The amount of damage birds do, and how to reduce it
The Bird Species Responsible
It is not easy to know how much damage is really caused by birds. In Japan, the Ministry of Agriculture, Forestry and Fisheries carries out a study each year. The major pest birds are: two species of crow ( Corvus spp.), the carrion crow and the jungle crow; tree sparrows ( Passer montanus); ducks (mainly spotbilled ducks, Anas poecilorhyncha); brown-eared bulbuls ( Hypsipetes amaurotis); grey starlings ( Sturnus cineraceus); and two species of doves and pigeons.
Crows are by far the most harmful, in terms of both the amount of produce they eat and the area damaged. The range of crops damaged by crows is very wide, including watermelon and fruit vegetables. Not only the ripe fruit is damaged, but seeds are often eaten after sowing. The area damaged by sparrows is almost as large as that damaged by crows. This is because sparrows eat mainly the ripening seeds of rice, and rice fields cover a large part of the agricultural area of Japan. In terms of the amount of crop damage, brown-eared bulbuls come second after crows. They eat citrus fruits and vegetables during the winter. Pigeons and doves damage legumes, while grey sparrows damage Japanese pears and other fruits.
Increase in the Number of Birds
Between 1974 and 1991, the amount of damage caused by sparrows in Japan showed a sharp drop. Since the main crop eaten by sparrows is rice, this probably reflects the decline in the area of paddy fields over that period. Damage by other birds increased, however, especially by the brown-eared bulbul. On the whole, crop damage by birds in Japan is tending to increase.
What are the reasons for this? First of all, the number of birds is increasing. Many farmers are using combines to harvest rice and wheat. Quite a large number of grains reaped in this way are left behind in the field. This gives birds an abundant and high-quality food supply that contributes to the increase in numbers, and keeps it stable.
Furthermore, many farmers are beginning to plant rice by direct seeding rather than by transplanting. The sown seed is a food resource for ducks if the paddy fields are flooded, and for sparrows and pigeons if the fields are drained.
In some cases, damage has occurred to new crops. One example is the brown-eared bulbul, which began to eat the leaves of various kinds of leaf vegetables. Bulbuls were formerly migratory birds, which overwintered in the southern part of Japan and bred in the mountainous and northern regions of Japan. In the 1970s, they became year-long residents and began to cause severe damage to winter cabbage and other leaf vegetables. In the case of Japanese pear, bird damage became much worse after the introduction of new varieties such as Kosui, which have a higher sugar content than traditional varieties.
Sometimes a new pest bird species appears. An example is the Chinese bulbul (Pycnonotus sinensis) which appeared in Okinawa for the first time in 1976 and began eating the leaves and fruit of vegetables.
How to Scare Birds Away
Japanese farmers use many methods to protect their crops from birds. The most effective way of doing this is to cover the field with netting. However, nets are costly and cannot be used in large fields. Another method is to reduce the bird population. However, all wild birds are protected in Japan, and cannot be hunted without special permission from the government.
For these reasons, the most common method of protecting crops is to scare birds away. Stimuli which rouse aversion in the birds are used to drive them away from fields. These stimuli can be classified into two main groups, visual and acoustic.
These include plastic bags, flags and streamers, and bird-scaring tapes. Plastic bags and flags set out in fields will flutter in the wind. They not only drive away birds already feeding in the field, but deter birds on the wing from coming down to land. Streamers made out of plastic bags are effective in driving away brown-eared bulbuls which eat vegetable leaves.
Since bird-scaring tapes are only effective when they reflect the sunlight, they are not useful before sunrise. Their effectiveness rapidly diminishes if there is no other source of food for the birds near the field.
Since ancient times, scarecrows and effigies have been used to frighten birds away from crops. Recently, the mannequins used in shop windows have been used as scarecrows. They are very effective at keeping pigeons and doves away from soybeans. Models of eagles are also used, but they do not seem to be very effective.
Some years ago, farmers began using balloons with eyespots painted on them. These were based on the fact that birds often show an aversion to eyespot patterns on wings or on the larvae of various insects. However, our experiments showed that although the balloons were effective at first, birds quickly habituated to them.
Another widely used method is to hang up the dead bodies of crows and other birds in fields. However, this does not have much effect.
Devices which scare away birds with a sudden loud sound also have a long history. In the past, Japanese farmers often used Naruko. These were wooden clappers that were banged together by pulling a string, or by the wind.
In general, a device to scare birds with sound is effective over a wider area than one which uses visual effects. How-ever, this also means that the noise disturbs a wide area. Many of the noise-producing devices are also rather expensive.
Bangers and bird calls
The device most commonly used by Japanese farmers is a banger, which produces a loud noise from an explosion of gas.
Recorded distress calls and alarm calls of birds are often used to frighten birds away. Artificially synthesized aversion sounds are also used. They are amplified and broadcast through a loudspeaker.
The razzo is a bird-scaring device made in Germany. It shoots a wing-shaped object into the air, propelled by an explosion of propane gas. It combines at the same time an acoustic stimulus (the explosion) with a visual one (the movements of the winged object). The radius of its effective area is 50m or more in fields where visibility is good. It is very successful when used against pigeons and doves eating soybeans in open fields. However, in orchards where visibility is limited, it has a visual impact only on birds which are close at hand. Only the explosion reaches more distant birds. Under these circumstances, its effective area is limited.
Other Stimuli Used to Scare Birds
Transparent fine string stretched tightly over fields or rubbish dumps is sometimes used to keep crows away. This works because crows seem to hate their wings to touch the string.
A number of magnetic devices have recently been used in Japan. However, although birds are known to be able to sense magnetic fields, there is no evidence that magnetic fields repel them.
Damage to soybeans by pigeons and doves is less when wheat or barley is being harvested. If soybeans are sown at this period, there is reduced damage by birds. Damage can also be reduced by covering sown soybeans with a straw mulch. Deep sowing is also used to prevent damage to corn sprouts by crows.
A number of methods are used to keep birds away from crops. However, all these methods have an important defect. Birds quickly become accustomed to them. The only solution to this problem of habituation is to change tactics, and never rely on any one tactic for too long.
While birds eat and damage crops, they also eat insect pests, and play a role in preventing pest outbreaks. We should not try to reduce the number of birds too much. The best strategy is to drive birds away from fields in the most effective way possible, while allowing some birds to remain.
Index of Images
Figure 1 Balloons with Eyes Protect Rice Field
Figure 2 Traditional Scarecrow
Figure 3 Old Shop Mannequin Used As Scarecrow, Japan. Birds Fear These More Than Ordinary Scarecrows.
Figure 4 Black Plastic Streamer Helps Keep Bulbuls Out of Vegetable Field