Although territorial pairs of crows and magpies are usually the most serious nest predators, flock-living corvids can also do serious damage. In many cases the sheer weight of numbers means that they come across nests, even if by accident, at far too great a rate. This applies especially if you have large numbers of juvenile crows living nearby, perhaps obtaining their main living on a rubbish tip or outdoor pig unit.
Similar problems apply with moorland situations. Here, if there are no trees, crows will breed on the edges of the moor, defending their territories here, but foraging out onto the moor for grouse nests and other foods. Similar problems can occur with rooks and jackdaws. Wherever possible it pays to get permission from neighbours to trap the boundaries in these situations.
Remember that crow cages must only be operated under a General Licence (issued annually by Scottish Natural Heritage) by authorised persons who understand and comply with its conditions (see legal section for more details of the licences). An authorised person means the owner or occupier, or any person authorised by the owner or occupier, of the land on which trapping is taking place. Individuals are not personally required to be in possession of a licence. The licence can only be relied on in circumstances where the authorised person has satisfied himself that appropriate non-lethal methods of control such as scaring or bird proofing are either ineffective or impracticable.
In most cases Larsen traps are of limited use on open moorland. Even if corvids are naive enough to go in and join the decoy when attracted only by the need for company, the rate of catch at one or two birds at a time is too slow to be really effective. On the other hand, the urge to join a feeding flock is great. So a larger cage, with several decoys already in it, is likely to work well. This applies especially in the late winter period, when many keepers like to make a start on their crow control.
The exact design of these multi-catch cages is much less critical than with Larsens, and General Licences do not specify what size a multi-catch cage should be. The Wildlife and Countryside Act 1981 states that if any person keeps or confines any bird in a cage which is not sufficient in height, length or breadth to permit the bird(s) to stretch its wings freely, he shall be guilty of an offence and be liable to prosecution.
Designs and dimensions of traps are numerous. The size of the trap is relatively unimportant when compared to its siting, the latter being crucial to catching success. We recommend you do not site a cage near to a main roost or near to a used public access route. Not only might users of the route see and possibly disrupt your legal activity, but non-captive crows may become shy of the location when disturbed by people. Select a quiet location site on a known flight line eg. along a lochside, sea shore, beside a burn in a glen or even in open moorland where crows have good all-round vision.
Three basic types of multi-catch cages have come to the fore. The roof funnel, the ground funnel and the ladder letterbox. In all cases they should be covered with 35mm wire mesh, since smaller sizes trap songbirds and anything larger could allow jackdaws or magpies to escape.