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Summer is wardrobe time for birds

ebird.orgLike tree rings, the banding on a feather records its growth, but instead of seasonal growth it records the daily cycle of growth.

It’s that time of year again. As birds wrap up parental duties, they go shopping for new clothes.

Mid-June into early September is the time to molt for most small birds. While many undergo a limited molt in late winter to acquire bright breeding plumage, they molt no more feathers than necessary to look sharp. The summer molt or “basic” molt, in contrast, involves all feathers, including the costly wing and tail feathers. Unlike the limited late winter molt when food is also limited, the food needed to power a complete molt in summer is relatively abundant, and nights are warm enough that missing some insulating feathers does not risk hypothermia.

Yesterday I found a molted tail feather of a scrub jay in the backyard. The sky blue color was striking as always, and the exquisite fine structure of the barbs and the invisible barbules that hold the feather together always impress me.

As I examined the detail of the feather, the sunlight struck the feather just right. I could see subtle banding oriented perpendicular to the shaft and extending the length of the feather. The pattern is easier to see on the dull color on the back of a feather. It can be very faint and difficult to see on many. This banding has nothing to do with color pattern. It’s more like the tree rings you can see on a freshly cut stump. A tree ring records alternating spring and summer growth each year. The light portion of a tree ring represents rapid spring growth. Summer growth is less, producing the narrower, denser and darker portion of a ring.

Like tree rings, the banding on a feather records its growth, but instead of seasonal growth it records the daily cycle of growth. During the day when the bird is packing away food, the blood carries a rich supply of the building materials to construct a feather. At night when sleeping, the raw materials are somewhat depleted. During the day, the growth of the feather is robust, while at night feather growth produces slightly thinner and weaker growth resulting in alternating bands.

During harsh weather when a molting bird cannot forage as long or as efficiently, the thinner and weaker growth is more obvious and may produce what are called “fault bars,” lines of obvious weakness to the point that portions of the feather may break off. This can be particularly apparent in birds of prey if a molting bird has an unsuccessful day or two hunting, or in their nestlings if the parents are unable provide sufficient prey.

I have often been asked how long it takes to grow a new feather. I’m sure it is known, but I haven’t seen it written. So, when I find a feather and can see the banding, I start counting. I was able to count 25 growth bands on the jay feather I held, indicating it takes about a month to grow a tail feather. It probably takes a similar amount of time to grow flight feathers on the wing. These feathers are subject to considerable stress and need to be constructed well. I suspect body feathers and down grow much more quickly.

Stewart Janes is a retired biology professor at Southern Oregon University. He can be reached at janes@sou.edu.