A technique for estimation of Barn Owl (Tyto alba stertens Hartert, 1929) prey biomass with special reference to mandible length-body weight ratio of small mammals

Abstract

An investigation was made to study the relationship between mandible length and body weight in six species of small mammals, which are the chief prey base of the Barn Owl Tyto alba. Regression equations for four species of rodent pests, B. bengalensis, M. meltada, T. indica and R. rattus are given for the quantification of prey biomass by using mandible length as a function of body weight. The prey biomass for S. murinus (40g) and M. booduga (12g) can be estimated by using mean body weight values. KEYWORDSBarn Owl, Bandicota bengalensis, body weight, mandible length, Millardia meltada, Mus booduga, prey biomass, Rattus rattus, small mammals, Suncus murinus, Tatera indica, Tyto alba Owls are in general highly beneficial birds of prey to mankind since they are rodent hunters. They hunt rodents at night and help to keep in check the populations of these mammals. Among the owls, Barn Owls have global distribution matched by few other species in the world (Burton, 1984;Taylor, 1994), with 36 subspecies distributed throughout the world (Taylor, 1994). In the Indian subcontinent there are two known subspecies, Tyto alba stertens in peninsular India and Tyto alba de-roepstroffi, a rare subspecies restricted to the southern Andaman Islands of the Indian Ocean (Bunn et al., 1982;Ali & Ripley, 1983;Taylor, 1994).The literature reviews on major diet of barn owls from six continents were reported to be rodents, the most important small mammalian group. Out of 52 key studies, analysed on Barn Owls diet by Taylor (1994), the rodents constituted more than 50% of all prey items in 47 studies and they formed more than 75% of all prey items by number in 33 studies, suggesting thereby that Barn Owls are excellent rodent hunters in nature.The results of average daily food requirements of Barn Owls under wild conditions have shown that in two studies, they required more than 75g prey/day (Taylor, 1994;Kanakasabai et al., 1998); and in one study their requirements were more than 100g/day, indicating that Barn Owls are potential rodent eaters in nature.The predatory pressure of Barn Owls over different prey spectrum was studied by an indirect method, namely, the regurgitated pellet analysis, which is a reliable technique than other techniques as far as owls are concerned (Errington, 1932;Glading et al., 1943) and particularly for Barn Owls (Marti, 1987). The pH of the Barn Owls stomach is higher (less acidic) than that of many other predatory birds and hence most of the bones of ingested prey are left undigested (Smith & Richmond, 1972). Further, the skulls and mandibles of even the most delicate small mammal and bird prey are found intact in the pellets and can easily be identified (Taylor, 1994).According to Marti (1987), both prey frequency and biomass quantification methods are equally valuable because the former provides better information on the relative impact of a raptor upon various prey species, while the latter may give a more accurate evaluation of the relative importance of different prey species in the diet.The p...