Brian L. Sullivan
The adult White-crowned Sparrow's plumage varies among subspecies.
With fall migration underway, many of us encounter individual birds of the same species in various plumages, undoubtedly causing identification challenges and headaches. Some differences stem from sexual dimorphism (males and females look different) or seasonal variation (breeding vs. nonbreeding plumages), but much of the variation we see is age-related.
All birds share a portion of their lives between natal down and adulthood, the duration varying from species to species. The age at which a species first breeds generally occurs sooner for smaller birds (for example, in the first spring) and later for larger birds — in fact, many years later for some raptors and seabirds. This means that "immaturity” can last a month, several months, a year or several years. The plumages that a bird wears before reaching sexual maturity also vary (think of the variation in large gulls!), and these are what we typically encounter in the fall and winter of a bird’s first year.
All birds have a juvenile plumage (sometimes called "juvenal plumage”), acquired usually just before leaving the nest, though some patterns look more familiar to us than others. Juvenile plumages occur for a variable amount of time depending on the species’ natural history and breeding biology.
In some cases, juvenile plumages appear very briefly, perhaps for a few weeks or months, and are largely unfamiliar to birders. In other cases, juvenile plumage remains for almost a year before being replaced through a molt in the first summer, and these plumages look more familiar to us. With all this possible variation, identifications can become confusing. Let’s take a look at some examples of how birds’ plumages change throughout their first year of life.
Most songbirds, from warblers to sparrows and jays, usually have a distinctive juvenile plumage held briefly after leaving the nest. Generally drabber than that of an adult, this plumage is quickly replaced over a few weeks or months with a more durable plumage.
By the first fall, we find it a real challenge to distinguish juveniles from adults in many species, and the process often requires detailed study or in-hand examination. Other species molt into an immature plumage, different from the juvenile and adult plumages, which can be worn until the first spring or through the first summer depending on the species’ breeding biology.
Species that breed in their first spring/ summer usually molt into an adultlike plumage in the first spring. Those birds were photographed on the same day in July, yet they look very different. The drab, dusky grayish bird is a juvenile, and the brighter blue "field-guide ringer” is the adult.
The grayish plumage of a juvenile Western Scrub-Jay will be replaced during the late summer and early fall, resulting in a bird that appears mostly adultlike — and difficult to distinguish from a true adult in its first fall. These molts are often incomplete, meaning that some feathers remain from the juvenile plumage, usually wing and tail feathers, and a few upperwing coverts. The shape of songbirds’ juvenile tail feathers often looks different, averaging narrower and more tapered at the tips than those of adults.
With careful examination, birds can be aged by these differences through the first fall, winter and spring. Juvenile feathers usually appear worn and faded next to the freshly molted feathers, and these plumage contrasts, or molt limits, also help age birds.
Now consider the plumage progression of White-crowned Sparrow. Its juvenile plumage might be unfamiliar to many birders, being largely streaked below. Note the lax quality of the plumage, especially on the flanks and belly. Like the scrub-jay, the White-crowned Sparrow’s juvenile plumage appears only briefly after fledging and is quickly molted but into an intermediate plumage for the first fall/winter.
Perhaps more familiar to birders is a first-fall White-crowned Sparrow. It has replaced the streaky juvenile plumage but has brown and tan crown stripes. A fall adult, the White-crowned Sparrow is difficult to mistake with its bold plumage pattern that changes little throughout the year.
In some large birds, such as raptors, juvenile plumage occurs for a longer period. In many raptors, most or all of the juvenile plumage remains through the first spring, then is gradually replaced by molt over the course of the first summer. This simplifies matters compared with songbirds.
Describing birds with terms like "juvenile,” "immature” and "adult” can help — but might prove idealistic. These terms mostly help birders understand and interpret what they’re seeing in the field and help us communicate about a particular bird’s appearance.
In reality, the transitions between some of these plumages happen over a period of months, so remain aware that we often encounter birds during molt periods. A good resource about plumage progression and molt is "Molt in North American Birds” by Steve N.G. Howell (Houghton Mifflin Harcourt, 2010). Highly recommended, this book provides an excellent overview of how these processes differ in birds — and why we birders should strive to better understand them.
Excerpt from WildBird, November/December 2011 issue, with permission from its publisher, I-5 Publishing LLC.