Effects of gypsy moths on population dynamics of cuckoos and other forest birds

The invasive gypsy moth (Lymantria dispar) periodically outbreaks and defoliates large areas of temperate forest in the northeast United States and southeast Canada. Outbreaks potentially influence bird populations in two different ways: positively, as a temporarily abundant food source, and positively or negatively, as a physical ecosystem engineer that alters forest structure through defoliation. Few birds eat gypsy moth caterpillars, but both Yellow-billed (Coccyzus americanus) and Black-billed Cuckoos (C. erythropthalmus) are known to preferentially consume large numbers of them during outbreaks. Using detrended and standardized abundance data from the North American Breeding Bird Survey and 29 years of U. S. Forest Service defoliation surveys, we show that both cuckoo species were abundant in defoliation years, but Yellow-billed declined immediately, while Black-billed declined more slowly over the two years following defoliation. Both species were also less abundant than average three to four years after defoliation. This seems be due to regional immigration-emigration dynamics in response to defoliation; Yellow-billed Cuckoo populations were less synchronized with other populations 100-250 km apart than with populations at greater distances. We examined 24 additional eastern woodland birds in different feeding and nesting guilds and found that nine exhibit population changes in the years following defoliation, presumably due to gypsy moth-induced habitat changes. These nine include three cavity-nesters, two understory-nesters, three canopy-nesters, and nest-parasitic Brown-headed Cowbird (Molothrus ater).