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...an immature is not always a juvenile.
When talking about the age of a bird, the terms juvenile and immature are not interchangeable. Strictly speaking, one should only refer to a bird as a juvenile during the period when it wears its first complete set of feathers. Once a bird begins to replace feathers from the original set, it is no longer a juvenile. Depending on the extent of the first molt, the subsequent plumage aspects may be referred to as formative, first basic, or, in the case of species that require several years to reach a definitive adult plumage (i.e. gulls, terns, raptors), first-cycle.
For most species, the juvenile plumage is very short-lived. This is particularly true of songbirds. Many migrant songbirds hatch, grow a set of feathers, fledge, and then replace a majority of the original feather set before they ever leave the nesting grounds. For many such species, wing and tail feathers are the only elements of the juvenile plumage that are retained. It is simply too taxing on the bird to replace the longer, more durable flight and tail feathers so soon after growing the first set.
If you see a hatch-year thrush, warbler, or sparrow in July and then see that same bird in September, its appearance will have changed dramatically. Think of what an American Robin, Yellow-rumped Warbler or Dark-eyed Junco looks like when it is still being fed by an adult. By the first of October, you won't be seeing spots on the breast of the robin and the uniform streaking that marked the underparts of the warbler and the junco in July will be long gone.
The convenient thing about songbirds is that they tend to transition from juvenile plumage into an appearance that is very much like that of an adult, or at least a winter adult. Unless you understand how to use molt limits, visually separating many six-month old songbirds from their parents will be difficult. However, some species exhibit intermediate aspects that are distinctly different from both the juvenile and adult plumages. Birds in these plumages should not be referred to as juveniles. It is best and most accurate to describe such a bird as an immature or, perhaps, a sub-adult.
The term juvenile is most often misapplied to gulls, eagles and hawks. These groups of species take multiple years to acquire definitive adult plumages. Unlike the majority of songbirds, they will go through a series of distinctive plumage cycles all of which might be considered to be immature or sub-adult. Three-year and four-year, white-headed gulls offer probably the best known example of plumage cycles.
When white-headed gull species hatch, most grow a set of feathers that is mostly brown or gray-brown, rather uniform, and spangled or mottled. Sometime during the fall or early winter of their hatch year, they will undergo a preformative molt. The most obvious feather replacement will be on the back/mantle, head, and breast. The juvenile mantle feathers, which have dark dusky-brown or gray-brown centers and pale buffy or grayish edges, will be replaced by medium to pale gray feathers that are uniform in color with little if any interior pattern. By the end of the first winter, the heads of birds in this group will be extensively white with some darker streaks and smudges rather than almost entirely brown or gray-brown, as it was in juvenile plumage. Finally, the breast, which was densely streaked and looked mostly dark in juvenile plumage, will end up mostly white or off-white with more limited to non-existent streaking.
Despite the extensive alteration of the head, back, and body pattern, the wings will continue to look brown and mottled because the flight feathers have not been replaced during the preformative molt. Over the next 8-9 months, the first set of flight feathers will usually become worn and in some cases heavily sun-bleached. They will look old and tattered compared to the newer feathers acquired during the preformative molt.
Another groups of birds with distinctive juvenile plumages are shorebirds. Like gulls, they leave the breeding grounds in juvenile plumage and hold that first plumage aspect well into their first fall. Most species, particularly the long-distance migrants, will not start their preformative molt until they reach their wintering grounds.
When the initial North American field guides appeared, it was as if juvenile birds and their plumages did not exist. They were generally not illustrated and if they were the illustrations were not properly labeled and juvenile plumages were not discussed in the text. The first Peterson guides actually offered black-and-white illustrations of "fall" shorebirds. For the most part, these illustrations showed juvenile birds, but did not label them as such or account for the fact that one might also see alternate and basic-plumaged adults during Fall.
The 1st edition of the National Geographic Guide (now in its 6th edition) followed the lead of British and European field guides and became the first North American field guide to thoroughly illustrate, describe, and properly label the juvenile plumages for shorebirds and other species groups. The Sibley Guides raised the bar again by accurately showing the date range (in months) that one can expect to see each particular species in its juvenile plumage. Having access to illustrations or photos that show all the different aspects that a bird might present and understanding the time of year that they might look a certain way is vital when it comes to identifying most birds. As an example, if you see a bird in January that looks like the juvenile junco in the top image, you can be pretty comfortable in concluding that it's not a junco based on seasonality. In fact, Sibley indicates that birds of this appearance are only likely to be seen from May thru August.
As you can see from the three Western Sandpipers in the photos above, understanding how a bird's appearance varies with age and time of year is critical. If you saw the birds in the top image together on the same day–as I did– it would be easy to conclude that they were of different species. On occasion, a winter/non-breeding aspect Western (like the Baja bird) will show up with the southbound migrants (alternate-plumaged adults and juveniles) in August or September. Imagine a newer birder seeing a flock with birds representing all three plumages/aspects (it happens), and then being told by an experienced birder that all of the birds were of the same species.
If you intend to properly convey what you've seen to other birders, it helps to describe them in terminology that will be understood and terms that accurately represent the age-class that you observed. When describing a bird as a juvenile, it is important to understand what that label implies. It's perfectly okay to not know, or not be certain of a bird's age, as it's not always readily evident. While some might figure out that you mean immature or subadult when you call a bird a juvenile, others may be left searching for your discovery armed with a false notion of what they are seeking.
Finally, this subject cannot be discussed with out addressing the question of "juvenile" vs "juvenal," which causes confusion for some. Juvenile can serve as both a noun or an adjective, whereas juvenal is strictly an adjective, most often applied to describing the first plumage of a bird. In my view, whether you say juvenile plumage or juvenal plumage, most birders will understand what you are trying to say. However, among experts there has been some disagreement regarding the importance of using "juvenal" when describing the first plumage of a bird or in describing the molts that produce or alter that plumage. For example, a bird replacing its juvenal/juvenile feather set might be described as undergoing a post-juvenal molt. However, if you are talking about the bird itself, it can only be referred to as a juvenile. You might say, "I saw a juvenal-plumaged Song Sparrow, but it is not appropriate to say, "I saw an adult Song Sparrow feeding two juvenals." For a lively and thought-provoking discussion, I would recommend reading Joe Morlan's thoughts on this topic. He wrote a letter to the editor of Birding in response to a letter to the editor that Steve N.G. Howell had written in 2009, in which Howell endorsed the idea of abandoning the use of "juvenal." Morlan Letter to Birding I don't have a dog in this fight, but for the sake of simplicity I use "juvenile" as both the noun and the adjective when discussing birds.
Glossary of potentially unfamiliar terms:
Note: Most of these definitions below come directly from "Molt in North American Birds" (Howell 2010). This is the most recent, comprehensive treatment of this topic and is a great place to start if you are interested in learning more about molt sequences. I have chosen to use Howell's precise definitions in an effort to avoid introducing any further confusion to a topic that already creates confusion for many birders.
Alternate Plumage -- Any second plumage in a cycle in addition to–and which alternates with–basic plumage. Attained by a prealternate molt.
Aspect -- The overall appearance of a bird, which can be a composite of basic and alternate plumages. For example, larger white-headed gulls with clean white manifest a breeding aspect, even though they may not be breeding or may have started their prebasic molt. European Starlings in glossy, mostly unspotted plumage manifest a breeding aspect, even thought this is simply their worn basic plumage.
Basic Plumage-- The plumage attained by the prebasic molt (which is complete or nearly so)and presumed to be homologous in all birds.
Cycle -- A regularly repeated phenomenon, such as a plumage cycle. A basic plumage cycle extends from the start of one prebasic molt to the start of the next prebasic molt. The first plumage cycle extends from the acquisition of juvenile plumage to the start of the second prebasic molt.
Definitive Plumage -- A plumage whose aspect does not change with time.
Formative Plumage -- Any plumage present only in the first (that is, formative) year of life and lacking a counterpart in subsequent cycles. Attained by a preformative molt. Most conventional "first basic" plumages are formative plumages.
Four-year -- Refers to a group of large gulls that go through four complete plumage cycles before attaining a definitive adult plumage. May or may not happen within four calendar years.
Immature -- A general term for any nonadult plumage, including juvenile plumage.
Juvenal -- An adjective that is interchangeable with "juvenile" when describing the first plumage of a bird or the molts that produce or alter that plumage. Howell (2010) does not use this term, calling it a "psuedo-academic distinction that is unnecessary."
Juvenile -- A bird in juvenile plumage (see definition below)
Juvenile Plumage -- The first plumage of "true" or vaned (nondowny) feathers; often the plumage in which a bird fledges. Considered synonymous with "first basic plumage" in recent molt studies and attained by the prejuvenile (first prebasic) molt.
Molt Limits -- (called "Molt Contrast" by Howell) The point of contrast between two generations of feathers in a non-molting bird; for example, between the alternate and basic primaries of a tern.
Plumage -- There are two meaning for this word. In the Humphrey-Parkes System, refers simply to a generation of feathers attained by a molt, and the color of the feathers is not relevant. In everyday usage, refers to a bird's coat of feathers and may be termed "male plumage," "immature plumage," "breeding plumage," etc; thus the color and pattern of the feathers are relevant.
Preformative Molt --The molt by which formative plumage is attained. In traditional Humphrey-Parkes terminology this was called the first prebasic molt.
Three-year -- Refers to a group of medium-sized gulls that go through three complete plumage cycles before attaining a definitive adult plumage. May or may not happen within three calendar years.