably, our results support the novel egg protection against wind hypothesis since the burying layer efficiently prevented the eggs from rolling out of wind-swayed nests. The difference our study found between the two Remiz species highlights that one behaviour, egg burial, can serve different evolutionary functions between closely related species.