Thursday, 26 February 2015

The diet of breeding Grass Owls in Thailand.

Grass Owls are members of the Barn Owl family, Tytonidae, distinguished by their habit of roosting and nesting in tall grasses or other ground-cover plants, rather than trees. Until fairly recently all Grass Owls were thought to belong to a single species, but there are now two species recognised, the African Grass Owl, Tyto capensis, which is found across much of Africa, and the Eastern Grass Owl, Tyto longimembris, which is found in south China, Nepal, India, Myanmar, Southeast Asia, Indonesia, the Philippines, New Guinea, New Caledonia and Australia. The Eastern Grass Owl was first discovered in Thailand as recently as 2006, where a small colony was found nesting in a small area of marsh grassland called Nong Lom in Mae Jan District in Chiang Rai Province in the north of the country. 

In a paper published in the Raffles Bulletin of Zoology on 17 February 2015, Akalak Kunsorn of the Department of Biology at Chiang Mai University and the Environmental Ornithology Research Unit at Mahidol University, Siriwadee Chomdej of the Department of Biology and the Materials Science Research Center at Chiang Mai University, Narit Sitasuwan, Prasit Wangpakapattawong and Chatmongkon Suwannapoom, also of the Department of Biology at Chiang Mai University and Brett Sandercock of the Division of Biology at Kansas State University, describe the results of a study of the diet of the Grass Owls of Nom Long carried out by examining Owl pellets from the site during the breeding season.

Eastern Grass Owl, Tyto longimembris, in flight. Rob Hutchinson/The Owl Pages.

Birds lack teeth with which to process food like mammals, instead grinding it up in a muscular gizzard before it is passed to the stomach. As flying animals they are obliged to keep their weight to a minimum, and therefore need to dispose of inedible parts of their prey as quickly as possible, and so both Owls and Raptors regurgitate pellets comprising indigestible items such as fur and bone without passing them through the stomach and gut.

As well as Grass Owls the Nom Long site is also home to four species of Harriers; the Eastern Marsh Harrier, Circus spilonotus, the Pied Harrier, Circus melanoleucos, the Hen Harrier, Circus cyaneus and the Western Marsh Harrier, Circus aeruginosus. Grass Owl pellets tend to be larger and more elongate than Harrier pellets, but to avoid confusion Kunsorn et al. collected pellets only from the nests of Owls.

Kunsorn et al. carried out DNA tests on bones recovered from Owl pellets at Nom Long, in order to determine identify animals being consumed by the Owls. They were able to identify three species of Rodent by this method, the Black Rat, Rattus rattus, the Asian House Mouse, Mus musculus, and the Ricefield Mouse, Mus caroli. In addition wings and tails of Barn Swallows, Hirundo rustica, were also found in the Owl nests, suggesting that these Birds were also being consumed.

Pellets of the Eastern Grass Owl, Tyto longimembris, collected from nests at Nom Long. Kunsorn et al. (2015).

The diet of Eastern Grass Owls has previously been studied in southern Taiwan, where they were found to be consuming Lesser Rice-field Rats, Rattus losea, Ricefield Mice, Mus caroli, Formosan Blind Moles, Mogera insularis, House Shrews, Suncus murimus, Shrews Crocidura spp. and juvenile Formosan Hares, Lepus sinensis, as well as small Birds. This is a far larger range of prey than consumed at the Thai site, which Kunsorn et al. attribute to the more varied hunting grounds available to the Taiwan Owls. Nom Long is largely surrounded by grasslands and wetlands, with some Orange orchard about a kilometre away to the northwest, while the Taiwan site was within easy reach of shrubland, Rice paddies, Bamboo forest, Sugarcane fields, Mango orchards and Human settlements.

See also…

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