Capture of Sandhill Cranes Using Alpha-chloralose: A 10-Year Follow-up

In 1990, the International Crane Foundation began capturing Greater Sandhill Cranes (Grus canadensis tabida) with alpha-chloralose (AC) for long-term ecologic research. (Alpha-chloralose [C₆H₁₁Cl₃O₆], is a chloral derivative of glucose that requires authorization for use by the U.S. Department of Agriculture, Animal and Plant Health Inspection Service, Wildlife Services, and consideration of potential drug withdrawal times in species that may be hunted.) This method is associated with lower morbidity and mortality rates than most alternatives used for small groups of cranes, such as rocket nets or leg nooses (Hayes et al. 2003). Exertional myopathy (EM), however, is a potential complication of capture in cranes (Windingstad et al. 1983). An epidemiologic analysis by Hayes et al. (2003) showed captures with AC that resulted in lighter sedation scores or were conducted in August or October (compared with September) were associated with increased risk of EM. Beginning in 2002, captures were limited to September and early October to reduce the risk of EM. Field staff also emphasized improving reliability of precapture baiting of target groups of cranes and were trained to apply supportive care measures consistently, especially administration of subcutaneous crystalloid fluids, to mitigate potential deleterious effects of the long-lasting sedation from AC. We assessed the effects of these management changes on capture outcomes.

We reviewed records of 317 cranes (141 males, 159 females, 17 unknown sex) captured near Briggsville, Wisconsin (43°36′N, 89°36′W) between 1990 and 2011. The general methodology for oral delivery of AC has been described (Bishop 1991; Hayes et al. 2003). Capture efficacy (percentage of capture attempts when all cranes in a targeted social group were successfully immobilized), sedation score (scale of 1–5 modified from Hayes et al. [2003]: 1  =  light, 2  =  light-moderate, 3  =  moderate, 4  =  moderate-deep, 5  =  deep/very deep), prevalence of EM (based on clinical signs [Businga et al. 2007] and postmortem analysis [Windingstad et al. 1983]), and overall capture-related mortality were compared before and after management changes, 1990–2001 and 2002–2011, respectively. Time in confinement (elapsed time between capture and release) controlling for sedation score, prevalence of EM, and mortality were compared between birds that received up to 30 ml/kg subcutaneous lactated Ringer's solution (LRS) at capture and those that did not. Chi-square analysis was used to assess associations among categorical variables and the Student's t-test and two-way analysis of variance were used to assess factor effects on continuous variables. Values of P<0.05 were considered statistically significant.

The cranes were captured in 132 events; a mean of 2.9 cranes were captured per event (two to four birds were targeted in each attempt). The efficacy of the technique increased from 59% (48 groups in 81 attempts) during 1990–2001 to 72% (37 groups in 51 attempts) during 2002–2011. The majority of cranes were scored as moderately to deeply sedated (Fig. 1). The mean (±SD) sedation scores of cranes captured during 1990–2001 (3.37±1.3) did not differ significantly from cranes captured 2002–2011 (3.59±1.2; P = 0.14). The proportion of cranes diagnosed with EM decreased from 7 of 188 (3.7%) to 3 of 129 (2.3%) after changes in 2002, and the overall capture-related mortality rate decreased from 9 of 188 (4.8%) to 4 of 129 (3.1%).

The overall mean total time in confinement was 16.7±7.1 hr (range 6.5–34.8 hr) but exhibited a bimodal distribution, with releases occurring either after 8–9 or 23–24 hr of recovery to avoid releases at night. Fluid administration (P<0.01) and sedation score (P<0.01) significantly affected confinement times of the cranes. Cranes provided LRS were released earlier under most sedation scores, up to 7 hr (P = 0.01; Fig. 2). Deeply sedated cranes were typically held in a soft-sided enclosure to the following morning regardless of fluid administration. No association was found between LRS administration and EM (P = 0.75) or mortality events (P = 0.96).

Carefully narrowing the weeks when AC was used to capture sandhill cranes based on the analysis of Hayes et al. (2003) was associated with modest improvement in efficacy of group capture events. We believe the improvement in capture efficacy is best explained by more predictable crane feeding behavior. Baiting in September and early October coincides more closely with natural increased feeding before migration, and in more instances, all members of a family group ingested treated corn and achieved adequate sedation levels for capture. Although we predicted sedation scores would increase as well, they did not, affirming that AC produces a range of sedation effects in cranes when delivered in an open system. The overall variability in sedation scores pre- and post-2002 did not seem to be related to staff experience: turnover and training of new staff occurred throughout the study.

Not unexpectedly, LRS administration reduced confinement times by several hours in cranes with all but the highest sedation score. Support of the circulatory system by administration of fluids is believed to improve metabolism of AC and help ameliorate potential acid-base and electrolyte imbalances in immobilized cranes, fostering more rapid recovery. Previous serial venous blood sampling of AC-captured sandhill cranes showed some cranes may experience abnormally low base excess at 8–22 hr postcapture (ranging from −9.0 mmol/L to −13.0 mmol/L, respectively) suggestive of metabolic acidosis (Langenberg et al. 1998). The confinement times of heavily sedated cranes given fluids were not diminished enough to allow release before dark, but they may have recovered more quickly as well.

Although morbidity and mortality declined with the AC management changes, poor capture outcomes can also be influenced by individual susceptibility, such as from preexisting diseases, or accidents. We documented one case of gapeworm (Cyathostoma sp.) parasitism that led to the death of a sedated bird from tracheal obstruction and at least two accidental deaths (domestic animal attack and drowning). We predicted that more consistent fluid administration might have a protective effect, particularly for reducing EM-associated morbidity and overall mortality, but our data do not support this conclusion, although the small number of cases makes statistical inferences difficult.

No negative effects were observed from the management changes. Overall, capture of cranes focused during September through early October was associated with improved efficacy and lowered morbidity and mortality, and provision of fluid therapy reduced time to recovery and the need for prolonged confinement. This method appears to be an improved alternative for the capture of small groups of sandhill cranes.

This project is the result of an International Crane Foundation (ICF) Preceptorship in Avian and Conservation Medicine awarded to L. Schneider. We thank the many organizations and individuals that have provided support for long-term Sandhill Crane research in Briggsville, Wisconsin. We also thank the staff of the ICF Field Ecology Department for assistance. All captures were conducted under an approved institutional animal care and use protocol (ICF 007). This is publication number 13 from ICF's long-term Sandhill Crane research program.