Setting the birds free is certainly not the aim, as you say Dylan. Mist nets are used all over China for bird control, and probably at other airports in countries where it is considered there is not the money to implement more effective techniques. I have worked in this field since the mid-1990s, and have not seen any decline in their use in China during that period.
While mist nets may reduce numbers of certain species such as pipits, larks, wagtails and smaller waders, in many cases it does very little to impact the real problem species. For example, where a problem species is something like Black Kite, mist nets would do nothing to impact numbers and thus the bird strike risk. In general, it is large and/or flocking species that are the key issues at airports worldwide, as the potential for damage to engines is greatest from these birds. Mist nets are unlikely to do anything to control their numbers, except perhaps via the indirect impact on their prey species. But this is not likely to be significant. Control of smaller species is best done via habitat management.
Unfortunately, airports, who pay for bird control, want to do it as cheaply as possible. However, it is airlines that pay for the effects of bird strikes. Until this dichotomy is addressed, many airports will do the minimum possible. Although one would anticipate the legal process should close the loop, in many cases it does not.
Incidentally, HK International Airport does not use mist nets - you will be relieved to hear!
GeoffC
