Abstract

Intraabdominal radio transmitters are used preferentially over externally mounted transmitters, because they appear to have less impact on behavior, health, survival, and reproduction in ducks.^4,9 Inhalant anesthetics (isoflurane and methoxyflurane) are often used to anesthetize birds during surgery but require expensive equipment, such as a vaporizer and oxygen-delivery system.^7,9 Propofol is a new, rapidly metabolized intravenous anesthetic that provides smooth induction and recovery, excellent muscle relaxation, and short duration of anesthesia.¹⁰ Female canvasback ducks (Aythya valisineria) were assigned randomly to one of three treatments: propofol (n=39), isoflurane (n=39), and control (flushed, not captured; n=40). Rapid recovery characteristic of isoflurane anesthesia precludes placement of ducks into their natural environment prior to complete recovery. However, the use of propofol allows recovery to take place on the nest. Overall nest abandonment differed among treatment groups (χ²=6.115, 2 df, p=0.047), with lower-than-expected abandonment in control and propofol groups. No significant difference in abandonment was detected between propofol and control groups (χ²=1.22, 1 df, p=0.27), confirming that the relationship was driven by the higher-than-expected abandonment in the isoflurane group. A comparison with results from Arnold et al.,¹ where birds were captured and nasal-marked, revealed that surgery resulted in elevated abandonment (χ²=11.75, 2 df, p=0.003). Abandonment rate in the propofol group did not differ (χ²=1.12, 1 df, p=0.29) from rates reported by Arnold et al., suggesting that the former relationship was again driven by the isoflurane group. Stress at time of recovery may be responsible largely for nest abandonment. Propofol requires minimal equipment, reduces anesthetic cost, and decreases the risk of investigator-induced abandonment, making it an ideal anesthetic for field studies of waterfowl.

Introduction

Intraabdominal radio transmitters are used preferentially over externally mounted transmitters, because they appear to have less impact on behavior, health, survival, and reproduction in ducks.^4,9 Inhalant anesthetics (isoflurane and methoxyflurane) are often used to anesthetize birds during surgery but require expensive equipment, such as a vaporizer and oxygen-delivery system.^7,9 A simpler anesthetic technique, using methoxyflurane placed on a gauze, has also been employed in reducing stress and disruption of normal behavior after handling and external transmitter attachment.⁸ Methoxyflurane decreases nest abandonment after transmitter placement, but inability to control depth of anesthesia may result in complications and mortality.^5,8 Violent recoveries associated commonly with methoxyflurane anesthesia make this technique unsuitable for over-water nesting species, such as canvasback ducks.⁸ Methoxyflurane also poses a significant human health risk.³ Recovery from isoflurane anesthesia is too rapid to allow placement of ducks into their natural environment prior to complete recovery. This may cause unacceptably high abandonment when attempting to place birds on their nests following surgery, although this has not been tested. Propofol is a new, rapidly metabolized, intravenous anesthetic that provides smooth induction and recovery, excellent muscle relaxation, and short duration of anesthesia.¹⁰ Preliminary work in the laboratory suggested that recovery from propofol would allow placement of lightly anesthetized birds on their nests following surgery.⁶

Materials and Methods

In most brood survival studies, females are equipped with transmitters during mid- to late incubation to reduce nest abandonment rates, which in turn lowers (a) human-induced reproductive failure, (b) lost information, and (c) equipment costs.⁸ Therefore, female canvasbacks were captured at 15 to 18 days in a 25- to 26-day normal incubation period.² Female canvasbacks were assigned randomly to one of three treatments: propofol, isoflurane, and control (flushed from nest but not captured). Sterile dummy silicone implants weighing 15 to 18 g were surgically implanted while birds were anesthetized with either propofol (group 1, n=39) or isoflurane (group 2, n=39). Birds in the control group (group 3, n=40) were considered to be normal with minimal intervention since they were flushed only from the nest.

Propofol (group 1) was delivered through a 24-gauge IV catheter placed in the medial tarsal vein. Ducks were induced with 10 mg/kg of propofol given slowly over 1 minute. Depth of anesthesia was assessed, and additional boluses (1–2 mg) were given until the bird could be intubated with a non-cuffed endotracheal tube. Anesthesia was maintained with additional boluses (1–3 mg) given as needed, and birds were ventilated throughout the procedure with a 0.5-L pediatric self-inflating resuscitation apparatus.

Isoflurane (group 2) was delivered through a non-rebreathing system by an Isotec 3 vaporizer (Ohmeda, BOC Health Care, West Yorkshire, England). Anesthesia was induced with isoflurane starting at 1% and stepped up to 5% with an oxygen flow of 2 L/minute. Following induction, birds were intubated with a non-cuffed endotracheal tube. Anesthesia was maintained at 1.5 to 3.5% with oxygen flow rate of 1 L/minute. Birds that became apneic were ventilated with a 0.5-L rebreathing bag attached to the circuit.

Anesthetic depth was assessed by monitoring (a) heart rate using an esophageal stethoscope, (b) nictitating membrane movements, (c) swallowing or coughing, (d) response to stimuli, and (e) movement. During anesthesia and surgery, heart rate and respiratory rate were monitored. Anesthetic depth was adjusted to maintain the bird at a constant level of anesthesia. Surgery was performed as described by Olsen et al.⁷ After discontinuation of anesthesia, heart rate and respiratory rate were monitored in both groups until normal breathing resumed. Oxygen flow rate was maintained at 1 L/minute for birds receiving isoflurane, and ventilation was provided as necessary for both groups. The endotracheal tube was removed when the bird began to lift its head to cough or swallow. Respiration was monitored for a few minutes following extubation to ensure that ventilation was maintained.

At the nest, group 1 birds were given an additional bolus of propofol. Respiratory rate was monitored, and depth of anesthesia was sufficient to allow removal of the catheter. Bleeding was stopped by applying Blood Stop Powder (Dominion Veterinary Laboratories LTD, Winnipeg, Manitoba, Canada) and manual pressure. Group 1 birds were allowed to recover on the nest, while group 2 birds were released on the nest after recovery.

Monitoring for nest abandonment in all groups was accomplished by recording nest temperature every 4.6 minutes for 6 days after treatment. Temperature was recorded on a HOBO XT Temperature Logger (Onset Instruments Corp., Pocasset, MA, USA) attached to a thermistor implanted into a dummy egg. The dummy egg was anchored to a metal rod measuring at least 15 cm and placed in the center of the clutch. Nests were visited 6 days after surgery to retrieve the HOBO Temp and to determine fate of the nest (abandoned, active, or destroyed). Nests were considered abandoned if the temperature pattern indicated that a bird did not return to the nest following surgery (group 2) or flushing (group 3). In group 1, nests were considered abandoned if the bird left the nest within a 6-hour recovery period and did not return.

Logistic regression and chi-square were employed to determine whether nest abandonment varied with year, treatment, and the interaction of these two variables. Results were considered significant when p<0.05.

Results

All birds survived surgery. One mortality occurred prior to surgery using propofol during a period when ventilation and monitoring of the canvasback duck was inadvertently stopped. This bird is not included in the analysis. One canvasback duck in the control group was found dead on the nest and was likely killed by a mink (Mustela vison).

In canvasback ducks, nest abandonment occurred in all treatment groups in both years (Table 1). No difference in nest abandonment between years was found (logistic regression, p=0.55), nor was there a year by treatment interaction (p=0.83). When these terms were dropped from the model, a weak treatment effect was detected (logistic regression, p=0.06), owing to the greater abandonment (p=0.02) in the isoflurane group. These analyses were appropriate because the data fit the logistic models adequately (model goodness of fit, p values >0.8). Categorical analyses confirmed these findings; there was a significant difference among treatments in nest abandonment (χ²=6.115, 2 df, p=0.047), with propofol and control groups having lower-than-expected abandonment. Since abandonment in propofol and control groups did not differ (χ²=1.22, 1 df, p=0.27), this indicates that the former relationship between groups was driven by the higher-than-expected abandonment in the isoflurane group.

Table 1. Nest abandonment of female canvasback ducks in late incubation following random assignment to three treatment groups

Number of females (% abandoned) and group sample sizes (n) for each year and years combined (Total) are shown.

Discussion

Human disruption of nesting waterfowl may lead to lowered production and survival of ducks, decreased sample size, and loss of transmitters. Time and stress associated with handling, capture, transport, surgery, and release can interfere with normal reproductive behavior and lead to nest abandonment.⁹ This study suggests that anesthesia at time of release reduces nest abandonment after surgery. Waterfowl anesthetized with isoflurane are completely aware at time of release, whereas birds anesthetized with propofol are allowed to recover in their natural environment without human disturbance. A study by Arnold et al.¹ reported a 9.9% nest abandonment rate (39 abandoned; 353 did not abandon) in female canvasback ducks following capture and marking. A comparison of nest abandonment in the two surgery groups in this study with results of Arnold et al. revealed that surgery produced higher abandonment rates (χ²=11.75, 2 df, p=0.003). However, when results for the propofol group were compared with findings of Arnold et al., no significant difference in nest abandonment (χ²=1.12, 1 df, p=0.29) was found, suggesting that the former relationship was driven primarily by the isoflurane group. Stress at time of recovery in the isoflurane group may be responsible largely for the higher nest abandonment rate.

Although nest abandonment in the propofol group (15.4%) wasn’t significantly different from results of Arnold et al. (9.9%),¹ it is unwise to conclude that surgery and anesthesia had no effect. Further investigation with larger numbers of females in treatment groups is required to resolve this issue. Since propofol provides little intra- and postoperative analgesia, supplemental analgesia is required for surgical procedures.¹⁰ In this study, bupivacaine was administered prior to surgery to provide both intra- and postoperative pain control. The effectiveness of bupivacaine to provide analgesia is undetermined in avian species, and further investigation is required.

Acknowledgments

Funding for this project was provided by Delta Waterfowl and Wetlands Research Station, Ducks Unlimited Institute for Wetland and Waterfowl Research, and Western College of Veterinary Medicine Wildlife Health Fund (University of Saskatchewan). Support in kind was provided by the Canadian Wildlife Service and Malinkrodt Groups Inc. I would like to thank Robert Brua, Robert Clark, Marnie Cooper, Nigel Caulkett, Shannon Lind, and Margaret Yole for their assistance and advice on this project. I am also grateful to the numerous field assistants who located the nests and provided technical support during this study.

Literature Cited

1.  Arnold, T. W., M. G. Anderson, R. B. Emery, M. D. Sorenson, and C. N. De-Sobrino. 1995. The effects of late-incubation body mass on reproductive success and survival of canvasbacks and redheads. Condor. 97:953–962.

2.  Bellrose, F. C. 1976. Ducks, Geese and Swans of North America. Stackpote Books: Harrisburg, Pennsylvania.

3.  Best, J. L. and C. J. McGrath. 1977. Trace anesthetic gases: an overview. J. Am. Vet. Med. Assoc. 171:1268–1269.

4.  Greenwood, R. J. and A. B. Sargeant. 1973. Influence of radio packs on captive mallard and blue-winged teal. J. Wildl. Manage. 37:3–9.

5.  Lacki, M. J., B. N. Smith, W. T. Peneston, and F. D. Vogt. 1989. Use of methoxyflurane to surgically implant transmitters in muskrats. J. Wildl. Manage. 53:331–333.

6.  Machin, K. L. 1997. Determination of the efficacy of anesthetic agents on captive and free ranging ducks. MSc Thesis, University of Saskatchewan, Saskatoon. Pp 49–87.

7.  Olsen, G. E., F. J. Dein, G. M. Haramis, and D. G. Jorde. 1992. Implanting radio transmitters in wintering canvasbacks. J. Wildl. Manage. 56:325–328.

8.  Rotella, J. J. and J. T. Ratti. 1990. Use of methoxyflurane to reduce nest abandonment of mallards. J. Wildl. Manage. 54:627–628.

9.  Rotella, J. J., D. W T. P. Sandowski, and J. J. DeVries. 1993. Nesting effort by wild mallards with 3 types of radio transmitters. J. Wildl. Manage. 57:690–695.

10.  Sebel, P. S. and J. D. Lowdon. 1989. Propofol: a new intravenous anesthetic. Anesth. 71:260–277.