This was not something I hadn't heard of before. Part of me doesn't buy it, but it's interesting if true.
WSR-88D dopper radars have a bunch of settings that allow them to look at the data in different ways. They can tell velocity, measure reflectivity, and also measure the size of stuff in the air. Correlation coefficient is a measure of how uniform the stuff in the air is. The the more different it is, the higher the coefficient. It's really useful for tornadoes and hail storms for obvious reasons.
So could it be birds trapped in the eye? I was able to find one other source from a different hurricane saying the same thing. But given how it seems like it's the entire eye, that feels strange. Then again, I can totally see how birds would be blown into the eye and then travel with it.
Regardless, I thought it's really interesting. You can see a full animation of it here. On the menu on the left, you can see Correlation Coefficient along with the numbers 1-4. the numbers are for tilt, which is how far above the horizon the radar is looking. 1 is about .5 degrees above, 4 is about 5 degrees.
weather.cod.edu/satrad/nexrad/?parms=TBW-N1C-1-200-100-usa-rad
RT: https://poa.st/objects/0283de20-29b3-41cb-ba94-e3af6e62ac2d