Richard has pretty much nailed it. Adult Chiffchaffs undergo a complete post breeding moult, whereas young will only moult maybe a few greater coverts (if I remember my Svensson correctly). That's why many young Warblers look quite smart whilst adults can look very tatty before / during the moult cycle.
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No one on their death bed ever said they wished they'd spent more time at work. http://bitsnbirds.blogspot.co.uk
Hello Simon, I'm not going to go through the complex cycle of molting and variation etc as each species is different one from another and my own knowledge is limited. But reading your post, it appears most likely your warblers in question are going through the molt phase. At this time of year, most species are going through a change of feathers. This can change their total appearance ( ducks are a classic example as male mallards look nothing like adult males and this change in ducks is known as eclipse.) Sometimes they go through a cycle order such as their body first, tail and wings then the head (I have a picture of a young Robin in adult plumage except it's head alone which was still completely in juvenile feathers) I've seen jays with bald heads, crows with terrible looking wings, robins with no tails etc. My point being is your warblers will without doubt be in molt and can be a real challenge to identify as key features will fade or disappear. Anyway, I hope this information helps. Ta!
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Which bird is ideal for keeping cakes in? I asked. The answer: a Bun-tin.
http://www.flickr.com/photos/135715507@N06
I don't own any books with this sort of detail and I thought I would ask the forum members as well as hit google. My question is about a few Chiffchaffs I've seen lately [last fortnight] with scruffy heads, seeming to lack some feathers and therefore no easy to see supercilium. The effect is to make the birds' heads look more angular and the bill look bigger.
I had imagined this was juvenile birds just out of the nest without the plumage having grown fully, but don't they always say autumn juvenile birds look immaculate? Then I saw a comment on Birdforum about moulting and thought it might be moulting adults. So when do Chiffchaffs moult and what is a likely explanation for this occurrence. I have no fixed idea myself, so I won't be arguing, just hoping some of the very knowledgeable people on here will both recognize what I am describing and have an explanation!
