I dont know! I think the conventional wisdom is its an expression of frustration and/or excitement. But based on the odd sounds & squeaks my cat Orange makes, Ive sometimes had the impression that he was trying to talk to the birds in bird language. I think he might be saying something like "come here and let me eat you!"

Same cat person history here, although sadly their lifespans have been shortening due to that damn kidney disease carp.

My babies all chattered away in windows or sliders after sighting prey. (They were n.e.v.e.r. allowed outside.) Could it be that they are alerting fellow felines that 'dinner' may be near?

One little girl, a petite tuxedo princess, aka "Miss Jones", was so smart she knew that glass is liquid and therefore if she could just run fast enough she would actually run right through the glass! Well. We would here a l.o.u.d. thud, run into the next room and there she would be, shaking her head and getting ready for her next attempt--from farther back! Eventually she learned.

I miss my little cat people so much. All that special kitty love!

We've had several who did the chattering. One of them used to love to watch nature channel shows on TV. One time, she watched the show's subjects, a flock of geese, fly off to the right and disappear from the screen, so she ran to the TV and looked behind it. :)