⚖️ Weight Management Math · 8 min
Fat weight, flying weight, and the percentage between
Why this matters
Falconers talk about flying weight as a percentage of fat weight because raw grams mean nothing across birds. The percentage is the language, and it is the safety margin.
By the end of this lesson you will be able to
- Calculate a target flying weight from a fat weight and a percentage
- Express a weight change as a percentage of body weight
- Explain why the same gram loss means different things on different birds
The two anchors
Fat weight (or high weight) is what the bird weighs fed up and unflown, typically at the end of the molt. Flying weight is the weight at which it responds reliably and hunts well. The gap between them is usually expressed as a percentage.
Typical working figures, and these are illustrations rather than prescriptions, since every bird is its own case:
| Bird | Fat weight | Flying weight range |
|---|---|---|
| Female Red-tailed Hawk | ~1,300 g | roughly 85 to 92% of fat weight |
| Male Red-tailed Hawk | ~1,000 g | roughly 85 to 92% of fat weight |
| Female American Kestrel | ~125 g | roughly 88 to 93% of fat weight |
Target from a percentage
A female red-tail's fat weight is 1,300 g and you intend to fly her at 88%.
1,300 × 0.88 = 1,144 g.
The general form: target = fat weight × percentage. Going the other way, percentage = current ÷ fat weight.
A loss, as a percentage
Your red-tail was 1,180 g yesterday and is 1,144 g today. How much did she drop, and is that a lot?
36 g lost. 36 ÷ 1,180 = 3.1% of body weight overnight. On a red-tail that is a big single-day move and you would look hard at yesterday's ration and last night's temperature.
Your kestrel was 112 g yesterday and is 108 g today.
4 g lost. 4 ÷ 112 = 3.6%. The same proportional move, in a bird with a fraction of the reserve. This is the whole reason the manual singles out the Kestrel as the species most sensitive to weight fluctuation, and why poor weight control is what most often kills them.
The habit to build
Never read a weight change in grams alone. Divide it by the bird's weight and read it as a percentage. A 20 g swing is nothing on a gyrfalcon, a working adjustment on a red-tail, and an emergency on a kestrel.
Coming out of the molt
The manual's guidance for a bird kept through the molt is to add about 20% to her flying weight. If she flew at 1,144 g, that puts her molt weight at 1,144 × 1.20 = about 1,373 g. Note the direction: the 20% is added to the flying weight, not subtracted from a fat weight.
From the field
Your red-tail flies well at 1,150 g. One morning she reads 1,081 g. Do you fly? Work it: 69 g below, and 69 ÷ 1,150 = 6% below her known flying weight. That is not a sharp bird, that is a bird well under where you have any data on her. Feed her up, find out what happened, and fly her tomorrow.
This lesson is part of the NY Falconry Exam course. Open it in the course to answer the retrieval checks that go with it, track what you have read, and quiz the whole unit. Lessons, quizzes and the glossary are free.