It's late summer, and feathers are everywhere. The woods and fields look like there's been a pillow fight on a very large scale "… and the pillows are losing.
In the last two days, I have found feathers from an acorn woodpecker, house wren, western scrub jay, turkey vulture, barn owl and northern flicker, plus a couple I can't identify.
Even the male Anna's hummingbird at my feeder sounds funny. He's missing a primary in each wing, and the holes make a very loud whirring sound. I can just see the tip of the new feather beginning to grow in the blur of wings at the feeder.
For birds, August is the time of year most put on fresh clothes. The new set of feathers is usually not flashy. These are work clothes, the drab, everyday plumage suitable for travel (migration) and toughing out the long winter. Ornithologists even refer to this plumage as "basic" plumage — as opposed to the bright "alternate" plumage many birds don for the breeding season.
As soon as the young are fledged and on their own, adult birds begin to molt. The food they once fed to their young is now used to fuel the growth of a complete new set of feathers. This is quite the task. A set of feathers may weigh as much as 25 to 40 percent of the lean-dry weight of a bird. Without dwelling on what "lean-dry weight" means, it is enough to appreciate that molting is a huge undertaking for a bird. Besides the energy required, molting requires a lot of protein because feathers are made mostly of the protein keratin.
The need for protein explains why you see hummingbirds behaving rather strangely in summer. At times they rise deliberately from their perch, fly in seemingly random directions and appear to bounce off invisible walls like some character in a video game. They are plucking miniscule gnats from the air. This addition to their normal diet of nectar and sugar water provides the raw materials needed to build new feathers. I wonder how many gnats it takes to make a new set of feathers. It must take thousands.
There are two reasons why birds molt in the dog days of summer. First, it is warm, allowing birds to conserve needed energy, plus the loss of insulation comes with less of a penalty when nights are warm.
The second reason is the amount of food available in summer. If you doubt how much food is in field and forest, consider the number of uninvited visitors a relaxing moment on the back patio produces in the evening. Also, any gardener will launch into long-winded complaints of the abundance of garden pests that make it difficult to nurture roses and tomatoes. The bottom line is that energy and protein availability are high and other energy demands on birds are relatively low.
As soon as the molt is complete in late August and early September, many of our summer birds devote "surplus" energy into the rigors of migration. They are hoping for a well-earned rest in a gentler climate.
Stewart Janes is a Biology professor at Southern Oregon University. He can be reached at firstname.lastname@example.org.