Our observations revealed that sundering occurs before transporting the earthworms to the nest, which may be consistent with all four hypotheses. Earthworm sundering comprises simple prey-splitting behavior, and the Fairy Pittas brought all pieces at the same visit to the nest (JP personal observation). Our observations reveal that earthworm sundering has no direct relevance to the removal of certain body parts of prey, such as removing inedible parts of prey (Kaspari 1991; Sodhi 1992), and it does not have a function of making the prey load lighter to reduce the costs of prey transport (Sodhi 1992; Rands et al. 2000). As even long unsundered earthworms can be carried in a compact food load if they are bent and held by the beak at multiple points (e.g., Fig. 1B), the transport of prey hypothesis seems unlikely to be the sole explanation for earthworms sundering in the Fairly Pitta. The presence of the effect of nestling age on sundering is also inconsistent with transport of prey mechanism, which does not predict such an effect. However, we cannot entirely reject this hypothesis based only on our observational data.
The observation that longer worms were more likely to be sundered than the smaller ones may be viewed as inconsistent with the “hunting of multiple prey” hypothesis because the ability of a worm to escape from a pile of worms may not necessarily be dependent on the worm size, but if it does then it is more likely that the smaller worm may be quicker at moving away and disappearing from the field of view among leaf litter on the ground than the larger one. Nevertheless, as the handling of a detected and attacked earthworm is clearly easier and more efficient when no other food items are held in the beak, it seems beneficial for pittas to deposit their prey items (earthworms) at one spot on the ground instead of carrying them in the beak while foraging (see Online Resource 2). These observations appear to be partly consistent with the idea that the function of prey depositing may be an increase in foraging efficiency, and that the function of prey sundering may be to reduce the prey’s mobility and to decrease the risk that prey will escape after being deposited on the ground. Experiments on earthworm mobility and foraging efficiency of pittas are needed to fully evaluate this hypothesis.
The results also revealed that the sundered worms were more often delivered to young rather than to old nestlings and that sundered sections were shorter when nestlings were younger. As one of the predictions from the “decreased detectability hypothesis” is more frequent sundering when the brood is older, it appears that this hypothesis alone cannot fully explain the observed pattern. The significantly higher frequency of sundering prey and shorter sundered section length for younger nestlings, who were observed to have difficulties swallowing large items, are all consistent with the provisioning of small nestlings hypothesis, and are reminiscent of results from other species of birds that do not feed their nestlings with earthworms (e.g., Barba et al. 1996; Banbura et al. 1999). The relatively abrupt decrease in sundering during ~ 3 days starting on the 4th (Nest 4) or 6th (Nest 4) day of nestlings’ life (Fig. 2C) is consistent with the idea that after a specific growth stage is reached, the nestlings may be able to swallow most of the unsundered earthworms. This hypothetical stage may correspond to the slowdown of body mass growth and the increase in the growth rate of feathers as suggested by Jiang et al. (2017), who observed that the feathers on wings, back, and head of nestlings start growing at the age of about 7 days after hatching. However, to further evaluate the feasibility of this hypothetical threshold, detailed data are needed for many more nests.
The lack of significant effects of time of day and inter-visit interval on sundering and on the prey length (albeit the non-significant effects may be included in the top models) do not contradict any of the four hypotheses. As each piece of a sundered worm has to be kept in the beak separately during transport, it is obvious that sundered worms were more often observed in loads of fewer worms. As it presumably takes less time to collect a load of fewer worms, we expected a negative relationship between the inter-visit interval and the probability of earthworm sundering. The results partly suggest such an effect: the best model included marginally non-significant (p = 0.060) effect of inter-visit interval (albeit the second best model without this effect did not differ dramatically in the fit to the data from the best model: ΔAICc = 1.56 in Table S5).
Trade-offs may exist between costs of prey preparation to parents and benefits to the nestlings from being fed the prepared prey (Ponz et al. 1999), as well as between costs of prey preparation versus costs for carrying unprepared prey (Rands et al. 2000). However, our observations indicate that the act of sundering of an earthworm (unlike preparing a prey with a hard exoskeleton) is quick and easy i.e., not costly in terms of handling time or energy. Therefore, the aforementioned tradeoffs may not directly apply to the Fairy Pitta and possibly other vermivores who sunder earthworms.
In summary, this is the first quantitative analysis of the specific prey preparation behavior known to occur in avian vermivores provisioning their broods: earthworm sundering. Although our observational data and preliminary analyses do not completely allow us to reject any of the four hypotheses considered, we suspect that the main function of sundering by the Fairy Pitta is to increase the probability that a nestling is able to quickly swallow the prey. More extensive data collection from various avian vermivores, such as pittas, antpittas, or thrushes is needed to understand the general adaptive explanations for earthworm sundering, an understudied method of prey preparation by parent birds.