7 October 2000

Repel the raiders

In the first of a new series on furred and feathered fiends, Tom Allen-Stevens sets his sights on pigeons.

THERE are formulations which rid crops of aphids but leave the bee population intact, chemicals that will cripple the feeding mechanism of a slug and others that can home in on a narrow spectrum of weeds. But when it comes to larger pests were often at a loss as to how to control them.

Pigeons cause an estimated £3m worth of damage to crops every year, but a satisfactory way of keeping them off has yet to be found. Thats in spite of a recent HGCA-funded project. Two research programmes have been under way to evaluate reducing the palatability of oilseed rape, and both are now drawing to a close.

The first looked at applying a repellent known as cinnamamide to the crop. The properties of this naturally-derived chemical first came to light in the 1980s. Researchers at Central Science Laboratories (CSL) were investigating bullfinch damage to pears and were interested why some varieties were more susceptible than others. Cinnamamide is closely related to the plants natural defence, and it proved to be the key ingredient. This prompted colleagues researching pigeon damage in oilseed rape to investigate its application.

The aim of the research, carried out by CSL and the John Innes Centre (JIC), has been to find a formulation suitable for spraying on to a crop. "The nature of the formulation has been on trial as much as its efficacy. It must give up to four weeks protection during the winter months," reports CSLs Dr David Cowan.

Field-scale trials have been carried out at various locations around the country, along with the second leg of the research programme: a variety bred by JIC with a high level of glucosinolate. "High levels of volatiles are released by the crop when its damaged, for example by pigeons. This makes the leaves less palatable," explains JICs Richard Mithen.

The results are a mixed bag: when measured in January, cinnamamide-treated plots were found to be less damaged by 25%, compared to control plots. The combination of the cinnamamide applied to the high glucosinolate line resulted in a 30% reduction, compared to untreated Apex. The formulation has also been improved: it will now last for weeks rather than days. But there were no significant yield advantages at harvest.


"The 25% damage reduction is implicitly significant for cinnamamide, but oilseed rape is very good at recovering. One of the difficulties has been that pigeon damage is very difficult to predict, and therefore hard to scientifically measure. Having treated and untreated plots in the same field may also have been instrumental in drawing the birds in – pigeons may not have touched a wholly-treated field," says Dr Cowan.

If neither breeding nor chemistry can offer a reliable deterrent, its a question of scare tactics to shift the birds off your land. But how can you keep them off? CSLs Dr Ian Inglis has been researching the feeding and flocking habits of pigeons on a 1,000ha study site around Newmarket. The key, he says, is to give them a scary stimulus as close as possible to being hunted.

So is the shotgun the best tool for the job? "Shootings still a good scary technique. Its best done in the summer months because then youll hit half of a breeding pair, which will further reduce the winter population."

The snag with shooting is that only a co-ordinated, dedicated approach will be effective at keeping the birds away, and that takes time. Other scarers, such as gas guns and scarecrows, can also be effective, but depend on how they are used, says Dr Inglis. "An effective way of using a gas gun is to put it in or near a fake hide, and put a real gun in there occasionally. Otherwise they get used to the noise and dont find it a threat. This could be labour-intensive, but it works."

Peaceful Pyramids or Flash Harrys and kites can work in the short term. "Theres some evidence to suggest that once birds get used to them, they act as a signal for food," maintains Dr Inglis. Using tick beans as bait, the research team found they could actually attract birds towards the scarers. Pigeons would land next to a flashing pyramid and look for beans, even at sites where no beans had been placed.

As for scarecrows, Dr Inglis has found that those that pop up periodically are better. "Having a scarecrow pop up at the same time as a gas gun going off tends to be quite effective."

ADAS John Garstang agrees that the best way to use scarers is try as much as possible to catch your prey off guard: "That means moving them around. With gas guns its also a good idea to change the time interval between blasts." But he warns against the time interval being too short as this will encourage the birds to ignore the noise.

"The best deterrent is location: in fields next to roost areas, like woodland, youll get them down in droves. Failing that, make sure you have an even establishment."

This is where lo-till drilling can come unstuck: the long stubble in direct drilled crops will help keep pigeons off, but Autocast crops can be patchy, he says, attracting them in. "Try to keep the seedbed as even as possible so that the resulting crop looks as green as possible in winter."

Ironically a lush, green crop over winter often results in too thick a canopy in spring, with resulting yield penalties, reports ADAS John Spink. "It was to replicate pigeon damage that we tried mowing the crop, but found that this actually increased the yield." He warns that pigeon grazing tends to take out the growing point of the plant, however, so is not an effective form of canopy control. "Pigeons are not very civilised birds. Maybe we can train them to graze the crop properly."