What is com.apple.bird? Why does it grow so big? When trying to clean up their hard disk, some users discover ~/Library/Caches/com.apple.bird is pretty big, sometimes in the order of many gigabytes. What is it?
Bird is part of iCloud and the files in the folder might be owned by any app using iCloud.  I wrote a simple Python script to try and find out who owns the files in the directory:

$ python blame-bird.py 
4R6749AYRE.com.pixelmatorteam.pixelmator            0.00MB
com.apple.shoebox                                   0.00MB
com.apple.TextInput                                 0.00MB
iCloud.com.apple.iBooks                             0.00MB
57T9237FN3.net.whatsapp.WhatsApp                 6904.66MB

Accounted for: 6904MB.  Still unaccounted: 1879MB


So on my Mac WhatsApp is the biggest culprit.  I filed a bug report with them (although I am not sure if it's really their fault).