Superbowl

Birds have abnormally high risk of death related to anesthesia.

Reported mortality rates within 48 hours of anesthesia are relatively high in avian species (1.76%–3.95%) compared to feline (0.24%), canine (0.17%), and human (0.005%–0.02%) patients. One retrospective case series assessing outcomes following inhalation anesthesia in birds demonstrated an anesthesia-associated mortality rate of 7.7%.

(Source)

I'm not a vet, so I don't know exactly why that is, but there are a few things that I can hypothesize.

Even most big birds just look big. Flighted birds need to use their own power to stay off the ground. Like a rocket, every gram counts. Owls are about 40-60% feathers, so what you are seeing is about half air by volume. Which such low actual mass, measuring anesthesia is many times more critical to avoid death.

Wild animal medicine is new. Before the 1970s, people did not do this kind of stuff. Our founder at the rescue just did this stuff "for fun" but started picking up jobs from the State Game Commission because people didn't really have wild animal rescue as a career. We don't know how all forms of treatments affect different species, and combined with birds being non-mammalian animals, we really don't have great medical info on all of them and how to treat them without harming them all the time.

Animals also can't tell us their symptoms or medical histories. This chart discusses the anesthesia risk levels that my first link above talks about here:

A total of 1820 anesthetic records were included over a 3-year period. A total of 81 patients (4.45%) died during anesthesia or within 48 hours of cessation of anesthesia. Patients assigned an ASA grade of 1 had a mortality rate of 0% (95% confidence interval [CI] 0.0–0.2), and patients assigned an ASA grade of 2 had a mortality rate of 0.6% (CI 0.2–1.3%). Patients assigned ASA grades 3, 4, and 5 had mortality rates of 5.9% (CI 4.3–8.0), 18.8% (CI 13.4–25.7) and 50.0% (CI 29.9–70.1) respectively. Patients assigned an ASA grade of 3 were found to have 11.5 times the odds of a mortality event (95% CI 5.0–31.8) compared to ASA grades 1 and 2. A further increase in odds of 40.0 times (CI 16.9–113.8) was identified in patients assigned an ASA grade of 4, and 185.2 (CI 57.6–668.1) identified in patients assigned an ASA grade of 5. Age, sex and weight were shown to have no statistically significant impact on odds of death.

Without knowing what underlying conditions may exist, it's hard to judge the dangers of anesthesia. I haven't handled too many raptors yet, just a Screech Owl and Cooper's Hawk, but as much a fit as they throw when you try to grab them, they tend to calm down once they see you're not going to eat them, and they stay rather still in confusion more than anything. It's pretty obvious at that point they won't "win" a fight and their best bet is to keep from getting hurt, which is coincidentally our goal. We want to keep beaks and talons from cutting us, and we want to ensure their eyes, feathers, and brittle bones don't get damaged.

So for a procedure like this, as long as they aren't having the animal freak out and risking it being hurt, the risk of anesthesia complications is much higher than the risks of maintaining physical control over the bird.

I acknowledge this may be a much bigger answer than you wanted or needed, but I felt just saying "nah, that's dangerous" wasn't really satisfactory. 😄

Plus I think it is all really cool info, and I try to inspire you guys to see this stuff as very important work that needs your support!