Effects Of Droughts On Migrant Numbers

In much of Africa north of the equator, as emphasised above, rainfall during the northern summer largely determines the state of the vegetation and the extent of wetlands in the following dry season. It thus influences the food supplies of many birds, whether they consume plant or animal matter. But rainfall also varies greatly from year to year, and over much of the region since the late 1960s has in most years been well below former levels, owing to a failure of the rainbelts to extend so far...

Facultative movements in relation to weather

Facultative movements are also shown by species that obtain their food from water or the ground, and which must therefore move in response to freezing temperatures, snow or drought. Such escape (or weather) movements occur as soon as waters freeze or snow covers the ground, cutting off food supplies. The dates of such movements therefore vary between years, depending on the weather, and in mild winters need not occur at all. Even in the arctic, some waterfowl and gulls remain in autumn to feed...

Laboratory Research On Physiology Migratory Restlessness And Directional Preferences

Laboratory research on bird migration began in the mid-1920s and rapidly gained ground. Hundreds of experiments on migratory physiology, orientation and other aspects have now revealed most of the relevant physiological processes and controlling mechanisms, at least in broad terms Chapters 11 and 12 . An important discovery was that, at migration times, migratory birds in captivity developed migratory restlessness Zugunruhe in German , in which they hop and flutter around their cages, an...

Evolution Of Alloheimy

Whatever the pattern of geographical segregation among wintering migrants, if such differences are genetically influenced, through migratory behaviour, it is not hard to envisage how they might have come about Figure 23.9 . Imagine that populations which breed in separate areas come together on common wintering grounds. If they were limited by food in the wintering area, and individuals of one population were better adapted to that area, they would in time be expected to eliminate individuals...

Alternative Models Time And Energy Considerations

While one idea assumes that wintering areas nearest the breeding areas are best, and that further migration results from competition, an alternative 'time allocation' model emphasises the benefits and costs of migration in influencing the optimal wintering area. Imagine that, within the potential wintering range, sites progressively further from the breeding range are more benign, so that day-to-day survival is higher there, but that migration costs increase with increasing distance from...

Departure from breeding areas

In many species of birds, failed breeders that are freed from parental duties leave their nesting areas earlier than successful breeders, which can depart only when their young are grown. In consequence, in years of widespread breeding failure, as sometimes occur in arctic-nesting shorebirds, the bulk of the post-breeding migration occurs noticeably earlier than usual. It illustrates the dependence of post-breeding migration time on previous events in the annual cycle. More significantly,...

Migratory fattening and restlessness

Within the migration season, migratory flights may be influenced by prevailing weather and food supply Chapters 4 and 27 . The role of feeding conditions in affecting autumn fattening rates, body condition and departure dates has been established in the field for a range of species from Sedge Warbler Acrocephalus schoenobaenus Bibby amp Green 1981 to Greylag Goose Anser anser van Eerden et al. 1991 . In general, individuals that fatten most rapidly leave first, whether in autumn or spring, or...

Sitefidelity and dispersal

There is probably no single aspect of the entire subject of bird migration that challenges our admiration for birds so much as the unerring certainty with which they cover thousands of miles of land and water to come to rest in exactly the same spot where they spent the previous summer or winter. Frederick C. Lincoln 1935. In many bird species, as may be judged from ring recoveries, individuals tend to breed in the same general region where they were raised and once they have bred, they often...

Raptors and other soaring birds

Birds East Africa

If they had continued for a fortnight in the same strength as on that day, we could surely have said that they were in greater number than all the men living on the earth. . . . they are seen to pass in this way as thick as ants, and so to continue for many days. Belon 1555 , writing of the migration of Black Kites Milvus migrans on the Black Sea coast. In some large bird species, the spread wings span a large area relative to body weight, and provide good lift in rising air currents. This is...