Recruitment in Rocky Mountain Greater Sandhill Cranes and Comparison with Other Crane Populations

Abstract

The sandhill crane (Grus canadensis) has the lowest known recruitment (% juv in fall and winter of any avian species hunted in North America. Long-term recruitment data provide insight into a population's productivity, ability to support harvest, and responses to management programs. During 1972-92, we annually collected age ratio (n = 191,968) and brood size (n = 12,239) data for greater sandhill cranes (G. c. tabida [greaters]) of the Rocky Mountain Population (RMP) at their migration stopover in the San Luis Valley, Colorado (fall), and at sites in New Mexico (winter) and southeastern Arizona (winter). We collected similar data (1987-92) for lesser sandhill cranes (G. c. canadensis [lessers]) from the Midcontinent Population (MCP) wintering in flocks mixed with RMP cranes, and for greaters of the Lower Colorado River Valley Population (LCRVP) wintering in western Arizona (1973-75 and 1989-92). Annual recruitment in the RMP averaged 8.1% (SE = 0.6) in fall and 8.0% (SE = 0.5) in winter over 21 years; mean brood size was 1.25 (SE = 0.02) in fall and 1.23 (SE = 0.02) in winter. In years with higher percent juveniles, more pairs fledged multiple young (P < 0.01); second and rarely third chicks in broods composed 20% of RMP recruitment. Recruitment and brood size had a downward trend (P < 0.01) over 21 years. During 1986-92, recruitment and brood size were below average (P < 0.01), apparently due to drought in breeding areas. We estimated survival rates of RMP cranes to be 0.95 during 1972-85, 0.94 during 1985-90, and 0.91 during 1990-92. Recruitment, survival estimates, and count data during 1985-92 suggest that the RMP is stable at 20,000-21,500 or declining slightly and cannot sustain increased harvest. Recruitment in MCP lessers averaged 11.2% (SE = 0.34) and mean brood size was 1.14 (SE = 0.02). Mean recruitment in LCRVP greaters was 4.8% (SE = 0.93), and mean brood size was 1.14 (SE = 0.028). Mean recruitment in most other North American crane populations was 5-14%. The whooping crane (G. americana) has the highest long-term (55 yr) recruitment (13.9%) of any North American crane population. Recruitment data alone cannot be used to estimate rate of increase and are difficult to interpret without additional demographic information. However, perennially low recruitment (≤ 5-6%) approximates the lowest documented mortality rates in any North American crane population, and should be a cause for concern, particularly in hunted populations. Management of crane populations can be improved by annually gathering data on recruitment, population size, and hunter harvest, and by obtaining survival estimates from long-term studies