The role of birds in nature refers to the position and role of birds in different ecosystems (such as forest ecosystem, grassland ecosystem and farmland, lakes, oceans and other ecosystems). Birds are important members of the ecosystem. Although they may not have a significant impact on productivity, they have density restrictions on the prey they eat. In addition, birds are also responsible for transporting seeds and nutrients, participating in the energy flow and inorganic circulation in the system, and maintaining the stability of the ecosystem.

Birds prey on insects and small rodents, and their role in maintaining the ecological balance of nature cannot be directly perceived, because the environmental factors involved are very complicated. Only through strict experimental design, heavy field investigation, laboratory work and careful analysis can we get more correct results. In recent years, the quantitative study of bird predation and the study of bird predation characteristics at home and abroad have properly revealed the role of birds in the ecosystem, especially the position and role of insect-eating birds in the forest ecosystem.

First of all, the predation of birds

Birds have strong flying ability, high speed and wide range of activities. The migration of birds, the territory in breeding season and the cluster after breeding season make the bird community structure very complicated, and the species and quantity fluctuate greatly. This gathering and movement of bird population can make its number soar and "clean up" or "destroy" some prey in some areas.

Because birds eat a lot and metabolize quickly, most insect-eating birds don't roar like food-eating birds, and the speed of food passing through the digestive tract is also very different, which is closely related to physiological needs, feeding methods, feeding seasons, bird's age, bird's state and the nature of food itself. The difference of food digestibility may make the ratio of some foods overestimated or underestimated in laparotomy. In order to compensate for these differences, the correction coefficient (Table 1) should be adopted when estimating food composition. The species and quantity should be identified according to the characteristics of insect remains, capsules on the head of larvae, shields on the chest and buttocks, residues on the chest and abdomen, and residues on the upper jaw.

Table/kloc-Estimated consumption of 5-6-year-old Picea crassifolia by birds at 0/20th day larval stage and15th day pupal stage (only per hectare)

(From Crawford et al., 1983)

Any bird, even at the same time, will never only feed on the same insect. Its food composition includes not only harmful insects, but also beneficial insects and some insects parasitized by other algae, which increases the complexity of analyzing the inhibitory effect of birds on harmful insects. However, some recent research results show that birds tend to avoid parasitic insects. In this way, birds and parasitic insects may be complementary rather than competitive. There are also some birds that show a preference for certain pests or insects, such as cuckoo's preference for caterpillars of all sizes. Some birds like to eat adults, which is very important in pest control, because the death of adults often has an important impact on the next generation of population.

The food eaten by birds has obvious seasonal changes. When food is scarce in winter, many insect-eating birds turn to plant food. The food types of birds have also changed in different stages of their life history. In the early days, even birds that feed on grain must feed their chicks with animal foods such as insects to ensure their normal development and survival. Generally speaking, the types, states and periods of food that birds prey on are consistent with the food resources that nature can provide, which is the result of long-term evolution and adaptation of birds and the environment.

Birds can affect the number and dynamics of insect populations, and similarly, the species and density of insects also have an important impact on birds' predation. Most birds have functional and quantitative responses to prey population density. The former refers to the behavior changes caused by predators trying to attack more prey when the prey density increases. The latter is the change of predator number caused by the increase of prey density. Changes in numbers can be caused by reproduction or "invasion". The functional and quantitative responses of birds to insect density can be summarized as "the direct influence of birds on insects". Birds can also indirectly affect the insect population, mainly by changing the micro-habitat of prey, making it more sensitive to weather, parasitism and possible diseases and viruses.

It is very important to understand the predation characteristics of birds and the interaction between birds and prey populations for understanding the status and role of birds in the ecosystem. Whether or not the density of main prey is suppressed by birds' predation is also a very necessary parameter for comprehensive management of harmful insects.

Second, the control effect of birds on forest pests

Birds play an important role in preying on many forest pests. Limited by space, we only give some noteworthy examples to illustrate the role of birds in preventing and controlling forest pests in various fields.

(a) Direct influence of birds on leaf-eating pests

Dendrolimus punctatus is the main leaf-eating pest in China. The investigation in Anji, Zhejiang Province shows that birds in different periods and plots have different predation effects on Dendrolimus punctatus (Table 4). In the breeding season and forest edge, the predation effect of birds can reach 18.5% and 22.2%, while the predation effect of other plots is only 4.7%. After the breeding season, the predation of the same forest edge decreased to 8.7%, which was about 1/3 of the breeding season. In Zhangpu County, Fujian Province, the predation rates of insect-eating birds on Dendrolimus punctatus larvae over 4 years old were 65438 09.73% (3rd generation) and 48.75% (4th generation), respectively. Among the predatory actions of various natural enemies, insect-eating birds have the greatest control effect on Dendrolimus punctatus population. By studying the fecundity of great tits, the food composition of chicks and the proportion of Dendrolimus punctatus in food, it was found that the proportion of Dendrolimus punctatus in chicks' food changed with the periodic change of population density from high (1985) to low (1987) and then gradually increased (1988) (Table 5). It is worth noting that when the population density of Dendrolimus punctatus is low, although the breeding population density of great tits, the number of brood eggs and the proportion of Dendrolimus punctatus pieces in young food also decrease correspondingly, the predation effect is increased by 1 1.24 times compared with that when the population density of Dendrolimus punctatus is high.

Table 4 Number of Dendrolimus punctatus inside and outside the bird pen (from Chu, 1987)

The proportion of Dendrolimus punctatus in the daily average food of great tits in the initial incubation period.

(From Chu Guo Zhong, 1989)

Table 6 Annual changes of growth parameters of newly hatched chicks of great tits

(From Chu Guo Zhong, 1989)

Picea asperata is the most important leaf-eating pest in spruce fir forest in North America. It is estimated that birds consume about 13% of larvae in the descending stage after the outbreak of leaf roller. When there are 1235000 ~ 247 1000 leaf rollers per hectare, the control degree of birds is 3.5 ~ 7%. According to the analysis of stomach contents, it was found that 40% of the food in birds' stomachs was rice leaf roller when a large number of rice leaf rollers occurred. The results of four years' analysis showed that the proportion of leaf roller moth in the stomach of birds was 7 ~ 46%. When the leaf roller moth occurs, birds have both functional and quantitative reactions. Birds that used to feed on the ground and tree trunks and eat grass seeds changed their eating habits, such as orange thrush, rushes, etc., and woodpeckers and woodpeckers began to eat the eggs and pupae of leaf curl moths. Several kinds of warblers show the greatest quantitative response. Although the number of some warblers decreased, the number of chestnut-breasted warblers (tree warblers) increased by 12 times, and that of orange-breasted warblers (brown tree warblers) increased by 9 times. Of course, when it happens in large quantities, the density of leaf roller moth increases by 8000 times.

When the moth (Choristoneura pinus) was 4-5 years old, many birds changed their eating habits and began to eat this rich food. Predation occurs after mature larvae until pupa and adult stages. In the second year, a large number of blackbirds entered the 240-hectare plot. At that time, there were about 24,700 Dendrolimus punctatus per hectare, and the predation of birds increased from 40-45% in the first year to 60-65% in the second year. Tits eat only 0.3 ~ 2.6% of the crown gall midge larvae, but they eat about 65,438+00% of emerging adults, mainly females, so about 20% of adults are preyed. In Hungary, domestic sparrows and tree sparrows can kill more than 98% of adults, which may play an important role in controlling the American white moth. In Japan, tree sparrows and gray sparrows kill about 40 ~ 50% of American white moths that appear before dusk. Because the mating activity of this moth occurs between sunset and sunrise, the death caused by bird predation may have an important impact on the population dynamics of this insect. When someone is studying tits and chrysanthemums, it is found that tits regularly feed on the larvae of young forest cone moth (Ernarmonia conicola), and the predation intensity varies with the density of larvae in the cone, which seems to have a critical value (every 50 cones 10 larvae). Exceeding this critical value is beneficial for birds to prey. Although the great tits may destroy more than 50% of the overwintering mature larvae, the population will still increase in the second year.

(2) The direct and indirect effects of birds on bark beetles.

Woodpeckers are the main natural enemies of bark beetles. There are mainly hairy woodpeckers (Dendrocopos villosus), velvet woodpeckers (D. pubesens) and three-toed woodpeckers (Picoides tridactylus). They show functional and quantitative reactions. Quantitative reaction is particularly important, especially in local areas. According to records, the density of woodpeckers has increased by 50 times in a burned forest land, which sometimes exceeds the increase of bark beetles in a small forest land. When the red-winged beetle (ru-fipennis) generally occurs, woodpeckers eat 20-29%. From medium density to high density, woodpeckers can kill 24-98% beetles by feeding themselves and other means (such as bark drying). Woodpeckers' foraging activities can promote bark drying. In addition to woodpeckers, there are 8 species of birds that prey on Tomicus piniperda 10% adults. The number of woodpeckers preying varies from 5% to 86%. The results reported in different regions are so different, which may be due to the different climate, outbreak degree and process, and the number of generations that occur every year, so the feeding activities of woodpeckers have different effects. After the outer bark is pecked off, the remaining bark will dry quickly and be more susceptible to bad weather and diseases. This mortality rate is often greater than that eaten by woodpeckers.

There are 4 families, 1 1 species of birds feed on the short-billed dragon. Six species of birds (excluding woodpeckers) prey on adults, and the predation rate is between 8-26% in three years. Hairy woodpecker, velvet woodpecker, North American black woodpecker (Dryocopus pileatus) and white-headed woodpecker (Picoides albolervatys) are the main predators of Dendroctonus valens larvae in central Sierra, California, USA. At the beginning of the outbreak, the predation rate was 32%. The number of woodpeckers has increased in the big occurrence area. Initially, because of immigration, and later because of the death of trees, the availability of woodpecker nests and habitats was increased. When woodpeckers look for bark beetles, they need to peel, pierce or cut holes in the bark. There are a considerable number of beetle eggs and about 58% larvae in the bark stripped by woodpeckers, and they all die in winter. After being pecked by woodpeckers, the bark becomes thinner, and the temperature and humidity are very different from the original bark, which reduces the protective effect of bark on the remaining bark beetles and creates spawning opportunities for parasites with short spawning period. All these indirect effects increased the death of bark beetles (see figure).

(3) The direct influence of birds on stem borers and weevils.

Woodpeckers are also the main natural enemies of stem borers. The predation rate of tiger skin beetle larvae can reach 32% and 65%, and that of Zelkova schneideriana larvae can reach 39%. The predation rates of longicorn beetles were 65438 03% and 65% respectively at two investigation sites. It was also found that 98% of the food of the spotted woodpecker was longicorn larvae, which could reduce 12%. Woodpecker is also the most important natural enemy of Syringa wasp, which can cause 67 ~ 8 1% death. In the Netherlands, woodpeckers have functional and quantitative responses to pine elephants. When the woodpecker population is low, the woodpecker's prey is small. With the increase of woodpecker larvae, 95% of larvae and pupae may be pecked by woodpeckers, especially in the food shortage stage. However, wood beetles on tree roots often cannot be hunted.

(d) Transmission of avian and insect pathogens

It has been proved that birds can spread insect pathogenic viruses by eating infected insects. Among 49 piles of bird droppings collected from trees, 44 piles were infectious, and 89.9% larvae died of NPV infection. Birds are passive carriers of the virus. In winter, the virus can spread to at least 6 kilometers away from the center. Of course, due to different climatic conditions, not all bird droppings infected with insects are active.

In a word, the forest is an important place for birds to live. Both birds and pests are members of forest ecosystem, and after long-term evolution and natural selection, a complex predation system has been formed. Birds are an important part of predator-prey system. Random predation by birds can play a functional control role in the ecosystem and play an important role in the population dynamics of many forest insects. Birds are not only the direct death factors of pests, but also indirectly affect pest populations by spreading insect pathogenic microorganisms or changing micro-habitats to affect parasites and predators. When the population density of pests is low or increasing, the effect of birds on reducing the number of pests is much more obvious than when the population density is high. The main function of birds is to block or prevent the occurrence of pests, or to extend the interval of occurrence. However, birds alone cannot disintegrate the pest population that occurs in a considerable range. When pests occur, the reproductive potential, feeding habits and territorial relationship of birds will reduce their influence as predators. If the occurrence of pests is small, birds can also play a control role due to the invasion of resident birds or the inflow of large groups of migratory birds, which can accelerate the decline of pest populations that have reached the peak.

Third, other functions of birds.

Raptors (eagles, owls, falcons, eagles, etc.). ) often feed on rodents in forests, grasslands and farmland. Some crows and shrikes can also prey on mice, and together with other natural enemies, they can suppress the number of mice. In a well-structured forest environment, natural enemies can restrain and delay the sharp increase in the number of rodents, and also reduce the density of increased rodents. Someone found the bodies of 1348 mice in the stomachs of 360 owls. Someone studied 19 000 owl food balls and found 46 179 small mammals, almost all of which were mice. A five-year study on the feeding habits of the long-eared owl confirmed that its main food was mice, and only 1.3% was birds. Through the analysis of the food residue of the wintering long-eared owl in Wuchang, Hubei Province, it was found that 70.3% were small rodents, mainly Apodemus agrarius. From 1982 to 1986, 923 pieces of food ... and 29% were collected in 58 moire nests.

Many birds, especially vultures, owls and other raptors, as well as seagulls and crows, have the habit of eating carrion. They have made special contributions to eliminating sick animals and carrion and eliminating organic pollution of the environment. Birds can eat insects that parasitize the larvae inside and outside livestock, and starlings and tick-eating birds can alleviate ticks and other parasites that harm livestock and wild animals.

Many birds are pollinators of flowering plants, especially some tropical birds, such as hummingbirds, bees, sunbirds, woodpeckers, embroidered birds, snipes, toucans and parrots, and are often important pollinators of some flowering trees and shrubs. Without these birds, the ecological balance of nature may be seriously damaged. In addition, many birds store seeds. Jay usually hides cones under leaves, moss and rocks. A group of Jay (about 30-40) once collected 200,000 pine cones and took them away from 1 km. In the low-humidity forest in the Malay Peninsula, there are 7 species of pigeons that feed on at least 22 species of banyan fruits, and green doves specialize in eating the seeds of fig trees, which are spread far away by excreting feces. Birds can spread seeds for a long or short distance, and many migratory birds still have viable seeds in their digestive tract, which may be a little farther. It has been proved that some hard-shelled plant seeds germinate more easily after passing through the digestive tract of birds. Some birds that feed on insects, such as woodpeckers, thrushes, tits and other important seed-spreading birds. Oak woodpeckers (Melanerpes formicivorus) dig holes in various trees and wooden poles and fill the holes with nearby cones. Not all the cones stored by birds can be found again by them, and these forgotten fruits are often a cause of forest expansion.