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274 THOMAS H. POGSON AND SUSAN M. LINDSTEDT increased concentration of cranes there in the and wildlife managers near and beyond the limits 1980s compared to the early 1970s. of the wintering areas allowed us to more accu- Permanent and seasonal wetland habitats in rately delineate the boundaries of the regions used the Central Valley were lost at the rate of 2,100 by the population. ha per year between 1954 and 1985 (Frayer et al. 1989). Throughout the Central Valley, mead- ows and pastures have been converted to crop- Four sources of error may have affected our es- lands (Gilmer et al. 1982). In the Thornton re- ti mates of the population's size: 1) counting the gion alone, 900 ha of meadows and native same individuals more than once within regions, pastures were converted to croplands between 3) including the same individuals in estimates 1976 and 1983 (T. Pogson, unpubl. data). His- from more than one region, 3) confusing Lesser tonically,these sites were flooded annually to cre- Sandhill Cranes with Canadian and Greater ate waterfowl hunting areas which provided noc- Sandhill Cranes during counts, and 4) failing to tumal roosting sites for cranes(T. Pogson,unpubl. count all members of the population. Nine radio- data; C. D. Littlefield, pers. comm.). Most lo- tagged cranes changed nocturnal roost sites on cations converted to croplands in the Thornton 15% of 103 consecutive nights. Although this is region were still being flooded in 1983-1984 and a small number of individuals to describe the 1984-1985 to create waterfowl hunting areas and behavior of the population, the movements of were still used by Sandhill cranes as nocturnal radio-tagged cranes among roosting sites suggest roosting sites and diurnal loafing areas. In other that summing the number of cranes counted at areas of the winter range, however,seasonal wet- roosting sites on successive days could be a source lands and other uncultivated habitats that pre- oferror in ourestimates.We controlled this source viously were flooded to create waterfowl hunting of error by not summing the peak number of areas have been converted to croplands (Frayer cranes from two adjacent count sites when de- et al. 1989), and are no longer available to wet- clines in the number of cranes at one site were land species. It is likely that this habitat destruc- associated with an increase at the adjacent site. tion contributed to the concentration of cranes Marked cranes were never observed at more than at Thornton where roosting sites and feeding site while conducting mid-day counts. Because habitat were abundant during the winters of 1983-- counts were conducted in the Butte Sink before 1984 and 1984-1985. conducting counts at Thornton in October and In December 1983, we found 1,500 Greater November 1983,and marked cranes moved south Sandhill Cranes in areas of the winter range not from the Butte Sink to Thornton beginning in searched by Littlefield and Thompson(1979)but November during both years, it is possible that: which had been used by cranes for at least 30 1)some individuals were counted in both regions, years (J. Shanks, J. Mello, F. Pelladini, pers. or 2) that cranes moved out of the count areas comm.). These cranes account for most of the and were not detected. If the former was an I'm_ remaining difference between the 6,012-6,810 portant source of error, estimates calculated for large cranes we estimated in the Central Valley October (6,012) and November (6,057) should in 1983-1984 and the 2,359-2,553 cranes re- have been higher than the estimate for December ported by Littlefield and Thompson (1979) in (6.617), when counts at Thornton were con- December 1969, 1970 and 1976. We conducted ducted before the counts in the Butte Sink. Thus, extensive roadside surveys primarily in the it seems unlikely that counting the same indi- northern portion of the Central Valley during two viduals in more than one region was an impor- winters, while Littlefield and Thompson (1979) tant source of error in our estimates. We located conducted approximately 16,000 km of roadside only 82-86% of the color-banded and radio- surveys in the entire Central Valley,Carrizo Plain, tagged cranes known to be alive in both 1983— and the Imperial Valley during each of three win- 1984 and 1984-1985, suggesting we may have tors (C. D. Littlefield, pers. comm.). Littlefield missed 14-18%of the population by not locating and Thompson (1979) did not search the pe- all the sites used by large cranes. Because this is ripheries of the wintering areas, and they did not only a crude estimate of the error caused by not have the benefit of using radio-tagged cranes to locating all wintering sites, we did not expand locate cranes in relatively isolated areas. Our in- our estimates to account for the percentage of terviews with local residents, waterfowl hunters, the population that we may have missed. We VI-1.34