Grays Lake Ecosystem. Breeding Ecology of Sandhill Cranes at Grays Lake National Wildlife Refuge, Idaho

Authors(s): J. Ball, A. Henry and J. Austin

Publication:

Publisher:

Publication Date: 0000-00-00

Type:

Location:

Abstract: Historically, Grays Lake supported the largest breeding concentration of greater sandhill cranes (Grus canadensis tabida) in the Rocky Mountain Population. The purpose of this study is to collect current information on population numbers and breeding success to compare with data collected in the 1950's and 1969-1971. The 4 main objectives of this project are to 1) determine the number of breeding and non-breeding sandhill cranes; 2) determine nest success and evaluate the factors that affect nest success; 3) describe habitat use by breeding sandhill cranes; and 4) determine how many pairs successfully raise chicks and how many migrating cranes use the Grays Lake valley. Data was collected during 1997-2000. In 1997-2000, sandhill crane nests throughout much of the Grays Lake basin were located by using a spotting scope and binoculars. Nests in tall vegetation or areas not easily viewed were located either on foot or by canoe. For each nest, we collected information on nest height and width, nest site vegetation, water depth, number of eggs, incubation stage, and nest fate (successful, destroyed, abandoned, or other). Data used in this study include those collected on cranes nesting in experimental units (12 Fish and Wildlife Service units where the Habitat Management is conducted), as well as on other public and private lands (non-experimental units) in the valley. We monitored a total of 129 nests in 1997 (60 on experimental units and 69 on nonexperimental units); 131 in 1998 (66 and 65), 143 in 1999 (54 and 89), and 173 in 2000 (53 and 120). Apparent nest success rates of all nests averaged 43% in 1997, 66% in 1998, 55% in 1999, and 59% in 2000. Rates in experimental and nonexperimental units differed by <7% in 1997 and 1998; larger differences in 1999 (44% vs. 61%) and 2000 (45% vs. 65%) were likely due to very high (>90%) nest success in areas searched only in those 2 years. Rate of renesting seems to vary among years with lower rates corresponding to years of higher nest success (4.6-9.2% of nests in 1997, 1.5-4.5% in 1998, and approximately 10% in 1999 and 2000). Greater than 57% of monitored crane nests were located in Baltic rush/spikerush and wet meadow plant communities, in water depths averaging <10 cm. Nest success seems to be higher when the nests are more isolated by deep (>40 cm) water. Spring population surveys were conducted from 17 April to 11 May, 1998-2000, to determine number and distribution of breeding and non-breeding sandhill cranes in the Grays Lake basin. All cranes seen in the marsh and in adjoining pastures and uplands in the valley were counted during surveys. Mean population counts for 1998-2000 was 728 (n =14, range 613 to 831). Population counts were usually highest at the end of April then counts gradually declined as nesting began in early May and visibility began to deteriorate with rapid growth of vegetation. Each year the number of actual pairs was consistent among counts at 230-250. Fall population counts were conducted from mid-August through the end of September, 1998-2000. Counts in mid-August include primarily resident sandhill cranes and their young. Juvenile cranes are distinguished from adult cranes by plumage characteristics in order to determine the proportion of pairs at Grays Lake that successfully raised chicks. Counts during mid-August averaged 729 (n =5, range 628-847) for 1998-2000; 4.9% (1998), 4.4% (1999), and 2.0% (2000) of identifiable cranes were juveniles. Crane numbers started to increase after 20 August as cranes moved into the valley from other areas. Peak numbers (1,203-1,217 in 1998 and 1,485-1,574 in 1999) were counted during mid-September. In 2000, peak numbers (1,579-1,674) occurred in early September. Most cranes departed soon after 23 September, similar to records for earlier years. Data is currently being analyzed; reports and final manuscripts will be completed by early summer 2001. Proposals are currently being submitted for continued ecological and management related research of sandhill cranes.

Keywords: animal, breeding, bird, management, Aves, ornithology, Gray's Lake, Idaho, population, mortality, nest, nesting, habitat, migration, crane, sandhill crane, Grus canadensis tabida