Hi Kwan,
If it is possible, see whether the no of prayer birds (munias, tree sparrows and white eyes) can be counted. A HKU student and KFBG did it in 2005 and 2008/09, respectively. The long term trend may indirectly indicate the activities of bird release in HK.
Just the no of cage is ok. It is known that each cage usually contains about 40 birds.
Cheers,
Captain