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Predation by Weasels (Mustela nivalis) on Breeding Tits (Parus Spp.) in Relation to the Density of Tits and Rodents

Euan Dunn
Journal of Animal Ecology
Vol. 46, No. 2 (Jun., 1977), pp. 633-652
DOI: 10.2307/3835
Stable URL: http://www.jstor.org/stable/3835
Page Count: 21
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Predation by Weasels (Mustela nivalis) on Breeding Tits (Parus Spp.) in Relation to the Density of Tits and Rodents
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Abstract

(1) Annual and seasonal variations in predation rates by weasels (Mustela nivalis L.) on tits were studied in Wytham Wood, Oxfordshire. Four Parus species of tits breed in the nest boxes which have been provided for them in the wood since 1947. (2) Weasels are by far the most important cause of nest failure in the wood and on average 20.8% of nests were preyed upon by them each year from 1947 to 1975. (3) There were marked annual fluctuations in predation rate, varying from 0-50%. Predation was insignificant until 1957, two years after myxomatosis cleared the wood of rabbits. This caused widespread predation pressure on small rodents and may have led weasels to explore nest boxes as an alternative source of food. (4) The nesting density of tits and the density of rodents, measured the same summer, together accounted for over 60% of the annual variation in predation rates; both prey types appear to be equally important. The possible influence of weasel density on predation rates is also discussed. (5) Because predation on tit nests is proportional to nesting density, Coal Tits Parus ater L. apparently suffer less predation than the other species by starting to breed first when few other nest boxes contain eggs. The staggered laying also results in interspecific variation in the stages (laying, incubation, nestling) at which nests are most likely to be preyed upon. (6) Using the combined nests of Blue Tits and Great Tits as a `prey population', it is shown that a high nest density hastens the onset of predation but if rodent density is high this tends to retard the rate of predation on tit nests. (7) This effect of prey density on the timing of predation accounts for seasonal variations in predation rates between 1959-63, described by Perrins (1965), and explains the decline in predation on nestlings after 1963. The relationship found here may also help to explain why Krebs (1970) found that predation on nestlings was not density dependent. (8) The breeding strategy of the tits is discussed and found to be little defence against predation by weasels. Laying date may, however, affect the incidence of adult mortality but there is insufficient information on survival to calculate overall fitness.

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