Population Limitation in Birds

Ian Newton, Keith Brockie

• Publisher: Academic Press
• Year: 1998
• ISBN:
  9780080879239
• Topic: Technologie et ingénierie
• Subtopic: Élevage d'animaux

This book meets the demand for a comprehensive introduction to understanding the processes of population limitation. Recognized world-wide as a respected biologist and communicator, Dr. Ian Newton has now written a clear and detailed treatise on local scale population limiting factors in birds. It is based almost entirely on results from field studies, though it is set in a contemporary theoretical framework. The 16 chapters fall under three major section headings: Behavior and Density Regulation; Natural Limiting Factors; and Human Impacts. Population Limitation in Birds serves as a needed resource expanding on Dr. David Lacks research in this area of ornithology in the 1950s. It includes numerous line diagrams and beautiful illustrations by acclaimed wildlife artist Keith Brockie.

• Provides a sorely needed introduction to a long-established core subject in ornithology
• Focuses on local scale factors
• Written by a well-known biologist and effective communicator
• Includes numerous line diagrams and beautiful illustrations by acclaimed wildlife artist Keith Brockie

Chapter 1

Preview

This book is about bird numbers, and about why these numbers vary in the way they do, from year to year or from place to place. It is therefore concerned with the various factors that influence bird numbers: with the role of food-supplies and other resources, of predators, parasites and pathogens, and of various human impacts. These are all matters of fundamental interest to ecologists, including those involved with the management of bird populations, whether for conservation, crop protection or sport-hunting. Although birds form a tiny fraction of all animal species – perhaps less than 0.2% of the total – we have no reason to suppose that the numbers of birds are limited in ways different from those of other animals. Yet for some aspects of study birds offer distinct advantages.

In the first place, most bird species are diurnal, and can be readily distinguished from one another in the field. This makes them easy to watch and to count. Indeed, in the breeding season many species obligingly force themselves upon our attention by singing their species-specific songs. Secondly, most birds can be trapped and marked with leg rings or other tags. Such marked birds then remain as identifiable individuals throughout their lives, enabling their movements and life-histories to be followed. Many species, too, are large enough to carry radio-transmitters, facilitating detailed studies of their day-to-day activities. Thirdly, most bird species raise their young in discrete nests. Their individual reproductive rates can therefore be measured in a way that is not possible for most other creatures with no parental care, and in which the young from different parents soon intermingle. Fourthly, the highly developed social structure of birds has enabled the role of territorial and other dominance behaviour in population regulation to be studied in greater detail than for most other animals. All these features combine to make birds popular with their human observers. In some parts of the world, through the combined efforts of birdwatchers, changes in the numbers and distributions of birds have been monitored with precision over long periods and wide geographical areas. For these various reasons, then, the population ecology of birds is perhaps better known than that of any other group of organisms, and the general principles that have emerged have much wider relevance in the science of ecology.

The aim of this book, as stated in the Preface, is to review current ideas on population limitation of birds, illustrating the issues involved with examples from different species and from different regions of the world. For the most part, different aspects of the subject are covered in different chapters. The purpose of this introductory chapter is to provide a preview, outlining the scope and conceptual framework within which subsequent chapters lie.

BIRD NUMBERS

Compared with many other organisms, birds are well-known taxonomically. On recent estimates, there are about 9672 known species of birds in the world (Sibley & Monroe 1990). Each decade, usually through human action, a few species may become extinct, while a similar number of previously undescribed species may come to light.

These bird species vary from extremely widespread and abundant, numbering many millions of individuals, to extremely localised and rare, consisting of a few individuals on the verge of oblivion. Among landbirds, the most numerous species probably include the House Sparrow Passer domesticus and European Starling Sturnus vulgaris, both of which are widespread in the Old World and, through introductions, in the New World too. They also include the Red-billed Quelea Quelea quelea, a seed-eating weaver bird which breeds over much of Africa south of the Sahara. The total populations of all these species have been estimated at more than 500 millions. Nightly roosts of more than a million individuals are regular in Quelea and Starlings, as well as in Sand Martins Riparia riparia in Europe and Red-winged Blackbirds Agelaius phoeniceus in North America. The record for individual roosts, however, is probably held by the Brambling Fringilla montifringilla, whose nightly gatherings in areas of good Beech Fagus sylvatica crops in central Europe have sometimes numbered more than 20 million individuals, as noted at least from the 15th century onwards (Jenni 1987).

Many species of seabirds range widely over the oceans, but while nesting gather in vast colonies at traditional locations, where they can be counted. Perhaps the most numerous seabird in the world is the Wilson's Petrel Oceanites oceanicus of the Southern Ocean which is thought to number more than 50 million pairs, distributed in a range of colonies. The second most numerous is probably the Arctic-nesting Little Auk Alle alle, with an estimated 30 million pairs. However, estimating the numbers of small birds that nest below ground in remote places is fraught with problems, and these figures are little more than guesses. Among more conspicuous seabird species, some individual colonies of Adelie Penguins Pygoscelis adeliae in Antarctica have been found to contain more than a million pairs, while colonies of Sooty Terns Sterna fuscata on some tropical islands may contain up to a staggering ten million pairs. Moreover, some seabirds in their colonies achieve phenomenal densities (Warham 1990). One colony of the Short-tailed Shearwater Puffinus tenuirostris was estimated to number 250 000 pairs on about 21 ha (at about one pair per m²). In the Sooty Shearwater P. griseus, an estimated 2.75 million occupied burrows were found on 328 ha of the Snares Islands (about 1.2 burrows per m²), and in the Great Shearwater P. gravis, two million occupied burrows were found on 200 ha of Nightingale Island (about one per m²). Even these figures are low, however, compared with those for a Galapagos Storm Petrel Oceanodroma tethys colony, in which 200 000 pairs occupied an area of 2500 m², giving a mean density of 8 pairs per m² (Harris 1969).

At the other extreme, some landbird species, restricted to small oceanic islands, have naturally small populations, consisting in some cases of fewer than 100 breeding pairs. This is especially true of some birds-of-prey, which live only at low densities, and have probably never been much more numerous than they are today. But most rare species owe their present status to human action. More than 1000 bird species (about 11% of the total) now give cause for concern, and in recent decades the populations of several have persisted for some years, at fewer than 20 individuals (Chapter 16). Extreme examples include the Mauritius Kestrel Falco punctatus (once reduced to four individuals) and the Chatham Island Black Robin Petroica traversi (reduced to seven individuals), both of which have since increased as a result of conservation measures (Butler & Merton 1992, Jones et al. 1995). For if rare species are protected, they usually recover again, providing that suitable habitat remains.

The overall abundance of any bird species depends on the extent of its geographical range, the amount of suitable habitat within that range, and the mean density achieved within that habitat. In many habitats, small birds tend to be more abundant than large ones. In various natural bird communities in North America, for which complete censuses had been taken, the most numerous species were those weighing around 40 g, and maximum densities dropped markedly in species that were either below or above this weight (Brown & Maurer 1987). The relationship held independently among species in different dietary categories, including herbivores, nectivores, omnivore-insectivores and carnivores. It also held among birds in Britain, but again only in broad terms: among birds in all weight classes some species were rare (Blackburn & Lawton 1994). Body size is thought to influence density partly because it affects resource needs. Individuals of larger species require more food and space, so must live at lower densities than small ones. Larger species also eat larger food-items, on average, and larger prey occur at lower densities than small ones, the link between body size and density occurring among a wide range of organisms and not just among birds (Blackburn et al. 1990, 1992). In fact, body size is related to so many aspects of species biology that it is hard to tell which are most relevant to density.

A broad relationship ...