It should come as no surprise when beginning birders lament that "the bird did not look like anything in the field guide." The amount of variation in a bird due to age, gender, or feather wear, or even where they live is incredible, and impossible to show completely in any one field guide. Then throw in the mix the fact that most birds change every feather at least once a year — some twice a year — and some take several years to finally look like they "should."
Plumage can vary because of gender, and birds that show gender differences are said to be sexually dimorphic. The male and female Williamson's Sapsucker are so radically different, that they were once thought to be separate species. This dimorphism is common in many birds, and is very evident in the ducks. The differences between male and female ducks can be greater than the differences between species.
Fox Sparrow
|
Fox Sparrow
|
Age is another common variation factor. In a flock of adult White-crowned Sparrows, only the birds themselves can tell each other apart sexually. We cannot, but the young birds look drastically different from the adults. Many birds also show geographic differences in plumage throughout their range, and some not very subtly, like the Fox Sparrow or the Dark-eyed Junco. The junco complex was formerly four distinct species, with four distinct plumages.
Dark-eyed Juncos
|
|
Some birds show different morphs not associated with sex or age. The light versus dark phase of Red-tailed Hawks is a good example. To make matters worse, there often exists intermediate forms between the two.
Molt is a complex and poorly understood phenomenon among casual birders . . . and many expert birders for that matter, but understanding molt cycles is a worthy study that can help you in the field.
Old feathers can be extremely worn and tattered, and must be replaced for the vital roles of flight and insulation, and usually takes place so that it does not coincide with other energy expensive events such as breeding or migration.
Most birds will lose and replace every feather once a year just after breeding. This is called a pre-basic molt and will leave the bird in basic plumage. The term basic is more acceptable now than the old terms of winter or non-breeding plumages. Chickadees and hawks, hummingbirds, swallows and thrushes, to name but a few groups of birds, will undergo only this one pre-basic molt, and their plumage will look unchanged throughout the entire year. They are always in basic plumage.
It may take several years of this pre-basic molt to reach the adult or definitive plumage. Gulls, for example, reach this definitive plumage in three or four years, and in eagles it can take up to five years.
Many passerine birds — buntings, tanagers and warblers for example — also have a partial molt of some body and flight feathers typically between January and April. This is the pre-alternate molt, and will leave the bird in alternate plumage, more accepted now than the term breeding plumage. You can watch this happen this time of year in our part of the world to the Yellow-rumped Warblers, who are putting on their alternate finery before they fly north to their breeding grounds.
The extent of these molts varies among species, and of course, there are variations on typical molting sequences. To use the ducks as an example again, the males undergo a partial molt each Fall into a female-like eclipse plumage. This coincides with the simultaneous loss of the flight feathers, and no doubt gives them some camouflage in this vulnerable time.
I've touched very briefly on the complexities of molt patterns. Different species will molt different feathers at different times and in different sequences. There are oddities like the Bobolink, that have two complete molts each year, and their fresh alternate plumage leaves them looking buffy colored, until feather wear exposes their striking black and white patterns. I'll leave it to the reader to research as they will.
I've said this before and it bears repeating. Look at every bird you see, even if it's not a "new" bird for you, and pay attention to variations in plumages among birds of the same species. It can only make you a better birder.
