To use gizzard analysis quantitatively, in the assessment of bird predation on insect populations, information is needed on the time that identifiable remains of the prey can be found in the gizzard and on the rate at which these remains disappear. The digestion rate, measured by the disappearance of mandibles or cremasters, has been studied for larvae and pupae of the spruce budworm, Choristoneura fumiferana (Clem.), when eaten by the olive-backed thrush, Hylocichla ustulata swainsoni (Tschudi).
The predatory effect of nine species of forest birds on the large-larval and pupal stages of the black-headed budworm was investigated for 3 years. Successive bird collections and subsequent gizzard analyses within a season permitted the development of response curves that show changes in consumption in relation to changes in prey size, density, and other characteristics within the season. Percentage predation varied from 3 to 14% over the 3 years and strongly suggested that birds act in a density-dependent manner.
Radioactive phosphorus was used as a tracer in assessing the number of red-legged grasshoppers (Melanoplus femurrubrum (De Geer)) eaten by the European praying mantis (Mantis religiosa L.) under seminatural conditions. The predator showed a functional response to changing prey densities with a continuously decreasing increase in the number of prey removed from the population.
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