Feathers
I would take the feathers as an insider joke on one of the development code names for 970 (ultralight).
I compared the photos with the feathers to the 970 die photo available from the same gallery.
The aspect ratio of the chip pictured with the feathers appears to match the aspect ratio of the die photo of the 970. The die photo is likely of a chip pulled from the line before the upper metal layers were deposited. This gives a picture which highlights the floorplanning.
Chips fabricated all the way to the C4s (like the one pictured with the feather) are pretty boring and uniform looking because most of what you're seeing are the C4s (solder balls), power grid, and clock grid.
As an aside, it is not clear to me whether that "die photo" is an actual photo. It looks suspiciously regular. Real die photos have variations in color between different instances of the same circuit because of the different light paths and interference patterns.
The "IBM 64-bit PowerPC 970 microprocessor chip die. (#88)" looks too regular. Notice how the different sub-blocks of the caches are all exactly idential to each other. This reminds me of the fake die photo plots we use internally for presentations, posters, etc. They are much more satisfying than an actual die photo because they reveal more of the internal structure...it's easier to point to "what I designed".
C4 = Controlled Collapse Chip Connection, introduced in products in 1964.
