In vacuuming the house in recent weeks we have noticed an increase in cat hair as our two healthy and happy indoor-only cats apparently shed their heavier winter coats for cooler summer ones.

In cats, as in all but a few mammals, there is little change in appearance between seasons. The exceptions that we know of include some northern mammals like weasels and snowshoe hares, that go from brown in summer to white in winter. In a great many bird species, on the other hand, there is a major difference in appearance as feathers are replaced as the birds go from immature to adult stages and then change seasonally as adults.

This process is called molt, and most of us may not give it much thought. One of the easy-to-see bird species in our area in which it is often easy to see the progression of molt is in the American goldfinch.

In the fall, male American goldfinches lose their bright yellow feathers and replace them with the same drabber, olive-green color worn by females and immatures. Presumably this makes them a little less vulnerable to predators than if they sported their bright yellow plumage through the winter months.

Like many people with backyard bird feeders, we have been graced with American goldfinches all winter, and so we have been able to watch day by day through April as patches of bright yellow feathers replaced the drabber ones of the males.

Herring gulls and great black-backed gulls replace only the head and body feathers in the late winter and early spring, but replace all the feathers, including the sturdy wing and tail feathers, in late summer and fall. In both of these species there is a four-year progression of molts.

Birds in their first year are very brown and mottled, each year gaining more adult-like back and wing feathers and whiter underparts and head. By the fourth year they look like the classic adult gulls that most people know so well, with gray backs in herring gull or black in great black-backed gull, and white heads, tails, and underparts.

In late spring and summer it is not uncommon to see young gulls with very bleached-out and ragged wing feathers as they approach the time when they will molt and replace them for the upcoming year.

Ducks, loons, grebes and a few other waterbirds have a very interesting molt strategy in that many species molt all the major wing feathers (called the “primaries”) at the same time. Without these wing feathers they can’t fly! This means that they have to be in a relatively safe place on water where they can swim, dive, or hide in vegetation to evade predators. The feathers grow in quite rapidly so that the birds are able to gain more aerial mobility within weeks. During this flightless period, male ducks also molt out of their fancy bright breeding plumage into the more cryptic browns and beiges of the females.

Male common eiders often move to offshore ledges in summer, where they may be less susceptible to predation as they go through this process. Next time you look in your closet for a change of clothes, consider the different suits worn by birds!