Insect-eating birds that feed nestlings with grasshoppers, beetles, or moths perform insect prey preparation before feeding nestlings so that the nestlings are able to swallow the prey. This behavior is generally not expected for soft prey such as earthworms. However, an overview of photographic evidence available online suggested that earthworms are sundered by parents before bringing the prey to the nestlings in a range of species from several families of worm-eating (vermivore) birds worldwide. In new research, Seoul National University Ph.D. student Jinseok Park and colleagues studied earthworm sundering performed by parents provisioning their broods at four nests of the fairy pitta (Pitta nympha) in the Republic of Korea; they found that fairy pitta parents sundered earthworms more often when nestlings were smaller and when the earthworm was longer.
A fairy pitta (Pitta nympha) stands outside the nest (located on the ground) and feeds sundered earthworms to its nestlings. Image credit: Sung-le Jang.
The fairy pitta is a small and brightly colored species in the passerine bird family Pittidae (pittas).
First described in 1850, it has a body length of up to 19.5 cm (7.7 inches) and is easily discernible for its plumage of seven different colors reminiscent of a rainbow.
The fairy pitta breeds in East Asia and migrates south to winter in Southeast Asia.
The bird’s diet mainly consists of earthworms, spiders, insects, slugs, and snails.
“Diet specialization that involves feeding on earthworms is observed in a range of avian species from several families worldwide, such as pittas, antpittas, and thrushes,” said Park and co-authors from Seoul National University, DGIST’s Laboratory of Integrative Animal Ecology, and the Museum and Institute of Zoology at the Polish Academy of Sciences.
“In those taxa, earthworms account for the largest proportion of their nestlings’ diet, but not many studies focused on the prey handling behaviors by vermivores that consume earthworms.”
“The fairy pitta is one of those vermivores,” the researchers added.
“It is a ground forager with unique dietary specializations that may require special behavioral adaptations.”
“The diet of young nestlings consists of over 80% of earthworms.”
From May to July in 2012, 2013, and 2017, the scientists studied a total of 4 nests of the fairy pitta in Hallyeohaesang National Park located in the southern part of the Republic of Korea, which corresponds to the northern part of the bird’s breeding range.
Based on direct observations and video recordings, a foraging pitta captured one earthworm at a time, put the single prey aside on the ground, and resumed searching for another prey, which was deposited at the same place on the ground.
After multiple prey items have been collected on the ground, the bird picked up all of them and carried the full load to the nest, and fed the nestlings.
The authors often observed that the birds sundered earthworms into two or more pieces before depositing them on the ground.
“I have observed that parents sunder earthworms more often when nestlings are younger and when the earthworms are longer,” Park said.
“Thus, we believe that the parent pittas sunder earthworms in order for the smaller, younger nestlings to swallow the food better.”
The team also considers three possible functions for earthworm sundering: (i) earthworm sundering might help parents to carry the prey to the nest; (ii) it may reduce the risk of visual detection by predators as the nestlings can quickly swallow sundered prey; or (iii) it may reduce the mobility of prey so that parents can deposit the captured prey on one spot of the ground while they hunt for more earthworms.
“These explanations can all contribute to the evolution of the earthworm sundering in the fairy pitta,” said Dr. Sang-im Lee, a researcher in the Department of New Biology in DGIST’s Laboratory of Integrative Animal Ecology.
The researchers analyzed photos from the photographic archives of the Macaulay library at the Cornell Lab of Ornithology and found that multiple species from several families worldwide also sunder earthworms.
“This is the first quantitative analysis of this specific prey preparation behavior known to occur in various avian vermivores,” said Dr. Piotr Jablonski, a researcher with Seoul National University and the Museum and Institute of Zoology at the Polish Academy of Sciences.
The study was published in the Journal of Ornithology.
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J. Park et al. Sundering of earthworms by the Fairy Pitta (Pitta nympha): an unexplored strategy of prey preparation for brood provisioning by avian vermivores. J Ornithol, published December 18, 2021; doi: 10.1007/s10336-021-01955-0
