Distribution, abundance, and migration timing of Greater and Lesser Sandhill Cranes wintering in the Sacramento-San Joaquin River Delta region of California

The Sacramento-San Joaquin River Delta region of California (hereafter, Delta region) is an important wintering
region for the Central Valley Population of greater sandhill cranes (Grus canadensis tabida) and lesser sandhill cranes (G.
c. canadensis), but basic information about the ecology of these birds is lacking to design a biologically sound conservation
strategy. During the winters of 2007-08 and 2008-09, we conducted roost counts, roadside surveys, aerial surveys, and tracked
radio-marked birds to define the geographic area used by sandhill cranes in the Delta region, document migration chronology,
and estimate subspecies-specific abundance. Radio-marked sandhill cranes arrived in our study area beginning 3 October,
most arrived in mid-October, and the last radio-marked sandhill crane arrived on 10 December. Departure dates ranged from
15 January to 13 March. Mean arrival and departure dates were similar between subspecies. From mid-December through
early-February in 2007-2008, the Delta population ranged from 20,000 to 27,000 sandhill cranes. Abundance varied at the
main roost sites during winter because sandhill cranes responded to changes in water conditions. Sandhill cranes used an area
of approximately 1,500 km² for foraging. Estimated peak abundance in the Delta region was more than half the total number
counted on recent Pacific Flyway midwinter surveys, indicating the Delta region is a key area for efforts in conservation and
recovery of wintering sandhill cranes in California. Based on arrival dates, flooding of sandhill crane roost sites should be
staggered with some sites flooded in early September and most sites flooded by early October. Maintained flooding through
mid-March would provide essential roosting habitat until most birds have departed the Delta region on spring migration.