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The success of Larsen traps
Larsen traps were designed by a Danish gamekeeper in the 1950s. In Denmark it has been suggested that this trap alone was responsible for a significant reduction in the national magpie population from 1965.
Larsen traps will catch all corvid pest species (i.e. crows, magpies, jackdaws, jays, rooks) at all times of the year, but their particular value is in catching crows and magpies when they set up their breeding territories. The Larsen trap contains a separate compartment for a ‘decoy’ bird, which is seen as an intruder in any corvid territory in which the trap is placed. Territory-holding birds attempt to drive it away and get caught in the process. In the original design, the trap mechanism involves a spring door to each catching compartment which, when set, is held open by a split perch. To enter the trap, birds the size of a magpie or crow inevitably drop onto the perch. The perch gives way, and the bird’s momentum takes it past the bottom of the door, which flips up – et voila! Later variants of the design have introduced other, alternative door mechanisms.
Larsens are live-catch traps. Why catch alive? Because of the risk of catching non-target wildlife other than corvids. Virtually all non-target birds are protected by law, and the licences allowing Larsen trapping stipulate that they must be released alive and unharmed. It is important to remember that in today’s countryside, the future of shooting depends on game management being conducted responsibly and professionally and in a way which delivers wider environmental benefits.
Having said this, we have experienced very few captures of non-target species in Larsen traps – another point in their favour. In a survey of over 10,000 birds captured, only 1% were non-target species. Finches and tits often visit them, but are too small to trigger the mechanism. Of course, many legally protected bird species, as well as gamebirds, suffer from corvid predation on their eggs or young, and the Larsen trap is potentially an effective tool in the conservation of these birds too. There is no ‘natural balance’ between corvids and the birds they prey on, because they also feed to a great extent on other foods provided, directly or indirectly, by man. Recent GWCT research shows that corvid control can contribute to the conservation of some, but not all, songbird species. It has also been proven to benefit wader species such as lapwing and curlew.
A second reason to catch corvids alive is that each may in turn be used as a call-bird to attract further captures. Because Larsen traps are small, they can easily be moved around to address further pairs of crows or magpies, and a few traps can therefore cover quite a large area. In this way, the whole effort quickly grows to an effective scale within a single breeding season.
When to trap and why
We suggest that trapping effort is restricted to spring and summer. This is the period of maximum prey vulnerability and the time when Larsen traps are most effective. In most areas, the overall population of crows and magpies is far greater than the number of breeding pairs. This is apparently because only a limited number of territories with a suitable nest tree site can be fitted into a given landscape. Non-breeding birds usually feed in flocks, roaming over areas much larger than the usual territory size, and using different foods from those of territorial birds. As a result, they may pose less of a threat to conservation. Flock birds should be thought of as a reservoir of frustrated would-be breeders or young birds. If a territory becomes vacant, it will normally be claimed by fresh birds of breeding age from this reservoir. However, until they are established, newcomers are less likely to find the nests of vulnerable species.
If you trap outside the period March to July, you will have to diminish the reservoir population over a very large area to cause any benefit to your wild bird population in the breeding season. Also, consider that when catching flock-living corvids you may actually educate other members of the flock and make them trap-shy. This might jeopardise your efforts in spring when it really counts.
Where to put the trap
If you are familiar with your land, you will know the specific trees that always seem to attract crow or magpie nests. If you are just getting to know the area, look out for nest-building activity from the beginning of March. Before bud-burst, magpie nests are very obvious in the trees. Crows and, to a lesser extent, magpies, often sit high in the trees near the centre of their territory, literally acting sentinel. You should aim to position the trap in plain view of sentinels, and in a prominent location within 100 yards of the nest site. If you haven’t time to watch out for nests or sentinels, concentrate on small copses and spinneys, thick hedges and woodland edges – but have you really got time to check the traps every day if you cannot undertake reconnaissance?
Avoid placing traps too close to rookeries, unless you want a full-time job dispatching or releasing rooks, in which case a multi-catch trap is a better option – please see our fact sheet on multi-catch traps for more information. We suggest that crows and magpies are your main target in the conservation of wild-breeding gamebirds and other vulnerable wildlife species.
Place the trap on the ground, especially for crows, which like to approach on the floor. However, when trapping magpies among bushes, or in a dense hedge, raising the trap above brambles gives it a better chance of being seen. Do not be afraid to experiment with setting traps in cover. A good call-bird will often reply to the calls of the territory holder even though the trap is out of sight. Indeed, this ploy can account for trap-shy individuals, there is also the benefit of the trap not being easily seen by people. Where local situations allow, for example, areas with low public access, placing the trap so the call-bird appears dominant, such as in direct sunlight if conditions are not too hot, rather than in the shade, may improve trapping success.
If you have not caught anything within two days it could be either because the birds are not yet fully territorial or, the trap is not close enough to the heart of the territory or, as can be the case with crows, the call bird is a young bird which possess little threat to the pair. In some cases, it may be best to move the trap and use it elsewhere. If you know that there are dominant birds which will not go in, rest the site for a few days and then bring back the trap with a new decoy.
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How to catch both birds of a pair
It is not strictly necessary to catch both birds of a pair. They defend their territory together, and to remove one will prevent the remaining bird defending the area against a speculating pair of intruders; in this way its breeding effort is disrupted. Furthermore, the male bird feeds the female while she is incubating the eggs – if he does not turn up, the hen bird must leave the nest.
If you keep up your use of Larsen traps throughout spring and summer, the establishment of territories will be continually disrupted and, while present, fresh birds will be pre-occupied with territorial defence and nest building. Their demand for food is never swollen by the need to feed young, and they have little time to watch the movements of incubating game and songbirds. In a well-run trapping programme, you will have caught each original pair before they have young, and then gone on to remove new ones before they are fully established.
Crows and magpies learn very quickly, and a bird that witnesses its partner being removed from a trap by a human and killed may subsequently be very shy of traps. If you catch a bird in one compartment of a Larsen trap, leave it disputing its case through the wire with the call-bird. Its partner will very often join in and get caught in the other compartment. This is one advantage of the three-compartment Larsen trap.
However, if you have not caught the other bird by nightfall, you probably never will, so take out the first capture. Do not leave trapped birds overnight without a perch and with no food and water until the next morning. Siting traps where you can view them with binoculars allows you to check without scaring potential captures. Do remember, however, that you are legally required to go to the trap and undertake a physical inspection of food and water at intervals of not more than 24 hours.
Adequate food, water, shelter and a suitable perch must be provided. An inspection must be sufficient to determine whether there are any live or dead birds or other animals in the trap and any dead or sickly decoy birds must be immediately removed from the trap. Do not therefore rely on a binocular check.
Catching the difficult ones
Perhaps as a result of a previous fright, some magpies and crows can be very shy of a Larsen trap. Crows, in particular, will often dance around a trap but refuse to jump on. Moving the trap deeper into the territory is the most likely route to success. Otherwise it can pay to lift the trap up onto a bale or some other solid object. Raising the decoy in this way sometimes infuriates the territory holder into jumping on. Do not just add legs, though – if the territory holder can see the decoy through the floor of the trap it may attack from below and never land on the top.
Another popular choice is to use a side-entry Larsen. There are various versions available from different manufacturers, many of which work well. However, please be careful to choose a trap of adequate size – we specifically warn against reducing any of the dimensions of compartments of the Larsen trap design dimensions as offered by the GWCT. They are the minimum needed for the welfare of captive birds.
Other single-compartment traps with no decoy compartment, known variously as Larsen Mates, Larsen Pods or ‘clam traps’, have become very popular. The intended use is either with bait alone (e.g. a good option to help catch the first call-bird); or to catch birds that are unwilling to enter the Larsen trap catching compartment. These single-compartment traps are now explicitly lawful in Scotland, but in England and Wales their use is currently a grey area. We hope they will be explicitly approved in both England and Wales in future GLs.
How to dispatch a captured corvid humanely
Any birds to be killed under General Licence must be dispatched humanely.
The General Licences clearly describe humane dispatch as taking all reasonable precautions to ensure that any killing of birds under licence is carried out by a single, swift action as soon as reasonably practicable after discovery. If you are right-handed, hold the bird as described in our handling guidelines in your left hand. With your right hand, grip the legs, tail, and wing tips together. In one movement, draw the bird out of your left hand, so that it doesn’t have time to bite you, and strike the back of its head very hard against the edge of a hard surface. Alternatively, you can use a short stick, or a fisherman’s ‘priest’. It takes a surprising force to kill one of these birds. Do all you can to make death instant. Breaking the bird’s neck after this makes certain that you have not just stunned it.
There is currently (May 2020) a question mark over dispatching birds in sight of others of the same species. Existing General Licences for Canada geese and Egyptian geese require that these birds must be killed out of sight of each other but there is no mention of whether this applies to other trapped species. GL33, the Standard Licence Conditions for trapping wild birds and using decoys under a Natural England licence, proposes that all trapped birds must be killed out of sight of others of the same species. It seems hugely anthropomorphic to suppose the decoy bird will care a jot about seeing the captured bird killed, when a few moments earlier that bird was trying to beat it up and drive it away.
GWCT advice at this stage is to take the precaution of killing all birds out of sight where possible until we have further clarification. With Larsen traps and the normal small number of captures, it doesn’t take much effort to turn your back or step behind the vehicle.
When you have finished trapping
|Keep in touch
|There is still much to be learnt about the efficient control of corvid numbers. If you have experiences at variance with the advice given here, or make any significant improvements to trap, bait, or strategy, please contact the Game & Wildlife Conservation Trust’s Advisory Service on 01425 651013 or firstname.lastname@example.org.
A number of incidents of accidental captures of protected species have been reported when traps have been left out. As a result, the licence requirements now state that the trap must be rendered incapable of catching or holding birds when not in use, examples of how to do this include securing the door in a fully open or closed position or removing the doors completely. Please remember that if a top entry trap is left with its doors closed, a heavy bird or animal landing on one of them could force the door down and trap itself.
We recommend that wherever possible, traps that are not in use are taken in for storage. This prevents the risk of accidental captures, avoids the chance of someone else setting or vandalising the trap and extends its working life.
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