Numerical and functional responses of breeding passerine species to mass occurrence of geometrid caterpillars in a subalpine birch forest: a 30‐year study

Abstract

In northern Fennoscandia, the geometrid moths Epirrita autumnata and Operophtera brumata have cyclicities in density with mass occurrence at 10‐year intervals. The larvae of Epirrita and Operophtera attain a size of 2–3 cm and 1.5–2 cm, respectively, and are nutritious food items for passerine birds. To examine whether these larvae have any numerical and/or functional influence on a passerine bird community (mountain birch forest in Budal, central Norway) during a 30‐year period (1972–2001), I estimated their abundance (number of larvae per 100 sweeps) in the birch canopy, and the densities of breeding birds in the passerine community. In addition, from 1972 to 1998, I monitored the nesting success of five of the bird species. The foraging pattern of the most abundant bird species and their gizzard contents (adults and nestlings) were examined in 1972–78 (covering population peaks of both the geometrids). Population peaks of Epirrita occurred in 1975–76, 1985–86 and 1996, and of Operophtera in 1976–77, 1986–87 and 1997–98. The passerine community consisted of eight species that were territorial in all 30 years, one species in 26 years, three species in 14–21 years and three species in 1–4 years. Only the Brambling Fringilla montifringilla population responded numerically to the fluctuations of Epirrita and Operophtera. Brambling was also the only species whose mean clutch size varied between years, and this correlated positively with the density of Epirrita. The mean annual nesting success of Willow Warbler Phylloscopus trochilus, Bluethroat Luscinia svecica and Common Redpoll Carduelis flammea tended to be higher in years with mass outbreaks of Epirrita, but was significantly so only for Reed Bunting Emberiza schoeniclus. The abundance of Operophtera larvae showed no influence on the nesting success of any bird species. The passerines foraged more frequently in the birch canopy in the Epirrita outbreak years (1975–76) than in the years before or after. Gizzard analyses of five adult passerine species and their nestlings showed that Epirrita was the main food item in 1974–76. Even though Operophtera occurred in large numbers in birch trees in 1976 and 1977, only a few larvae were found in the gizzards of the passerines. None of the passerines showed an increase in their population density in the year following the larval outbreaks, but the densities of Willow Warbler and Bluethroat increased in the succeeding year, indicating a higher return rate for these species. The study shows the existence of a dietary response and also indicates a reproductive response to the changes in the abundance of Epirrita in mountain birch forest. The lack of numerical response in the passerines (except the Brambling) to the fluctuation in Epirrita contrasts with the pattern described for passerine communities in northern temperate deciduous forests in North America, where Lepidoptera caterpillars periodically have mass outbreaks.