There are 25 species of bats in Hong Kong, accounting for almost half the number of local mammal fauna. Bats can be generally categorised into two groups – insectivorous bats and fruit bats, although there are exceptions to this. In Hong Kong we have one insect-eating species (Myotis pilosus) that also feeds on fish. However, during a joint field trip of the Fauna Conservation and Agriculture, Fisheries and Conservation Department (AFCD) staff, an exciting discovery was made!

The trip took place in early October 2022 when the two groups visited an ancient Study Hall in the northern New Territories that was inhabited by about 40 Himalayan leaf-nosed bats (Hipposideros armiger). A small clump of bird feathers was spotted on the ground amongst bat guano (droppings); the bats were roosting just above our heads on the wooden beams of the ceiling. We took a closer look at the bats, and to our surprise we spotted one of the bats still chewing a small bundle of feathers that then dropped to the floor among the guano.

The Himalayan leaf-nosed bat has been categorised as an insectivorous species; however, its close relative, the Diadem leaf-nosed bat (Hipposideros diadema), is known to show carnivory (1997), and in a separate study, bat predation of nocturnally migrating birds in the Mediterranean region was also reported for the aerial-feeding insectivorous bat Nyctalus lasiopteras (2001). Our fascinating observation might be the first ever record and sighting of this insectivorous bat preying on vertebrates, and also the first sighting of a bird-eating bat in this region.

During the site visit we collected some of the discarded feathers and passed the sample to our Conservation Genetics Laboratory for further investigation regarding the bird species. DNA was extracted from the feathers and then sequenced for a barcode gene. The sequence data of the feathers was compared with other bird records kept in a global DNA database called GenBank. The genetic analysis result indicated that the feathers belonged to a relatively uncommon autumn passage migrant/winter visitor – the Two-barred warbler (Phylloscopus plumbeitarsus).

The average length of this small bird is 12cm, which is actually just shorter than a full-grown adult Himalayan leaf-nosed bat (about 8-13 cm). Its weight, about 9g, is one seventh the weight of this bat species. That’s equivalent to a 70kg human carrying 10kg of food in their mouth. The bat species will often occupy night-time perches where large prey items are processed in between periods of hunting, and this may be what happened.

Although we cannot know for sure the circumstances of this bird and bat encounter, it’s possible that the bat opportunistically discovered the sleeping warbler while gleaning prey close to the branches of a tree.

A further study could involve an investigation of the bat droppings to see if DNA analysis confirms the bird had been digested. Although we did not witness the bat catching the warbler, the fact that carnivory was observed in a species very similar to H. Armiger suggests that this is what in fact occurred.

We hope this observation has created a starting point for future studies in terms of food webs, predator-prey relationships, feeding behaviour and morphological adaptations of bats. It certainly challenged our perceptions of this bat species. This is also a reminder for us to stay curious and pay attention to our surroundings; look a little bit closer and maybe you will discover some intriguing findings too!

Reference
Santana SE, Cheung E. (2016) - Go big or go fish: morphological specializations in carnivorous bats. Proc. R. Soc. B 283: 20160615. http://dx.doi.org/10.1098/rspb.2016.0615
Shi, B., Wang, Y., Gong, L. et al. (2020) Correlation of skull morphology and bite force in a bird-eating bat (Ia io; Vespertilionidae). Front Zool 17, 8. https://doi.org/10.1186/s12983-020-00354-0
Shek, C.T. (2006) - A Field Guide to the Terrestrial Mammals of Hong Kong.
Robinson, R.A. (2005) - BirdFacts: profiles of birds occurring in Britain & Ireland. BTO, Thetford http://www.bto.org/birdfacts, accessed on 01 December 2022
Pavey, C.R. and Burwell, C.J. (1997) - The diet of the diadem leaf-nosed bat Hipposideros diadema: confirmation of a morphologically-based prediction of carnivory. Journal of Zoology, 243: 295-303. https://doi.org/10.1111/j.1469-7998.1997.tb02783.x
C. Ibáñez, J. Juste, J. L. García-mudarra. et al.(2001) - Bat predation on nocturnally migrating birds. Environmental Science, Biology Proceedings of the National Academy of Sciences of the United States of America, Vol. 98 | No. 17. https://doi.org/10.1073/pnas.171140598
Robinson, M. & Webber, M. (1998). Observation on the diet of Myotis ricketti from Lao PDR; a piscivorous bat. Bat Res. News 39: 26.