Wild animals can expertly camouflage illness or injury. Wild animals can also become underweight or obese. A scale reading is the closest we can come to asking, "how are you feeling today?" so we weigh our raptors daily (or as often as we can) to track their health. But how do we determine their target weight?
When I started at the Museum in July of 2016, Carson the Red-tailed Hawk weighed 1,550 grams (3.4 pounds). I looked back a few months and saw she reached over 2,000 grams (4.4 pounds) in December. I wasn't too concerned about the seasonal variation in weight since birds tend to put on extra fat during winter. But on their own, the numbers are meaningless. I had to figure out if this was a healthy weight range for Carson by first looking to the species' natural history.
According to the Cornell Lab of Ornithology, female Red-tailed Hawks weigh from 900 to 1,460 grams. Carson was well over that average range - the first clue that her weight might be too high. But every bird is an individual. Maybe she just had "big bones."
A few months later we made the trek to St. Paul for an appointment with veterinarians at The Raptor Center. In addition to an overall health check, they took x-ray images to get a view underneath the feathers. I stood in the dark room carefully studying the computer screen as the vet pointed out organs, joints, and where her left wing had broken and healed incorrectly. The vet flipped between views, zooming in and out, then chuckled. "You can see her butt cheeks," she said, pointing to the fat around Carson's tail. That was our second clue that Carson was a little overweight. If the vets had seen her the previous winter at 2,000 grams, they likely would have classified her as obese.
We also interpret Carson's behavior to gauge her target weight. If Carson leaps on food before I have the chance to take my hand away, she's feeling a little too hungry and I'll give her extra food the next day. If, on the other hand, she is slow to take food and is not interested in training, she probably isn't too hungry. That's how the phrase "fed up" originated in falconry: a bird with a full belly doesn't have the motivation to hunt. In this case I would reduce her food.
Goal weights are always a moving target. Between field guide references, physical exams, and behavior observations, we are able to keep Carson at a healthy weight where she feels well-fed but is still excited to work for her food.
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