Survival and Fledging Rates of Whooping and Sandhill Crane Colts at Necedah National Wildlife Refuge, Wisconsin, USA

Abstract

Whooping cranes (WHCR, Grus americana) are federally endangered and a reintroduction effort was initiated in 2001 at Necedah National Wildlife Refuge (NNWR) in Wisconsin to establish the Eastern Migratory Population (EMP). However, despite seventeen years of management, recruitment remains low. Greater sandhill cranes (SACR, Antigone canadensis tabida) are biologically similar to WHCRs and have similar breeding ecology, potentially indicating that SACRs can be used as initial indicator of ecosystem quality for reintroduced WHCRs. I studied colt survival for both species of cranes at NNWR to determine if low recruitment is unique to the EMP. Additionally, water levels were decreased in some wetland impoundments to better mimic a natural annual water cycle. The objectives of this research were to determine (1) if summer drawdowns and reduced wetland water levels increase survival and fledging rates of WHCR colts and SACR colts, (2) the differences in overall colt survival rates between species, and (3) if wetland type influenced transition or survival rates. I hypothesized that (1) summer drawdowns would increase the survival of WHCR and SACR colts, (2) SACR colts would have a higher overall survival and fledging rate than WHCR colts, and (3) WHCR and SACR family groups would transition to drawdown areas more often when available, increasing with colt survival. VHF transmitters were placed on adults and colts of both species to determine daily colt survival, fledging rates, and locations during the 2017 and 2018 field seasons. I used a multi-state model to estimate daily survival rates and daily transition rates of crane family groups between full impoundments, experimental drawdown impoundments and natural wetlands. WHCR colts had higher survival and fledging rates than SACR colts in both years of the study. Many of the fledged colts were raised in areas with lower water, and survival was highest and most consistent across the two species in managed drawdowns. Crane family groups mostly stayed in one wetland type often located near their nest site, but there was a high probability of transitioning out of a full impoundment even if nests were initially located in them. Impoundment drawdowns that mimic a natural hydrologic cycle at Necedah NWR may improve EMP WHCR colt survival and fledging rates as well as eventually improve the population growth rate.