Cannon-netting

Cannon-netting is a method of catching large numbers of animals, often birds, usually to band them, or otherwise tag them, as well as acquiring biometric data (measurements), in order to find out about their movements, migration routes, survival rates and metabolism. It is often used where large numbers of birds, especially waders, terns and gulls aggregate on beaches at high tide, or in places (which may be baited) where waterfowl or cockatoos feed. Cannon-netting may be used as a capture-for-culling method for perceived pest species such as cockatoos. Mammals such as deer and macropods may also be cannon-netted.

Methodology

Cannon-nets are nets pulled rapidly by explosively-driven projectiles to cover a pre-determined area of ground and presumptively capture any birds (or other target animals) present before they have time to escape. An alternative is the use of rocket nets, the only significant difference being the method by which the projected nets are pulled over the catching area. In the case of cannon-netting the projectiles that are attached to the net are fired simultaneously from smooth-bore cannons by electrically detonated cartridges. A full-sized cannon-net may utilise four or more cannons to pull the net over the target birds. Smaller nets with fewer cannons may also be used.

Catching waders

When cannon-netting is carried out for the purpose of catching large numbers of waders or shorebirds for banding and release, it requires an experienced team to coordinate and manage not only the catch itself, but also the subsequent care and processing of the birds to their eventual safe release. Catching is usually conducted on tidal beaches with nutritious tidewrack, or adjacent to coastal mudflats, though occasionally other locations, such as salt lakes or sewage treatment lagoons, are used.

Planning. In the days preceding a catch, known and likely high-tide roosting sites are reconnoitered to establish the best prospective site for a catch, although a final decision may not be made until immediately prior to net-setting. Consideration of expected maximum tide height and wind direction may affect the precise placement of the net.

Setting the net. The net is usually set during low tide when the birds are dispersed and feeding. The mesh of the net must be free of any obstacles such as twigs, leaves and shells that might obstruct its deployment when fired. The bunched net is laid in a line with pegged 'jump ropes' attached to the back edge. Markers are set to indicate the extent of the catching area. The heavy iron projectiles are attached to the projectile ropes that will pull the leading edge of the net, and are inserted in the partly buried cannons behind the net. The previously cartridge-loaded cannons are connected by an electrical circuit to a firing box in a hide which may be some distance away, though with a good view of the catching area. After setting, the net is often concealed with sand, seaweed or other light debris.

Setting up the catch. If the birds do not fly or walk readily into the catching area of the net when coming in to roost, it may be necessary, through the judicious placement and movement of people or vehicles, to encourage the birds to move into position where they can be safely caught. Team members coordinate their efforts through radio contact.

Firing. The net is fired when it is judged that an appropriate number of birds is catchable, and that none will be endangered during firing. The number of birds caught should not exceed the capacity of the team to deal with them expeditiously.

Extraction. The birds are extracted as speedily as possible from the net and placed in temporary fabric holding cages to await processing in a sheltered environment. Depending on weather conditions, shadecloth may be used to cover the holding cages. The aim is to minimise stress to the birds.

Processing. This includes the attachment of individually numbered bands to the birds' legs, sometimes with site-specific colour-bands, plastic leg-flags, or light level geolocators to track migration routes. Measurements are made of wing-length, head and bill length, weight, as well as degree of moult or signs of breeding plumage. Retraps of previously caught birds have their band numbers recorded and are similarly processed. Evidence of age is noted in order to estimate seasonal breeding success. Sometimes blood samples or cloacal or tracheal swabs are taken for epidemiological studies, such as government programs monitoring the incidence of avian influenza. All data are logged on field datasheets and later entered on a computer database. Details of catch conditions are also recorded. Following processing, all birds are released.

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