The U.S. Fish and Wildlife Service has used spring aerial surveys to estimate numbers of migrating sandhill cranes (Grus canadensis) staging in the Platte River Valley of Nebraska, USA. Resulting estimates index the abundance of the midcontinent sandhill crane population and inform harvest management decisions. However, annual changes in the index have exceeded biologically plausible changes in population size (\u3e50% of surveys between 1982 and 2013 indicate \u3e20% change), raising questions about nuisance variation due to factors such as migration chronology. We used locations of cranes marked with very-highfrequency transmitters to estimate migration chronology (i.e., proportions of cranes present within the Platte River Valley).We also used roadside surveys to determine the percentage of cranes staging at the Platte River Valley but outside of the survey area when surveys occur. During March 2001–2007, an average of 86% (71– 94%; SD=7%) of marked cranes were present along the Platte River during scheduled survey dates, and 0– 11% of cranes that were present along the Platte River were not within the survey boundaries. Timing of the annual survey generally corresponded with presence of the greatest proportion of marked cranes and with least inter-annual variation; consequently, accuracy of estimates could not have been improved by surveying on different dates. Conducting the survey earlier would miss birds not yet arriving at the staging site; whereas, a later date would occur at a time when a larger portion of birds may have already departed the staging site and when a greater proportion of birds occurred outside of the surveyed area. Index values used to monitor midcontinent sandhill crane abundance vary annually, in part, due to annual variation in migration chronology and to spatial distribution of cranes in the Platte River Valley; therefore, managers should interpret survey results cautiously, with awareness of a continuing need to identify and understand components of variation
