Predation by Birds on Spruce Budworm Choristoneura Fumiferana: Functional, Numerical, and Total Responses

Abstract

This study examined the hypothesis that predation by forest birds restricts expansion of low—density populations of the spruce budworm Choristoneura fumiferana (Lepidoptera: Tortricidae). Bird populations were censused in spruce—fir stands of low to moderate spruce budworm density in northern New Hampshire and western Maine. Diets were determined from stomach contents of collected birds. Budworm population densities were estimated by sampling fourth—instar larvae and successfully emerged pupae (counted after moth emergence). Populations of spruce budworms ranged from 80 x 103 to >22 x 106 larvae/ha and from 0 to 1.2 x 106 pupae/ha. Birds are spruce budworms throughout the daylight hours. Blackburnian Warblers consumed the most Choristoneura (X = 28 420 budworms/ha), followed by Cape May Warblers (X = 26 220 budworms/ha), and grading down to no consumption by some species. In plots where spruce budworm densities ranged from low to transitional, the entire bird community as well as specific groups of birds, i.e., overstory warblers and Golden—crowned Kinglets; sparrows, vireos, and juncos; and nuthatches, thrushes, and Purple Finches, showed significant functional responses to increasing budworm density. When transitional budworm densities were eliminated from the analysis, only the warbler—kinglet group exhibited a significant functional response. Only two species, Canada Warbler and Golden—crowned Kinglet, showed numerical responses to increasing budworm numbers. The percentage of the budworm population consumed by birds decreased as budworm numbers increased. Birds consumed 84% of the larvae and pupae where budworm populations were low, 22% where budworm populations were intermediate between low and high density. Once fourth—instar larvae exceeded 106 individual/hectare, bird predation was ineffectual. The hypothesis that birds can effectively limit budworm increase is compatible with three recent theories to explain budworm outbreaks. The forest stands we studied were dense, even—aged, with little understory. Numbers of birds potentially can be increased by modifying these stands through silvicultural means (Crawford and Titterington 1979). Birds are capable of dampening the seriousness of spruce budworm infestations when habitats are suitable for supporting adequate populations of these effective predators.