A primer to ageing owls

In addition to putting a band on each owl we catch at MSRW’s owl-banding station on Point LaBarbe, we take a variety of measurements and we try to determine the age and sex of each individual. Determining the age of a bird helps us understand population dynamics of a bird population. For example, if we catch a high number of young of the year (aka hatch-year birds) in the fall, this tells us that reproduction was good last summer.

So you may ask, how does one age an owl? We take advantage of the fact that owls have an incomplete molt, which means that during their once-a-year flight feather molt, they only shed some of their flight feathers while retaining others for up to three years. Thus, for example, a two year old owl has both new feathers and one-year-old feathers in its wing. A three year old owl has new feathers, one-year-old feathers and two-year-old feathers in its wing. The only time an owl has a complete set of new feathers is during its hatch year, when it has to grow an entire set of feathers after hatching.

The different feather ages are especially easy to see in Northern Saw-whet Owls. New flight feathers contain a chemical called porphyrin which fluoresces bright pink when we shine a UV light on the wing.

UV light set-up to determine age of saw-whet owl flight feathers

Here is a photo of of the wing of a hatch-year bird with a full complement of new feathers. All of the new feathers flouresce bright pink.

Molt pattern of a hatch-year saw-whet owl. Photo by Kevin Perozeni.

This chemical breaks down as the feather ages and thus, older feathers do not fluoresce. In the following photo, a second-year bird has two age classes of feathers – the bright pink new outer and inner feathers which it has recently replaced and the pale one-year-old feathers in the middle. This is the classic molt pattern of a second-year saw-whet. When we see this pattern, we can be confident in calling a bird two years old.

Molt pattern of a second-year saw-whet owl. Photo by Kevin Perozeni.

After the second year, there is no regular pattern of molt and any feathers could be replaced at anytime. Thus whenever we see an irregular molt pattern, we can only say that the owl is older than two years. For example, in the following individual, the outer primaries and a group of feathers in the middle have been newly replaced.

Example of a molt pattern of an after-second-year saw-whet owl. Photo by Dawn Garcia

We can also use the UV light to examine feather age in other owl species, but more typically, we look at the shape and extent of barring on particular feathers. For example, the barring on the central tail feather of the Barred Owl we caught earlier this fall tells us that this bird was more than two years old.

Tail of a Barred Owl showing barring pattern of an older bird. Photo by Kandace Glanville

Owl-banding totals September 18th through October 15th:

Northern Saw-whet Owl: 359

Long-eared Owl: 3

Barred Owl: 1

-Nancy Drilling, lead owl bander