Bird Populations and Distribution
Despite the fairly large number of professional ornithologists and ecologists now scattered around the world, and the ever-increasing army of amateurs who volunteer to help with organized counts and records, the study of bird numbers and distribution could never be highly accurate. With wild, mobile subjects and widely varying coverage of often remote and inaccessible areas, the ornithologist must be content with his best estimate. In addition, local populations often fluctuate greatly through migration, seasonal food supplies, breeding success, natural disasters, pollution and habitat destruction.
Number of Species in the world
There are thought to be between 8,600 and 9,016 species in the world. A precise figure could never be given for new species continue to be discovered in remote or little-known areas while the status of others is very vague, sometimes bordering on extinction.
Some 265 species were listed as threatened by the ICBP (International Council for Bird Preservation) Red Data Book Endangered Birds of the World (2nd ed rev 1974—9) compiled by Warren B. King. But this is already very out of date and a third edition is being prepared. Added to this there is constant reappraisal of the classification of species by taxonomists and there has been a tendency for some widespread forms to be subdivided. During the years 1980—5 at least five checklists of the world’s birds were published and a study of these reveals some of the disagreements encountered in determining species and sub-species.
Number of species in Britain
Image Coursey: researchgate.net
Of some 232 species recorded as breeding in Britain, 207 are regulars and 25 irregulars, but these are likely to be added to in the near future as an ever-increasing number of birdwatchers report further colonization from neighbouring countries. On the other hand, others may be lost through long-term climatic or habitat change.
Including all breeding species, regular visitors and vagrants or ‘accidentals’ (irregular visitors), the British list stands at about 540, with records of some five further species awaiting official acceptance.
Total number of birds in the world
The question of how many individual birds are alive on Earth at any one time is yet to be answered with any certainty. It is hard enough to census or estimates the populations of individual species, except those which are very restricted in range and sedentary in habits, and the smaller and more secretive a bird the harder it is to study. In addition, there is tremendous seasonal fluctuation, hitting a low before hatching and peaking at the end of the breeding season, which itself varies according to geographical position.
At times of maximum numbers, juveniles would outnumber adults several times, thus giving a false impression of population security as many of the young die in their first winter. The annual death rate of adult passerines is 40—60%, but the percentage of eggs which reach full adulthood averages only about 12%.
Yet man remains fascinated by such matters, and to be able to gauge population trends reasonably accurately is a good way to keep an eye on the health of the world’s environment. Thus in 1951, Fisher made an educated estimate of 100,000 million birds, give or take a few hundred million. However considering the suggested large populations of a small number of species alone, it is tempting to suggest that Fisher’s estimate is extremely conservative.
Since then mankind has made great inroads into bird populations through tremendous habitat destruction and pollution, and to a lesser extent through more efficient killing for food and sport. This has been balanced to a certain extent by our introduction of some very prolific species into favourable habitats around the world and incidental assistance for some species provision of rubbish-dump food for gulls, lawns for blackbirds and road-casualty mammal carcasses for magpies and other scavengers. But it has to be suggested that the huge scale of habitat destruction must mean a steady overall decline for the world’s bird population.
Total Number of Birds in Britain
It would be hard to think of a country whose bird population is more variable than that of Britain for its geographical position brings huge numbers of passage migrants. And just as many come from most points of the compass to spend the winter in Britain because of its unusually mild coastal maritime climate, while many millions of Britain’s predominantly insect-eating summer visitors must go south in winter.
In addition, being an island with a long and intricate coastline, Britain is the base for innumerable sea-going birds, those which breed in Britain being relatively easy to monitor at their colonies.
Despite the problems, several attempts have been made to estimate the British summer landbird breeding population. In 1932 E. M. Nicholson suggested 80 million, and in 1939 James Fisher arrived at 100 million, upping it to 120 million in 1946. In 1974, using figures available from annual census returns and Ministry of Agriculture land-usage statistics, Fisher and James Flegg arrived at a figure of about
134 million for 1972.
In the last 10 years the army of amateur ornithologists has grown rapidly and through their co-ordination and enthusiasm future counts should become more and more accurate. Much of their work will revolve around the BTO’s (British Trust for Ornithology) Common Birds Census, and it must be remembered that only 100 of our 500-plus listed species contribute significantly to numbers. Some 50 of these make up about 75% of the total population.