Ecology and Vulnerability

American Woodcock

Background
The American woodcock is a migratory bird species that winters along the Atlantic and Gulf coastal plains from the Carolinas to Texas and breeds in the North American temperate zone, including Massachusetts^1,2.  Within temperate regions, it is dependent on shrub-dominated and early successional habitats, where the younger forest's dense vegetation protects them from predators and allows them to reach higher population densities than in more mature forest.  The replacement of this habitat type with mid-aged to mature forest over the past several decades has resulted in population declines of woodcock in the eastern U.S.^3,4.  Although woodcock may eat a variety of invertebrates, earthworms can be anywhere from 30-100% of their diet and woodcock distributions have been found to be associated with the availability of earthworms^4,5.  Because earthworms are most available when soil moisture is between 15 to 80%, woodcock are also associated with moist, rich soil conditions^5,6.

Climate Impacts
Studies have found phenological (seasonal timing) changes in some aspects of American woodcock life history.  Changes include earlier date of spring arrival in Massachusetts and New York⁷, earlier date of first arrival in Maine⁸, and earlier date of their breeding call in Wisconsin⁹.  Some of these researchers have attributed these changes to the effects of climate change.  One researcher noted that, taken to the extreme, such changes could result in some birds remaining at breeding locations year-round⁷.

One modeling study determined that although habitat is an important predictor of woodcock population numbers, the date of the start of the growing season is the strongest predictor, with a later date corresponding to lower numbers of American woodcock¹⁰.  The researchers speculated that this is likely related to the impact that growing season has on the availability of earthworms, a primary food source for woodcock.  Based on this variable alone, an earlier growing season as a result of climate change could potentially favor higher woodcock abundance in Massachusetts, but this would have to be accompanied by changes in forest habitat from mid-age and older forest to early or younger forest.  However, warmer temperatures, lengthened growing season, and increased evapotranspiration (loss of water from soil and plants from evaporation and transpiration) are all anticipated to result in increased short-term drought frequency in the Northeast¹¹.  Such droughts are defined by soil moisture content below 10% by volume.  These conditions may make earthworms less available to woodcock, which could potentially negatively impact populations.  Modeling conducted by the University of Massachusetts suggests that future climate scenarios should have a negative, though relatively small, impact on woodcock populations in the Northeast¹².