Clearing Up Bald Eagle Terminology: “Juvenile”, “Immature” and “Subadult”

An interpretive commentary on the proposal by Clark & Pyle (2015) to standardize the use of the term “pre-definitive” as a reference to all plumages prior to definitive. By EaglesExplored.com (October, 2025)

Preface

As plainly as can be stated, the Clark & Pyle (2015) paper referenced below is not principally about pressing the term “pre-definitive”.  It’s about the standardization of age-class plumage terminology for raptors. However, in a sea of papers about Bald Eagles and other raptors, it’s the first that I’ve ever seen that significantly confronts the ambiguity of using “immature” and “subadult”, and proposes a substitute term, “pre-definitive”, for standardization.

Commentary Article by Eagles Explored

If you’ve been using terms like juvenile, immature, or subadult to describe bald eagles before they reach full adult plumage, rest assured, you’re in good company. These labels are steeped in the record of peer-reviewed papers by giants in the biological world. They’ve helped generations of field biologists, banders, and educators communicate effectively, even if the definitions have sometimes varied.

In fact, Clark & Pyle (2015) highlight how these traditional terms can overlap or conflict depending on the source:

Variations in Traditional Terminology

“Juvenile” — Definition and Usage Across Raptor Literature

While less controversial than subadult or immature, the term juvenile still varies slightly in usage depending on context. Common definitions of “juvenile” include:

• The plumage worn from fledging until the first molt, typically lasting through the first year.
• The plumage displayed by a juvenile bald eagle is sometimes referred to as “juvenal” plumage (McCollough, 1989).
• A bird in its first calendar year, regardless of molt status.  Often used interchangeably with “first-year”.
• Unlike subadult and immature, the term juvenile is rarely applied beyond the first year.

“Immature” has been applied to:

• All non-adult plumages (Wheeler, 2003)
• Any plumage between juvenile and adult (Friedmann, 1950; Clark, 1999)

“Subadult” has been used to describe:

• The annual plumage immediately preceding adult plumage (Clark, 1999)
• All plumages between juvenile and adult (Liguori, 2011; Wheeler, 2003)
• All non-adult plumages (Newton, 1979)

The Fog of a Four-Tiered Plumage (or Aging) System

Have you ever used the terms juvenile, immature, subadult and adult to describe an eagle’s progression toward definitive plumage in adulthood?  Many have, and I admit to stumbling through those concepts along my eagle journey. After all, juveniles are dark, immatures develop lighter feathers throughout the belly, subadults start getting the white head, and adults are fully white in the head and tail, right?  Well, what might make sense in the scheme of things is not necessarily what science holds as a proper classification system. 

So, is there now, or has there ever been a four-tiered plumage classification system for Bald Eagles that uses the cited terms?  Probably so, but I haven’t seen it yet, exactly that is.  McCollough’s (1989) Figure-1 shows a comparison of molt and plumage classification systems developed by Southern (1967), Clark (1983) and Bortolotti (1984).  It’s Bortolotti’s system that comes as close as I’ve seen to using this terminology.  He uses “young immature”, “old immature”, “subadult” and “adult”.  That’s four stages.  And while the term “juvenile” is not cited, it’s not a stretch of credibility to imagine that first year eagles were often referred to as juveniles, “official” system or not.

Why “Pre-definitive” Matters

The cited inconsistencies between terms can lead to confusion, especially when trying to align plumage stages with molt cycles or calendar age. That’s where the term pre-definitive comes in.

To bring clarity, Clark & Pyle (2015) recommend adopting pre-definitive to describe all plumages prior to definitive adult, particularly when age cannot be determined. This terminology is grounded in molt strategy frameworks and avoids the ambiguity of older terms. It’s not a dismissal of tradition; it’s a refinement.

McCollough (1989) suggests that definitive plumage for Bald Eagles may be reached at 4½ to 5½ years of age, depending on individual variation and molt progression. So, the term pre-definitive elegantly covers everything from fledging through the final transitional plumages without forcing arbitrary age brackets or inconsistent labels.

Toward Clarity Without Oversimplification

The term pre-definitive offers a welcome umbrella.  It’s an inclusive reference to all plumages prior to definitive adult. It sidesteps the semantic fog of “juvenile,” “immature,” and “subadult,” and aligns with molt-based frameworks that prioritize biological process over tradition. But generality is not the end of the story. Beneath the surface of pre-definitive lies a rich tapestry of classification systems that tie molt, plumage, and age into coherent, scientific sequences. Among these, Clark & Pyle (2015) make the following recommendation in their paper:

“We recommend that standard age-class terminology be used in publications that involve raptor plumages. We propose using standardized terminology developed by Humphrey and Parkes (1959) and refined by Howell et al. (2003) for all raptors, and we propose age-coding terms recommended by Wolfe et al. (2010) and Johnson et al. (2011), as described herein.”

My Opinions and Conclusions

My opinions and conclusions?  I love the term pre-definitive and its broad application to early plumage.  I wholly support its use, particularly in scientific papers.  Yet for age-class terminology specific to Bald Eagles, I find McCollough’s (1989) plumage-molt-age matrix to be the most biologically faithful and field-practical. His work captures the nuance of individual variation while anchoring plumage stages in observable molt progression. So, while pre-definitive may serve as a linguistic bridge, it’s the underlying classification systems that give that bridge its structural integrity.

In everyday conversation, I suspect that I will still find myself wanting to use the non-specific immature-subadult terminology as a way to relate relative position in the molting sequence and aging process.  I’m an old bird and habits are hard to break.

This article is an opinion-piece by EaglesExplored.com.  Please delve into the science by reading from the following reference list, among others.  Feel free to contact us with any feedback or corrections.

Main Reference

CLARK, W.S. AND P. PYLE. 2015. A recommendation for standardized age-class plumage terminology for raptors. Journal of Raptor Research, 49(4), 513–517.

Literature cited as well as other applicable references

BAKER, K. 1993. Identification guide to European non passerines. British Trust for Ornithology, Thetford, Norfolk, U.K.

BLOOM, P.H. AND W.S. CLARK. 2001. Molt and sequence of plumages of Golden Eagles, and a technique for in hand ageing. North American Bird Bander 26:97–116.

BORTOLOTTI, G. R. 1984. Sexual size dimorphism and age-related variation in Bald Eagles.                 Journal of Wildlife Management 48:72-8 1.

-AND V. HONEYMAN. 1983. Flight feather molt of breeding Bald Eagles in Saskatchewan. In The Bald Eagle in Canada. (J. Gerrard and T. Ingram, eds.). Proc. of Bald Eagle Days 1983. Eagle Valley Environmentalists, Apple River, Illinois.

CANADIAN WILDLIFE SERVICE AND U.S. FISH AND WILDLIFE SERVICE. 1991. North American bird banding. Vols. 1 and 2. Environment Canada, Ottawa, Canada, and Washington, DC U.S.A.

CLARK, W.S. 1999. A field guide to the raptors of Europe, North Africa, and the Middle East. Oxford Univ. Press, Oxford, U.K.

 ———.1983. The field identification of North American eagles. Am. Birds 37:822-826.

 ———.2001. Aging Bald Eagles. Birding 33:18–28.

 ———. 2004. Wave moult of the primaries in Accipitrid raptors, and its use in ageing immatures. Pages 795-804 in R.D. Chancellor and B.-U. Meyburg [EDS.], Raptors worldwide. World Working Group on Birds of Prey and Owls, Budapest, Hungary.

 ———AND P.H. BLOOM. 2005. Plumages of Basic II and Basic III Rough-legged Hawks. Journal of Field Ornithology 76:83–89.

 ———AND B.K. WHEELER. 2001. A field guide to hawks of North America. Revised. Peterson Field Guide Ser., No. 35. Houghton Mifflin, Boston, MA U.S.A.

 ———AND P. PYLE. 2015. A recommendation for standardized age-class plumage terminology for raptors. Journal of Raptor Research, 49(4), 513–517.

FRIEDMANN, H. 1950. Birds of North and Middle America.  Falconiformes. U.S. National Museum Bulletin, No. 50, Part 2. Washington, DC U.S.A.

HOWELL, S.N.G., C. CORBEN, P. PYLE, AND D.I. ROGERS. 2003. The first basic problem: a review of molt and plumage homologies. Condor 105:635–653.

 ——— AND P. PYLE. 2014. Use of ‘definitive’ and other terms in molt nomenclature: a response to Wolfe et al. (2014). Auk 132:365–369.

HUMPHREY, P.H. AND K.C. PARKES. 1959. An approach to the study of molts and plumages. Auk 76:1–31.

JOHNSON, E.I., J.D. WOLFE, T.B. RYDER, AND P. PYLE. 2011.  Modifications to a molt-based ageing system proposed by Wolfe et al. (2010). Journal of Field Ornithology 82:421–423.

LIGUORI, J. 2011.Hawks at a distance. Princeton Univ. Press, Princeton, NJ U.S.A.

MCCOLLOUGH, M.A. (1989) Molting sequence and aging of bald eagles. Wilson Bulletin, 101(1), 1–10.

NEWTON, I. 1979. Population ecology of raptors. Buteo Books, Vermillion, SD U.S.A.

PYLE, P. 2005a. First-cycle molts in North American Falconiformes. Journal of Raptor Research 39:378–385.

 ———2005b. Remigial molt patterns in North American Falconiformes as related to age, sex, breeding status, and life-history strategies. Condor 107:823–834.

 ———.2006. Staffelmauser and other adaptive wing-molt strategies in larger birds. Western Birds 37:179–185.

 ———. 2008. Identification guide to North American birds. Part II. Slate Creek Press, Point Reyes Station, CA U.S.A.

 ———.A. ENGILIS, JR., AND D.A. KELT. 2015. Manual for ageing and sexing the landbirds of Bosque Fray Jorge National Park and north-central Chile, with notes on range and breeding seasonality. Special Publications of the Louisiana State University, Baton Rouge, LA U.S.A.

SOUTHERN, W. E. 1964. Additional observations on winter Bald Eagle populations: including remarks on biotelemetry techniques and immature plumages. Wilson Bull. 76: 121-137.

———.1967. Further comments on subadult Bald Eagle plumages. Jack-Pine Warbler 45:70-80

WHEELER, B.K. 2003. Raptors of western North America. Princeton Univ. Press, Princeton, NJ U.S.A.

 WOLFE, J.R., T.B. RYDER, AND P. PYLE. 2010. Using molt cycles to categorize age in tropical birds: an integrative system. Journal of Field Ornithology 81:186–194.