Abstract
Conditions in the GTNP breeding season can differ dramatically among years, even between adjacent years such as 2011 and 2012. Specifically, 2012 was the warmest and driest spring since the early 1990s, with snow melt-out date (SMOD) on April 17, a few days earlier than the long-term average, and 90% aspen leaf-out also earlier than the mean by nearly a week. In contrast, 2011 was the latest SMOD and aspen leaf-out in some 50 y. Birds settle in breeding habitat with somewhat altered preferences in early, warm and dry years such as 2012 compared to opposite conditions in the 2011. In 2012, many birds typical of lower, more open and less mesic habitats were found breeding in taller, denser and usually wetter habitats, a clear influence of the currently prevailing conditions in the park. A monitoring site of intermediate vegetation height and density, Site #9: RKO Dry Willows, is used to calibrate these habitat shifts.