The terms resident and sedentary are usually applied to birds that occupy the same general areas year-round, and to populations that make no obvious large-scale movements resulting in changes in geographical distribution. The term migration is less easily defined because it means different things to different people. Ornithologists tend to use the word only for return movements between breeding and non-breeding areas, but biologists working with other organisms often use the term more widely. For purposes of convenience in this book, I shall divide bird movements into six main types:
• First, there are the everyday routine movements centred on the place of residence, which occur in all birds, whether classed as resident or migratory. Typically, they include the flights from nesting or roosting sites to feeding sites, or from one feeding site to another, and can occur in any direction. In most birds these movements are short and localised, restricted to a circumscribed home range, and extend over distances of metres or kilometres. But in other species (not-ably pelagic birds) regular foraging movements can extend over hundreds of kilometres out from the nesting colony.
• Second, there are one-way dispersal movements. In both sedentary and migratory bird species, after becoming independent of their parents, the young disperse in various directions from their natal sites. Individual young seem to have no specific inherent directional preferences, so within a population, dispersal movements seem to occur randomly in all directions. In most bird species, dispersal distances can be measured in metres, kilometres or tens of kilometres, but in a few species (notably pelagic birds), such distances can be much greater (Chapter 20). Post-fledging dispersal of this type does not usually involve a return journey (see below), but in any case most surviving young subsequently settle to breed at some distance from their hatch-sites (called natal dispersal). In addition, some adults may change their nesting locations from year to year (breeding dispersal), or their non-breeding locations from year to year (here called non-breeding or wintering dispersal).
• Third, there is migration, in which individuals make regular return movements, at about the same times each year, often to specific destinations. Compared with the above movements, migration usually involves a longer journey over tens, hundreds or thousands of kilometres and in much more restricted and fixed directions. Most birds spend their annual non-breeding period at lower latitudes than their breeding period, but some migrate to similar latitudes in the opposite hemisphere where the seasons are reversed. Such migration occurs primarily in association with seasonal changes in food availability, resulting from the alternation of warm and cold seasons at high latitudes, or of wet and dry seasons in the tropics. Overall, directional migration causes a massive movement of birds twice each year between regular breeding and wintering ranges, and a general shift of populations from higher to lower latitudes for the non-breeding season.
• Fourth, there is another category of migration, which I have called dispersive migration, in which post-breeding movements can occur in any direction from the breeding site (like dispersal), but still involve a return journey (like other migration). Although these movements occur seasonally between breeding and non-breeding areas, they do not necessarily involve any change in the latitudinal distribution of the population, or any change in its centre of gravity. They are evident in some landbird species usually regarded as 'resident' (Chapter 17), and include altitudinal movements in which montane birds shift in various directions from higher to lower ground for the non-breeding season. In addition, many seabirds can disperse long distances in various directions from their nesting colonies to over-winter in distant areas rich in food, returning to the colonies the following spring.
• Fifth, there are irruptions (or invasion migrations), which are like other seasonal migrations, except that the proportions of birds that leave the breeding range, and the distances they travel, vary greatly from year to year (the directions are roughly the same but often more variable between individuals than in regular migration). Such movements are usually towards lower latitudes, and occur in association with annual, as well as with seasonal, fluctuations in food supplies. In consequence, populations may concentrate in different parts of their non-breeding ranges in different years. Examples include some boreal finches that depend on sporadic tree-seed crops and some owls that specialise on cyclic rodent populations (Chapters 18 and 19).
• Sixth, there is nomadism, in which birds range from one area to another, residing for a time wherever food is temporarily plentiful, and breeding if possible. The areas successively occupied may lie in various directions from one another. No one area is necessarily used every year, and some areas may be used only at intervals of several years, but for months or years at a time, whenever conditions permit. The population may thus be concentrated in largely different areas in different years. This kind of movement occurs among some rodent-eating owls and raptors of tundra, boreal and arid regions, and among many birds that live in desert regions, where infrequent and sporadic rainfall leads to local changes in habitats and food supplies (Chapter 16). Because these changes are unpredictable from year to year, individual birds do not necessarily return to areas they have used previously, and may breed in widely separated areas in different years.
These different kinds of movements intergrade, and all have variants, but in any bird population, one or two kinds usually prevail. Almost all bird species show post-fledging dispersal movements, in addition to any other types of movement shown at other times of year, and some species show both nomadic and irruptive movements (Chapters 18 and 19). Through migration, irruption and nomadism, birds exploit the resources of mainly different regions at different times. The birds thereby achieve greater survival and reproductive success (and hence greater numbers) than if they remained permanently in the same place, and adopted a sedentary (resident) lifestyle.
The main variables in these different types of bird movements include: (1) the directions or spread of directions; (2) the distances or spread of distances; (3) the calendar dates or spread of dates; and (4) whether or not they involve a return journey. They also differ in whether they occur in direct response to prevailing conditions, or in an 'anticipatory' manner, in adaptation to conditions that can be expected to occur in the coming weeks, and leading birds to leave areas before their local survival would be compromised or arrive in other areas in time to breed when conditions there are suitable. Each of these aspects of bird behaviour can be independently influenced by natural selection (Chapter 20), giving overall the great diversity of movement patterns found among birds, related to the different circumstances in which birds live.
This book is concerned with all these types of bird movements, but the emphasis is on the seasonal return movements of migration and irruption, which are by far the most spectacular and extreme. Migration itself varies greatly between species, as well as between populations, sex and age groups, in respect of distances travelled, routes taken, timing of journeys and behaviour en route. It is often useful to distinguish between 'short-distance' migrants that make mostly overland journeys within continents, and 'long-distance' migrants that make longer journeys between continents, often involving substantial sea-crossings. There is, of course, no clear division between the two categories, but a continuum of variation in the distances travelled and terrain crossed. Similarly, in terms of timing, some birds can complete their migrations in less than a day each way, while others may take more than three months each way, and may therefore be on the move for more than half of each year - most of the time they are not breeding.
In theory, some birds might benefit from remaining on the move at all times of year, for they could then take advantage of rich food supplies wherever and whenever they occurred. It is mainly the needs of breeding that tie birds to fixed localities for part of the year, because individuals need to remain at their nests, or visit their nests frequently, in order to feed their young. However, in some species, notably some seabirds, one parent can be away for long periods (often days, sometimes weeks at a time), while the other remains at the nest. This enables parents to collect food hundreds or even thousands of kilometres away from their nesting places. As their single chick grows, it may be able to survive on its own for long periods, enabling both parents to be away foraging at the same time. Some of the foraging flights of albatrosses undertaken while breeding can cover up to 15 000 km, a distance far greater than the total annual migrations of the vast majority of landbirds.
In many bird species, individuals do not breed until they are two or more years old. The immature, non-breeders of such species are not locality-tied in the same way as breeders, and are free to feed away from nesting areas throughout the year. It is not unusual in these species for adults and immatures to concentrate in different places in the breeding season, and in some such species the young remain in 'winter quarters' year-round, returning to nesting areas only when they are approaching breeding age (Chapter 15). This holds for many kinds of seabirds, shorebirds, large raptors and others.
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