Analysis of factors affecting bird predation on gypsy moth egg masses by using Holling's disc‐equation

Abstract

Summary Birds’ predation on gypsy moth egg masses deposited on tree trunks was quantitatively evaluated in a birch forest at Bibai, Hokkaido during the four winter seasons from 1974 to 1978. Holling's disc‐equation could be successfully applied to describe the relationship between the total egg consumption by birds and the density of egg masses. A rapid method for fitting the disc‐equation to observed data was proposed based on the study byShinozaki andKira (1961). Appreciable amounts of predation were observed only during the period when the depth of snow cover on the ground exceeded 1 m, while the egg masses deposited on lower parts of tree trunk below 1 m level suffered from only slight predation. It was therefore presumed that deep snow cover made the predator birds to change their feeding place from ground vegetation to tree trunks. The duration of the period with snow accumulation deeper than 1 m was introduced into the disc‐equation as an additional parameter. The values calculated by the resultant formulation agreed with the observed values in respective years satisfactorily. There were circumstantial evidences that the great tit, Parus major, was one of the important predators concerned.