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: