07 November 2010

Preformative Molt and Bonaparte's Gull

It is said that molt is the most understudied and underappreciated aspect of birds. Maybe. I remember reading a paper by Peter Pyle a few months ago where he mentions how it took him something like "ten years" just to wrap his mind around molt terminology. Wow! Over the last two years I've come to appreciate this bewildering subject and a basic understanding of the "modified" Humphrey-Parkes system has helped me understand birds just a little more. A deeper understanding of molt becomes integral when considering gulls and being familiar with the various molt strategies is a requisite for anyone with a veritable interest in this family.

Birds molt at different rates and frequencies but all generally follow one of four strategies. Determining which molt strategy (Simple Basic, Complex Basic, Simple Alternate or Complex Alternate) a bird obeys depends almost entirely on the events that take place in the first cycle.

Of the four molt strategies, the Complex Alternate Strategy (CAS) is, to me, the most unique. I have a particular interest in this strategy because it applies to all of the hooded gulls that occur in North America. Here, a formative plumage and an alternate plumage are found in the first cycle. These plumages are a result of a partial preformative molt and a partial prealternate molt.

Most birders generally know what is meant by a prealternate molt but I've found that many don't know the implications of the "preformative molt". In the past, ornithologists (including Humphrey and Parkes) overlooked the preformative molt and called it the first prebasic molt well. The juvenile plumage was not point zero in the study of molt. The former approach was easily justified because formative plumage can resemble that of a basic adult (Howell and Dunn, 2007). However, the preformative molt and prebasic molt are two different events. The modified H-P system suggests that a bird has arrived at 1st basic when in juvenile plumage. That is, the "1st" prebasic molt produces the juvenile plumage. This is nothing more than a nestling growing juvenile feathers.

The preformative molt is an additional molt that follows but is still contained in the first cycle; Unlike prebasic molts which reoccur, there are no preformative molts in any cycle thereafter.  Howell describes the preformative molt as a one-time event that acts as a "bridge" from juvenile plumage to 2nd cycle.

A good example of a common species that acquires formative plumage is Bonaparte's Gull. There are generally two waves of first cycle Bonaparte's that are observed in North America after the breeding season. The first wave begins in late July to early August and it consists of juveniles that appear to be in mostly fresh plumage like this individual:

Juvenile Bonaparte's Gull before acquisition of formative plumage; Chicago IL, 15 AUG 2010

By the end of August, these individuals become harder to detect because the preformative molt soon changes their appearance. The second and more significant wave of Bonaparte's typically peaks in late October (depending on migration routes and wintering latitudes). The first cycle individuals in this wave have concluded, or have come close to concluding, the preformative molt: 

 First cycle Bonaparte's Gull in formative plumage; Chicago IL,  2 NOV 2010

Some obvious changes have occurred but note that the juvenile flight feathers are not replaced as this is a partial molt. The juvenile tertials are still present as well. The upperwing coverts have mostly worn away their cinnamon/brown subterminal tips, but haven't been replaced. The head, neck and sides of the breast lose the brownish wash of the juvenile and this is a result of the body feathers molting. The brownish mantle and scapular feathers have also been replaced by the partial preformative molt. The bird now has an overall adult appearance. This is the stage that Humphrey and Parkes would have called 1st basic. 

If you're familiar with Jon Dunn's Advanced Video Series, then you'll note that the video on "small gulls" doesn't refer to juvenile plumage as 1st basic. It also fails to make mention of formative plumage as these videos predate the latest modifications to the H-P system. Again, a juvenile is equated with 1st basic in the modified H-P system. Although the replacement of some feathers occurs, the preformative molt should not be considered "the" gateway to 1st basic. This is a distinction that needs to be made when considering Complex Basic and Complex Alternate - the crux of the modified H-P system.

Same juvenile as first photo. Note the cinnamon-brown subterminal edging to the median/greater coverts. These upperwing coverts are not molted in the partial preformative molt. The November individual above with almost entirely gray median/greater coverts has the same upperwing coverts it had in juvenile plumage. Only now, the brown coloring on most of the coverts has faded on all but the lesser coverts; This is why a carpal bar is still evident in flight on individuals in formative plumage.  

First cycle Bonaparte's Gull in formative plumage; Carlyle IL; 14 NOV 2010

A slightly darker than average individual in formative plumage.  Note the juvenile wing and tail feathers produced by the 1st prebasic molt. The gray adult-like mantle and the white body feathers are a result of the preformative molt. In late winter, this species begins its 1st partial prealternate molt - a reoccuring molt in subsequent cycles.

I hope to follow up on this post with an explanation of the rest of the first cycle molts and plumages in first cycle Bonaparte's. Meanwhile, I'd like to recommend a few resources for anyone wishing to pursue this subject in more detail:
  • All You Ever Wanted to Know About Molt But Were Afraid to Ask, Part 1 (Birding, October 2003,pp. 490-496).
  • All You Ever Wanted to Know About Molt But Were Afraid to Ask, Part 2 (Birding, December 2003, pp. 640-650).
  • Gulls of the Americas; Howell & Dunn, 2007.
  • Identification Guide to the Birds of North America, Part 1 & 2; Pyle, 1997, 2008  
  • Molt in North American Birds; Howell, 2010.
I'd also like to leave you with one final thought that Steven N.G. Howell mentions in his latest work on molt: "We must accept, however, that even the most elegant human systems are imperfect attempts to put dynamic natural processes into boxes. There will always be exceptions. It is human nature to focus on exceptions, but to understand patterns it may be more helpful to focus on features shared by the majority of birds" (Howell, 2010). What if one day the modified H-P system is itself modified?


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  3. The most important thing about the preformative molt is that it is typically a partial molt.
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