Thanks everyone for your input, it has been very informative. I found the article in Bird Talk that briefly mentions this in relation to being overweight:

January 2008
Volume 26 #1
(Amazon on the cover holding a Dumbell - "The Diet & Fitness Issue")

"Battle of the Bulge" - p. 38

They show a picture of a lutino cockatiel with "cleavage." There's an arrow pointing towards the cleavage and the caption reads:

"In an overweight bird fat is stored in the area of the pectorals. Instead of the muscles being even with the keel bone, the tissue might bulge out on either side, resulting in "cleavage" that is palpable and often visible through the feathers."

Ok.... I think we are dealing with two different things here. A fat line, and a feather line. I have seen overweight birds and they are just as described by Sweet Marie and the BT article.... however, I belive that while it could be the reason some birds' feathers show a line down the middle, it is not necessarily the cause. One would need to feel the tissue and consult a vet to determine if the line visible on the ouside of the feathers is indeed caused by weight, or just by how the feathers lay.



(Thank you for the compliment on Tiki BTW Jac :-} )

Yes, I meant to echo that compliment- i saw the picture and thought "What a beautiful bird". Note that Tiki has a feather line. Lulu could possibly have a fat line, but feathers are deceptive, and I somehow doubt she does. But if you're not sure, examine your bird. If you feel the keel bone, if it protrudes a bit with healthy tissue on either side, your bird is not fat. But if you can't feel that bone, your bird is fat. I believe the so called 'fat line' is formed by the indentation caused by a submerged keel bone. If your bird has this, it's grossly overweight and may face some serious health issues, if not resolved.

I would think and I may be wrong but that the larger the bird the more of a feather line there will be, since feathers grow in tracks and these tracks are absent directly on the line of the keel bone it would make sense that birds with larger feathers like that of a macaw would naturally separate slightly without the aid of unwanted fat. While in a smaller bird like a conure it might be less evident.

Well in most birds. Their breasts have no feathers on the breast bone itself. So my only conclusion o this "mystery" is that since their are feathers on the right breast as there are on the left, they make, sort of a dip, when they meet, because no feathers are on the tip of the bone? Do I make any sense? Because even finches, when you blow on their breast, have no feathers right on he bone and a bit to the right and left. And a bird who has more rounded breast meat, is more likely to show this "line".

Exactly my point ;-}

Same here!