What is com.apple.bird? Why does it grow so big?
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).