Apple - 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).

'Bird' is part of the iCloud structure. You would have a large cache if there are pending documents to upload or process.

Over time, the com.apple.bird cache is generally kept cleaner than you're thinking. Mine is 400 KB [that's K, not even M]

You can diagnose this with brctl - which has a diagnose command and a log command. Unless you are sure you have a bug in the cache - I wouldn't delete the files as they might cause you at best - another upload of the original content and at worst - cause errors or data loss of iCloud backed documents.

With that large a cache you should be able to dump and then monitor whatever categories/classes of data have the most size.

brctl dump

It's exceptionally verbose and you might not want to spend time learning what it does - but the data is quite helpful in determining what's actually happening and gives you an option before reaching out to AppleCare or another support tech in cases where you don't see the cache clearing itself over time.