Greater Yellowstone Ecosystem Report
Deciduous trees and shrubs such as willows, cottonwoods and aspens, are common habitats in Jackson Hole, where their leaf-out dates, dependent on accumulated growing-degree-days (Gdd), vary year to year by well over a month. In 2010 these habitats had not yet leafed out by early June, and many trees and shrubs were not in full leaf until the middle of June. Leaf-out dates of temperate deciduous trees and shrubs are known to be determined by accumulated “degree-days” calculated as the number of degrees above some minimum (e.g. 5° C) summed over time since the winter thaw. Some resident birds apparently track these variations and are able to occupy habitats at higher densities in early years. Migrants, with no prior knowledge of conditions in Jackson Hole, typically return to breeding habitats on schedules adapted to match the average leaf-out dates. In both early and late years their densities are lower, compared to densities in average years.
Cody, Martin L.
"Variation in the Date of Leaf-Out in Jackson Hole's Deciduous Habitats: Effects of a Late Spring on Breeding Birds,"
University of Wyoming National Park Service Research Center Annual Report: Vol. 33
, Article 12.
Available at: http://repository.uwyo.edu/uwnpsrc_reports/vol33/iss1/12