You may have noticed that as the seasons change, birds begin to look different. What was a bright eastern goldfinch in the spring becomes a duller hue in the winter. A duck that had vibrant breeding feathers earlier in the year now looks a little drab.
That is because birds molt. Their feathers wear out due to the natural elements and bacteria and have to be replaced regularly or the birds wouldn't survive.
We wouldn't wear the same clothes every day, would we? Those clothes would wear out. Well, the same things happens to birds' feathers.
Molt is the growth of new feathers, not necessarily just the loss of old feathers. When birds get their first feathers it is also called molt, even though they don't lose any feathers in the process. The loss of feathers occurs when the new feathers that are growing push the old feathers out.
Molting timetables are different for every bird species and usually occur during times when food supply is at its greatest since most birds will be unable to fly for about two weeks as they molt. All birds molt annually, but some also molt parts of their body feathers more than that. Only two North American bird species have been found to undergo two complete molts per year --- Franklin's gull and the bobolink.
What stimulates molt is quite unknown to scientists and can have to do with bird hormones following breeding season or even a decrease in daylight length as summer ends and fall begins. Birds wouldn't molt their main wing feathers during migration because it could be harmful to their travels, but whether they molt before or after migration depends. According to "Molt in North American Birds" by Steven N.G. Howell, birds that migrate a short distance tend to molt at their breeding grounds before heading south, and those with a longer distance to travel will wait until they arrive in their overwintering areas. However, some do buck the trend and molt at a certain site during migration where they expect ample food will be.
You may not have ever seen a bird in molt, because those who can't fly or can't fly well, feel vulnerable and will keep to themselves during this time. You may have seen a bird in molt and not even realized it though.
There are two keys to identifying a bird in molt. The first is by gaps in wing feathers, which can be seen well when a bird is in flight. Another way is by the differences between feather colors. A bird in active molt may have clean and crisp feathers that contrast with drab, dirty, old feathers.
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