Recent research has shown that Carbon Monoxide is a cheap, humane and effective euthanasia agent. Tidemann and King 2009 (PDF, 180kb) in Wildlife Research 36(6) 522–527 CSIRO publishing - www.publish.csiro.au/journals/wr
Trapped mynas were previously killed humanely (euthanased) by enclosing
the holding section of the trap with a canvas sleeve and flooding
it with industrial grade carbon dioxide (PDF, 144k)
This method of euthanasia is widely viewed by animal welfare
agencies in Australia and elsewhere to be humane and accords
with the National Health and Medical Research Council of Australia’s
Code of Practice for the Care and Use of Animals for Scientific
and the policy of the Royal Society for the Prevention of
Cruelty to Animals (RSPCA) on the Control and Use of Feral Animals:
The killing of pest birds must be conducted in accordance with the Code of Practice (COP) and Standard Operating Procedures (SOP) for the humane capture, handling or destruction of feral animals in Australia.
The Methods of Euthanasia SOP (PDF, 288k) describes the general use of carbon dioxide as a method; the Trapping of Pest Birds SOP (PDF, 227k) has further specific information on the method for use with birds in traps, and it also provides advice on trapping. Both publications are available at: Department of Environment and Heritage (DEH) and the NSW Department of Primary Industries (DPI).