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Tit Predation and the Abundance of Ernarmonia conicolana (Heyl.) on Weeting Heath, Norfolk, 1962-63

J. A. Gibb
Journal of Animal Ecology
Vol. 35, No. 1 (Feb., 1966), pp. 43-53
DOI: 10.2307/2688
Stable URL: http://www.jstor.org/stable/2688
Page Count: 11
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Tit Predation and the Abundance of Ernarmonia conicolana (Heyl.) on Weeting Heath, Norfolk, 1962-63
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Abstract

1. Measurements of the abundance of larvae of the eucosmid moth Ernarmonia conicolana (Heyl.) and of predation by tits (Parus spp.) upon them, on Weeting Heath, Norfolk, in 1962-63, supplement and expand those given previously (Gibb 1958). 2. Samples of cones from each of 147 scattered old Scots pines (Pinus sylvestris) were examined for Ernarmonia conicolana and evidence of tit predation. Numbers of E. conicolana larvae varied on different trees from none to seventy-nine per fifty cones. 3. The larvae of E. conicolana were unevenly distributed on Weeting Heath. In general, numbers were greatest on trees with many heavily infested neighbouring trees within a 50 m radius. 4. Predation by tits varied from tree to tree according to the number of larvae per fifty cones (the larval intensity), reaching 60% on trees with fifty to sixty larvae per fifty cones. Abruptly increased predation with more than ten larvae per fifty cones suggests a threshold in larval numbers above which it became profitable for the birds to feed on them. 5. Confirming previous results, percentage predation was significantly heavier on larvae in cones with many than with few larvae per cone. Larvae were concentrated on outer-facing quarters of the cones, and suffered heavier predation there than did those on inner-facing quarters. 6. Mortality other than by tit predation is considered. About 14% of the survivors from tit predation failed to complete their life cycle. At least 3.7% of all larvae were parasitized by the ichneumon Ephialtes laticeps Ratz. 7. Numbers of cones with different numbers of pupal chambers closely conformed to Neyman's `contagious' distribution. It is inferred that very few developing larvae die as a result of intraspecific competition within the cone, although possibly short of food; and that total mortality of larvae is probably slight until they are attacked by tits. 8. Although predation by tits on the overwintering larvae is intensity and sometimes density dependent, and may account for more than 50% of the fully-grown larvae, probably neither this predation nor density effects between larvae are sufficient to regulate the population of Ernarmonia conicolana.

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