Ecology of Birds: Population
Another aspect of ecology is the study of factors which promote or limit the growth of populations. Sometimes fluctuations occur when, for various reasons, populations may increase rapidly and then a hard spell of weather may reduce the numbers appreciably. Sometimes fluctuations do occur when, for various reasons, populations may increase rapidly and then a hard spell of weather may reduce the numbers appreciably.
The fluctuations in the English populations of the Grey Heron have been studied since 1928, initially by Max Nicholson and local naturalists and more recently by members of the British Trust for Ornithology. Virtually every major slump in the Heron population has been linked with an excessively hard winter in which snow and ice have covered the pools and ditches in which they fish. After these slumps numbers increased again rapidly and then tended to flatten out at the previous level, where the size of the Heron population matches the availability of the natural resources. The census of the Grey Heron is one of the longest studies of a population ever made and what has already been discovered indicates its value. The study of any population or community of birds over many years in almost any habitat, would be of great value.
The numbers of some birds of prey and game birds, especially in the Far North, rise and fall in fairly regular cycles which are often linked with the cycles of abundance of the prey species. The Lemming, which is a small rodent, and which is preyed on by the Rough-legged Buzzard amongst others, becomes very abundant every three or four years. When the Lemming numbers crash the numbers of their associated predators crash too, which is when Rough-legged Buzzards may invade Britain in relatively large numbers. Incidentally this illustration emphasizes the point that it is the numbers of the prey that controls the numbers of the predators and not that the predator controls the numbers of its prey.
It used to be thought that numbers remained stable because the reproductive rate of the species was adjusted to its mortality rate. But Dr David Lack has shown in his book, Natural Regulation of Animal Numbers, that the reproductive rate of birds is probably the highest of which they are capable under the conditions in which they live. This means that a pair raises as many young as can be fitted into the time of the year when food is sufficiently abundant for raising young and that they lay on average the number of eggs which will give rise to the number of young that they can feed.
Many species of birds are able to adapt their clutch size to a particular condition. For instance, they may lay larger clutches at certain times of the year or larger clutches in some habitats than in others, or in some years than in others. The largest clutch size in the Wheatear tends to be laid in the first week in May which is normally the week when the largest number of clutches are laid. This is presumably because birds which laid at other times have been culled by natural selection because their offspring failed to survive. Similarly, Wheatears on the island of Skokholm, which is covered in grass, laid on average more eggs than a population of Wheatears on the stony shingle of Dungeness. The reason for this was probably that the Wheatear population of Dungeness had been artificially increased by the RSPB warden, Herbert Axell, who had placed nestboxes on the shingle especially for them. However, on the shingle there was very little grass available on the roots of which the moth caterpillar, one of the main items of young Wheatears, could actually feed.
Whilst birds raise as many broods as can be fitted into the period when food is sufficiently abundant for raising young, what they actually achieve depends on the age that they become sexually mature and how long they survive. Some birds become sexually mature in the year after they hatch, but gulls may take three years and eagles up to six years to reach maturity.
Small passerines that we see in our gardens tend to have an expectation of life of only about a year from the time that they are a month old. It is easy to assume that the Robin that you see and know so well is the bird you have known for years, whereas you may well have had a different one every year.
Birds die in a number of different ways, and jointly these factors, particularly those which are density dependent, counteract the fecundity of the species so that within the small fluctuations that occur the bird population remains relatively stable. Density dependent factors become more effective as the density of the population itself increases. The most likely causes of density dependent mortality are predation, lack of food and disease. An example of a density independent factor would be the mortality of young birds in the nest because of very heavy rain.
Predation, which is the killing of an individual of one species by the individual of another, may well be the most important mortality factor. Birds of prey and owls are most often thought of as predators but Magpies and crows may also be predators on other birds, especially during the nesting season. Foxes, rats, stoats, weasels, cats and man also join the band of creatures which are generally flesh-eaters and which feed on birds. Whilst predation may be rather final for the victim, it may well help the species as a whole, for generally weaker individuals tend to be killed.
Nests with eggs or nestlings particularly of those species which nest on the ground are very vulnerable. Even more hazardous are the lives of young birds a month or so after thay have left the nest and for many small passerines this is the period of heaviest mortality.
Although this mortality is not entirely due to predation. A shortage of food in the breeding season may well affect the number of young reared. A classic example is provided by the way some birds of prey stagger their laying in order that the eldest and strongest chick can survive in times when food is scarce. Occasionally when food is really short some birds of prey may not even nest.
Parasites can be another density dependent factor. The chief ectoparasites (those which live on the outer parts of the host’s body) are biting lice, fleas, blue bottles, flat flies, louse flies, ticks and mites, many of which overwinter as eggs or larvae in nests. Some lice and mites eat at the feathers, the remainder suck blood and if they are present in a nest or on the bodies of the nestlings in very large numbers they can kill or seriously weaken the nestlings. Furthermore, some of them transmit diseases. One feather mite regularly denudes the head or neck of Blackbirds in late summer. Whilst it could be said that probably more than 80% of the birds in the world have one or more types of ectoparasite, a large number also carry internally some microscopic organisms in the blood, such as protozoa, fungi, bacteria and viruses, known collectively as endoparasites. One of the commonest protozoan diseases is probably avian malaria, and it has been estimated that 97% of wild birds may be infected with it.
Another common endoparasite is the mould Aspergillus which attacks the windpipes, lungs and air sacks of birds, particularly ducks and causes heavy losses especially during wet weather. The contagious disease ‘foot pox’ causes wart-like growths of the feet and the base of the bill of the House Sparrow which may result in the loss of toes or feet.
Where birds feed regularly at bird tables they may become infected with Salmonella organisms which are transmitted through bird droppings. They can cause widespread deaths and it is important to see that your bird table is properly sterilized at least once a year. Avian tuberculosis is also transmitted through bird droppings and is another common cause of death amongst wild birds, particularly the larger ones. Many of these diseases are better known in domestic poultry; this is certainly true of Fowl Plague and Newcastle Disease which are often collectively known as Fowl Pest. The virus for Newcastle Disease has been isolated in a number of wild bird species which seem to be able to harbour it without ill-effect.
The density independent causes of death are legion. Whilst they are not always important in terms of affecting population, nevertheless the birdwatcher-naturalist could usefully record these deaths in the same way that he records the individual items of food eaten by a bird. Such records, if made consistently, could in time produce useful quantitative data.
Monitoring of the road deaths of birds several years ago showed that deaths of birds occurred most frequently when adults were feeding young in the nest, and at places where road verges were narrow or non-existent and the hedges came close to the tarmac – in fact deaths occurred most frequently in those parts of the road which were also dangerous in human terms.
The effects of oil which has been accidentally spilled or has been illegally discharged at sea and which can kill many thousands of birds and other animals is a continuing menace. Real catastrophes occur when one or more of the supertankers have an accident in an area inhabited by large numbers of seabirds and it is sad to think that such accidents continue to happen. The Amoco Cadiz disaster is proof enough.
The effect of polychlorinated biphenyls (PCB’s), which was noticed recently by the Beached Bird Survey, and the other closely related chlorinated hydrocarbon pesticides, such as Dieldrin, Aldrin, Heptachlor and DDT, has been to kill considerable numbers of birds throughout the world. So much so that ornithologists became seriously concerned over the decline in the populations of many species of birds of prey.
This sort of pollution by man-made chemicals, although in some respects gradually being brought under control, is inevitably going to recur as new products are released with inadequate testing and accidents happen. Wildlife once more will remind mankind what it is doing to the planet.
Natural events such as the various manifestations of the weather can cause the deaths of birds. A very wet spring can drown nests and eggs in holes, and can make the search for food and the feeding of the young such a daunting task that the parents give up. At the other extreme drought may also affect the breeding success of birds and their survival, even though comparatively few species need liquid water to drink and obtain the moisture necessary to them through their food. Another factor is that the ground may become too hard for thrushes to catch worms. P. & E. Willson, who carry out a Garden Bird Feeding Survey suggested in B TO News No. 84 that a marked decline in the number of birds coming to their feeding station in the autumn of 1976 could have been due to that summer’s drought. However, it could just be that the birds did not need to come to the feeding station as the weather was so mild. While on migration strong winds can divert birds to places a great distance from their normal wintering areas. Over a long stretch of the sea contrary winds can exhaust them: I have seen Starlings dropping exhausted into the North Sea only a few hundred metres from the Norfolk coast after battling from the continent against strong westerly winds.
Birds fly into wires, buildings and window panes. They usually fly into windows when they can see light at the other end of a room, presumably as they are under the impression that they are flying through and under cover.
Some causes of death are bizarre. I once rescued a Storm Petrel, which is a bird which only comes to land at night during the breeding season, which had flown onto the upper burrs of a Great Burdock and if I had not released it, would ultimately have died or been killed by some other animal as it hung there.
I have suggested you should record wherever possible the causes of deaths. Try to find out if some use can be made of the bodies of birds. Most museums like good skins so that they have no need to kill others for educational purposes. Scientists wanting bodies of birds, which have died or been accidentally killed, for chemical analysis, publish their requests in Bird Study, Birds or British Birds.
But, if you can find no one who has any scientific need for these bodies, see what you can learn from them yourself before you dispose of them. Look at and sketch them in your notebook; draw them carefully, having measured with a pair of dividers the shape and width of the various parts of the bird. How curved in the beak? Look at the lie of the feathers. How fresh are they? Was the bird moulting? You could also look for feather parasites, but if the body is cool most of them will already have deserted.
Then, with a sharp knife – a scalpel and surgical scissors are better still -you can open the bird up, identify the various internal organs, bones and muscles. If you do this often enough and learn what the inside of a bird looks like under normal conditions you may get some idea of the cause of death. There are great opportunities to learn from accidents.