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The effect of manual restraint on physiological parameters in barred owls (Strix varia).

Abstract: Manual restraint is commonly necessary when working with avian species in medical, laboratory, and field settings. Despite their prevalence, little is known about the stress response in raptorial bird species. To further understand the effect of restraint on the stress response in birds of prey, 12 barred owls (Strix varia) were manually restrained for 15 minutes. Physiological parameters (cloacal temperature, respiratory rate, heart rate) were followed over time and recorded at defined points during the restraint period. Heart rate decreased significantly over the restraint period by a mean [+ or -] SD of -73 [+ or -] 46 beats/min. Respiratory rate also decreased significantly (median: -11 breaths/min, interquartile range: -8 to -18). Cloacal temperature increased significantly over time in manually restrained owls (median: +1.5[degrees]C [+2.7[degrees]F], interquartile range: 1.3[degrees]C-2.1[degrees]C [2.3[degrees]F-3.8[degrees]F]). This study is the first to document stress hyperthermia in an owl species. Similar to another raptorial bird, the red-tailed hawk (Buteo jamaicensis), both heart rate and respiratory rate decreased and cloacal temperature increased over time in restrained barred owls. Barred owls appear to cope differently to restraint stress when compared to psittacine species.

Key words: stress-induced hyperthermia, handling, bird of prey, raptor, avian, barred owl, Strix varia


Manual restraint is a commonplace technique used when working with avian species, whether in a clinical or research setting. Many routine procedures, including physical examinations, injections, blood collection, gavage feeding, banding, wing clipping, and talon trimming require restraint. Chemical restraint in the form of sedation or anesthesia is often required for more invasive procedures and may result in alleviation of stress associated with manual restraint in birds. (1,2) Procedural sedation is often not utilized for quick tasks due to the time needed for induction and recovery. However, handling and restraint-induced stress in birds remains a constant concern. This stress response has the potential to result in serious adverse effects, especially in diseased animals, and has been likened to predation. (1,3,4)

The avian stress response is multifactorial, with rapid changes in both the nervous and endocrine systems resulting in secondary changes to physiological parameters. (4) Activation of the sympathetic nervous system results in immediate changes in heart rate and respiratory rate. (4,5) Conversely, the adrenocortical response plays more of a role in exposure to long-term stressors, and can result in adverse physiological changes, including suppression of the immune system. (4,5)

Heart and respiratory rates are parameters that are easy to monitor during routine restraint, and may provide a window into an animal's level of stress. While the exact mechanism remains unclear, core temperature has also been shown to increase during restraint in many avian, mammalian, and poikilothermic species. (6-12) Monitoring cloacal temperature, in addition to heart rate and respiratory rate can help gauge a bird's tolerance to restraint in order to prevent imposing undue stress during a procedure.

Birds of prey are often kept in zoological collections, used as educational animals, and encountered in rehabilitation or field research settings. Notwithstanding this prevalence, little is known about the stress response in raptorial birds. Manual restraint resulted in a significant increase in cloacal temperature in red-tailed hawks (Buteo jamaicensis), and was demonstrated to cause a tachycardia in both red-shouldered hawks (Buteo lineatus) and a ferruginous hawk (Buteo regalis). (13-15) However, differences in the physiologic stress Response appear to exist between species. In contrast to Amazona parrot species, manually restrained red-tailed hawks showed a decrease in respiratory rate over time and plateau of cloacal temperature. (1,13,16)

To the authors' knowledge, little is known about the stress response in birds of prey of the order Strigiformes. Most owl species are nocturnal, with unique anatomic features to facilitate this lifestyle. It is unknown whether owls react differently to manual restraint when compared to other bird of prey orders.

The objective of this study was to assess the effect of manual restraint on physiological variables, including heart rate, respiratory rate, and body temperature in barred owls (Strix varia), a commonly encountered owl species.

Materials and Methods


Adult barred owls (n = 12) of unknown age and gender, with mean [+ or -] SD body weight of 0.8 [+ or -] 0.07 kg (range 0.68-0.9 kg) were used in this experiment. The owls in this study were free-ranging animals undergoing rehabilitation at wildlife centers for orthopedic injuries, and were otherwise healthy. Only owls that were considered to be clinically normal, and at the end of the rehabilitation process, were included. Physical examinations and fecal parasite tests were performed on all birds, and they were considered to be in good health for the duration of the experiment. Animals were housed at their respective institutions and were fed diets of frozen-thawed mice. Animals were transported to the University of Wisconsin, School of Veterinary Medicine, on the day of each experiment. All procedures were approved by the Institutional Animal Care and Use Committee of the School of Veterinary Medicine, University of Wisconsin-Madison.

Study design

On experiment days, birds were transported in pet carriers to the University of Wisconsin-Madison. All birds were allowed a minimum of 60 minutes to acclimate prior to restraint. Each bird was then captured by hand using falconry gloves and then manually restrained within a towel. The towel covered only the dorsum of the animal, holding the wings in position against the body. Birds were held upright, with their bodies at a 45[degrees] angle with respect to the floor of the room (Fig 1). The legs of each owl were restrained at the level of the tibiotarsal-tarsometatarsal joints to prevent animal and handler injury. None of the birds had been habituated to restraint prior to the experimental trials.

A temperature probe was inserted into the cloaca 30 seconds after capture and the first temperature reading was recorded at 1 minute postcapture. Cloacal temperature, respiratory rate, and heart rate were recorded every 3 minutes for the duration of the trial, similar to other studies. (1,13,16) The restraint period lasted a total of 15 minutes, starting with the point of capture.

Data collection

Heart rate was determined by stethoscope auscultation over the pectoral muscles and respiratory rate was calculated by observation of keel excursions. Cloacal temperature was recorded in degrees Celsius with a digital thermometer (Fisher Scientific, Pittsburgh, PA, USA) traceable to the National Institute of Standards and Technology attached to a stainless steel, 3.2-mm-diameter, pediatric rectal probe (YSI 406; YSI Incorporated, Yellow Springs, OH, USA). The probe was coated with room-temperature lubricating gel and placed in the cloaca to a depth of 2 cm; temperature was recorded 30 seconds after inserting the probe. The stainless probe was disinfected after each experimental trial. The same individual measured and recorded all physiologic variables (G.A.D.).

Statistical analysis

A commercial statistical software package (SigmaPlot, version 12.5, Access Softek, Berkeley, CA, USA) was used to analyze the data. The data were tested for normality with a Shapiro-Wilk W test and for constant variance with the Brown-Forsythe test. Data that were not normally distributed or equally distributed were ranked prior to further analysis. A 1-way repeated measures analysis of variance was used with fixed effects of treatment group, time, and all associated interactions. A Tukey test was used for post-hoc, pairwise, multi-comparison procedures. Data were reported in mean [+ or -] SD or median with interquartile range (IQR) unless otherwise specified. Values of P < 0.05 were considered statistically significant.


Heart rate decreased significantly over time (Fig 2) by a mean [+ or -] SD of 73 [+ or -] 46 beats/min. Mean heart rate values from 6 to 15 minutes of the restraint period were significantly lower than at the 1-minute time point. Heart rates between 6 and 15 minutes were not significantly different between time points. Mean [+ or -] SD heart rates at the 1minute and 15-minute time points were 290 [+ or -] 20 beats/min and 217 [+ or -] 34 beats/min, respectively.

Respiratory rate decreased significantly (Fig 3) during the 15-minute restraint period (median [IQR]: -11 breaths/min [-8 to -18]. Between the 9- and 15-minute time points, respiratory rate was significantly lower when compared to the 1-minute mark. Median (IQR) respiratory rate at the 1minute mark was 39 breaths/min (33-42). Median (IQR) rates at the 9-, 12-, and 15-minute time points were 30 (24-30) breaths/min, 30 (25-30) breaths/min, and 24 (24-29) breaths/min, respectively.

Cloacal temperature increased significantly during the 15 minutes of manual restraint (Fig 4) by a median of+1.5[degrees]C (+2.7[degrees]F), with an IQR of 1.3[degrees]C-2.1[degrees]C (2.3[degrees]F-3.8[degrees]F). Median temperatures between 6 to 15 minutes were significantly higher than values at 1 minute of restraint. The median cloacal temperature at the 1-minute time point was 38.7[degrees]C (101.6[degrees]F)(IQR: 38.4[degrees]C-39.0[degrees]C [101.1[degrees]F-102.2[degrees]F]). Median values at the 6- and 15-minute time points were 40.3[degrees]C (104.5[degrees]F)(IQR: 39.9[degrees]C-40.5[degrees]C [103.8[degrees]F-104.9[degrees]F]) and 40.5[degrees]C (104.9[degrees]F)(IQR: 40.1[degrees]C-0.9[degrees]C [104.1[degrees]F-105.6[degrees]F]), respectively.


The findings of this study showed that manual restraint in barred owls results in significant changes in heart rate, respiratory rate, and cloacal temperature over time.

The decrease in heart rate over time in this group of barred owls most likely represents a return to the baseline or resting rate values present prior to capture. It is well documented that restraint or handling results in tachycardia in many bird species, including members of the orders Anseriformes, Sphenisciformes, and Galliformes. (6,7,17,18) Reports of tachycardia secondary to human disturbance have also been reported in red-tailed hawks, red-shouldered hawks, and a ferruginous hawk. (13-15) Similar to red-tailed hawks, heart rate dropped significantly over the restraint period in this group of owls. (13) This is in contrast to reports in other species. A manually restrained red-shouldered hawk showed persistent, marked tachycardia (up to 600 beats/min) during a 25-minute period. (15) In 2 species of Amazon parrots restrained over a 15-minute period, heart rate did not change significantly over time. (1,16) Heart rate in restrained chickens decreased over an 8-minute period, but the statistical significance was not evaluated. (19)

Although several studies have evaluated the effects of restraint or handling on heart rate and body temperature in birds, little information exists regarding its effect on avian respiratory rate. Restrained parrots (Amazona species) showed a significant increase in respiratory rate over 15 minutes. In red-tailed hawks, respiratory rate decreased over time during restraint, similar to the owls in this study. (13) The decrease noted in these 2 bird of prey species may equate to the normal return to predisturbance respiratory rates. This is in contrast to manually restrained psittacine birds, however, where respiratory rate continued to significantly increase over time. (1,16)

Restraint in this group of barred owls resulted in a significant increase in cloacal temperature. Hyperthermia secondary to restraint or "stress-induced hyperthermia" has been well documented in several bird species, including common eider (Somateria mollissima), Pekin ducks (Anas play-trhynchos), great tits (Parus major), pigeons (Columba livia), chickens, and 2 Amazon parrot species. (1,6,7,10-12,16) However, stress-induced hyper-thermia is poorly documented in bird of prey species. To date, stress hyperthermia has been documented in only one other bird of prey species, the red-tailed hawk. (13) Similar to red-tailed hawks, cloacal temperature in barred owls increased over time during restraint, but did not continue to significantly increase over the full 15-minute restraint period. (13) In 2 species of psittacine birds, cloacal temperature continued to increase significantly over time during 15 minutes of restraint. (1,16)

Median temperature increase (+1.5[degrees]C [+2.7[degrees]F]) in this group of owls was lower than reported mean values in restrained red-tailed hawks (+2.1[degrees]C [+3.7[degrees]F]), eider ducks (+2.0[degrees]C [+3.6[degrees]F]), and Amazon parrots (+2.3[degrees]C [+4.1[degrees]F]), but higher than mean increases seen in handled pigeons (+0.7[degrees]C [+1.2[degrees]F]), chickens (+0.5[degrees]C [40.9[degrees]F]), and Pekin ducks (+0.5[degrees]C [+0.9[degrees]F]). (7,11,12,16,20)

Direct comparisons of the results from this study with the majority of published research examining the effects of avian restraint are difficult due to differing methodology and species. The manual restraint period in this project was set at 15 minutes, in order to facilitate direct comparisons with a handful of other studies in red-tailed hawks and Amazon parrots. (1,13,16) When comparing changes in physiologic variables over time in restrained birds of prey (barred owls and red-tailed hawks) with the fluctuations noted in Amazon parrot species, obvious differences exist. In both the Hispaniolan Amazon parrot (Amazona ventralis) and the blue-fronted Amazon parrot (Amazona aestiva), both respiratory rate and cloacal temperature continued to rise significantly over time, which is the opposite of the observed changes in red-tailed hawks and the owls in this study. (1,13,16) Another distinction is in heart rate during restraint. In both barred owls and red-tailed hawks, heart rate significantly decreased over time during manual restraint whereas this parameter did not change significantly in restrained Amazona species. (1,13,16) Species-specific variation in the modulation of the stress response may explain the differences seen between birds of prey and psittacine birds subjected to restraint. In chickens, it has been hypothesized that some birds may be able to adapt their responses to stressful stimuli. (19) In one study, chickens with feather-pecking behavior had higher heart rates during restraint than those with fewer pecking tendencies. (19) It was suggested that this latter group of birds was able to respond to the manual restraint passively, which is marked by a parasympathetic effect on the cardiovascular system, rather than actively, which relies on strong sympathetic action. A similar phenomenon may have occurred with the barred owls in this experiment, which may explain the similarities with red-tailed hawks and differences with Amazon parrot species. Further research is needed to investigate species-specific differences in the avian stress response.

Some limitations exist within this study. Restrained owls were not manipulated other than for the intermittent recording of physiologic variables. It is highly possible that significant auditory or physical stimuli during the manual restraint period could have led to different results. Also, the gender of the owls was not determined, which could also have influenced the outcome of this study.


The goal of this study was to determine if manual restraint in barred owls resulted in similar physiological changes to those seen in other bird species. The results of this research show that restrained barred owls showed a significant decrease in heart rate and respiratory rate over time, with a concurrent increase in cloacal temperature. These data provide the first evidence of stress-induced hyperthermia in an owl species.

Grayson A. Doss, DVM, and Christoph Mans, Dr med vet, Dipl ACZM

From the Department of Surgical Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, 2015 Linden Drive, Madison, WI 53706, USA.


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Caption: Figure 1. Photograph of a manually restrained barred owl; note the temperature probe placed within the cloaca.

Caption: Figure 2. Mean ([+ or -] standard error of the mean) heart rate of 12 manually restrained barred owls. Heart rate decreased significantly over time. * Value differs significantly (P < 0.05) from initial measurement at 1 minute of manual restraint.

Caption: Figure 3. Median ([+ or -] 25th and 75th percentiles) respiratory rate of 12 manually restrained barred owls. Respiratory rate decreased significantly over time. * Value differs significantly (P < 0.05) from initial measurement at 1 minute of manual restraint.

Caption: Figure 4. Median ([+ or -] 25th and 75th percentiles) cloacal temperature of 12 manually restrained barred owls. Cloacal temperature increased significantly over time. * Value differs significantly (P < 0.05) from initial measurement at 1 minute of manual restraint.
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Title Annotation:Original Study
Author:Doss, Grayson A.; Mans, Christoph
Publication:Journal of Avian Medicine and Surgery
Article Type:Report
Date:Mar 1, 2017
Previous Article:Erratum.
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