Many bird species migrate in flocks. Individuals take off together and probably largely remain together during the flight, although they do not necessarily stay together for the next stage of the journey. Frequent calling, especially at night or in overcast conditions, may help to maintain flock cohesion and direction. It may also induce grounded birds in appropriate condition to join the passing stream (Box 4.2). In caged Bobolinks Dolichonyx oryzivorus, nocturnal call notes, recorded and played back, increased the nocturnal restlessness of birds in migratory condition (Hamilton 1962b). Such social influences may thus help to synchronise migration between individuals. Migrating in flocks may improve navigation through the collective efforts of individuals. It may also improve food-finding and foraging efficiency during stopovers, and give more effective predator detection and evasion.
Another common advantage of migrating in flocks is the energy-saving resulting from specific flight formations, such as the V-formation usual in geese, gulls, and others, or the dense flocks of some passerines and shorebirds. As described earlier, individuals gain lift by flying in the upward-directed air flow near another bird's wings, and thus save energy (Chapter 3). Some usually-solitary raptor species are also seen in flocks on migration. While this may result as much from the sharing of narrow migration corridors as from social attraction, individuals are clearly influenced by the behaviour of others in moving from one thermal to another (Chapter 7). In effect, each bird watches and follows others along the migration route.
As is evident to any bird-watcher, typical flock sizes, densities and flight formations differ greatly between species. Among passerines, species that travel in level flight, such as Starlings Sturnus vulgaris, usually migrate in dense flocks, whereas in species which migrate by undulating flight, individuals keep further apart. Most birds seem to migrate in single species flocks, but some often travel in flocks with other species of similar flight speed. Various thrush species often travel together, and so do various tits. Among nocturnal migratory passerines, individuals usually travel singly, or in loose aggregations, with individuals more than 50 m apart, maintaining contact by calls. In most species, flocks seldom exceed a few tens of individuals, but in some species flocks comprise hundreds or thousands of individuals (Gatter 2000). Moreover, at times of peak migration, flocks in many species follow in such quick succession that they almost run into
Box 4.2 Social influences and migration waves among Chaffinches
At the Courland Spit on the Baltic coast, Chaffinches Fringilla coelebs were collected at different stages of a migratory wave and their carcasses were examined to determine the fat content and the amount of food in the gut (Dolnik & Blyumental 1967). These results were then related to the volume of migration, and a remarkable and consistent pattern emerged.
Throughout the migration season, wave after wave of migrants passed through the area. On the first day of each wave only very fat birds flew: they began their movement around sunrise, without feeding beforehand, and continued for about 4 hours. There was then a pause of 1-3 hours, after which the movement was resumed and continued for 2 hours in the evening. On the second day, the volume of migration reached a peak; again the movement began at sunrise and at first only fat birds flew. As the day progressed, however, the migrating flocks contained increasing numbers of lean birds which, unlike the fat ones, usually had fresh food in their stomachs. This implied that lean birds began their flight later in the day than fat ones and after feeding. During the day the lean birds stopped to feed again: at the same time they attracted down some fat birds, although on this day the latter did not normally feed. By the afternoon, all the lean birds had stopped to feed, and in the evening fat birds were the only ones left flying. On the last day of the wave the migration did not begin at sunrise but only after the birds had fed. Fewer birds participated; they flew with frequent stops and at lower altitude. On this day, almost all the flying birds contained little fat, some feeding occurred throughout the day, and the movement did not reach a minor peak in the evening. Some flocks flew in the reverse direction. Each migratory wave usually lasted three days, but varied from one to seven. After it was over, the pause usually lasted three days, but varied from one to eight, depending partly on the weather. During each migratory wave, birds were estimated to cover up to 500 km, in which time individuals expended 2-3 g of fat. Together with stopping time, this amounted to about 500 km per six days, which was consistent with the migratory progress of Chaffinches recorded from ring recoveries (Chapter 8). No sex or age differences were noted among the birds caught on different days of a wave.
In their explanation of this pattern, Dolnik & Blyumental (1967) attached great importance to the pull that flying birds had on others which at that time were physiologically less ready to migrate. Since the first birds to fly were the very fat ones, it was presumably the presence of many fat birds, which started to fly under a common stimulus (such as favourable weather), that began each wave. Once started, however, the stream of flying birds stimulated others to join, and the larger the stream of flying birds the greater the pull. The expenditure of fat by the fat birds, and the frequent stops by the lean ones, explained the picture observed in succeeding days of the wave. When most of the fat birds had depleted their reserves, their stopping pulled the rest down and the wave was brought to a standstill. Movement was resumed after the birds had built up their reserves again, which presumably depended partly on feeding conditions
(Chapter 27), and inclement weather could further delay departure. This type of pattern may partly account for the greatly varying fat levels found in migrants caught together on migration at the same place.
The frequency with which the migratory waves appeared thus depended primarily on the time needed for spent birds to replace their fat, modified by variations in the weather which also affected the urge to fly. It was the stimulus of movement by the fat birds on others less fat that caused a large part of the population to move together and produce the wave-like pattern. Although birds moved at various stages of fatness, the amount of fat carried by a bird affected the timing and duration of its flights; and in general the fattest birds made the fastest progress.
The birds in this study were making a diurnal overland journey (along the coast) in a region where short-term variations in weather were less extreme than further west in Europe. Possibly in some other regions, the more variable weather has so much influence on migration as to obscure any underlying pattern in the behaviour and physiology of the birds themselves. This work does, however, help to explain why movement does not occur on all days when conditions are apparently ideal, and why it sometimes occurs on days when the weather is less good. The value to the birds in this behaviour probably lies in the advantages of travelling together, including predator avoidance and judgment pooling, as described in the text (see also Chapter 9).
one another. This is true even of large species, such as geese and swans, which sometimes appear as an almost continuous parade of flocks of 10-100 individuals. Despite the differences between species, the causal factors behind specific flock sizes and formations are poorly understood, apart from the obvious points mentioned above that flocks in general provide greater protection from predation, and in some cases improved navigation and reduced individual energy costs.
The benefits of migrating in flocks may explain why many birds do not start migration from their nesting places, which are scattered over a wide area, but first assemble at particular staging sites, often used year after year, and from which birds depart over a period of days or weeks. This behaviour is especially obvious in shorebirds and waterfowl, but occurs in many others, including cranes, gulls, terns and shearwaters. Although related to weather, the departures and arrivals of individuals are partly under social influence. Through social interactions, including vocal activity, individuals of flocking species seem to communicate their readiness to migrate, and thereby synchronise their departures. Some species display pre-flight intention movements, and intense calling, with repeated take-offs and landings, before finally setting off. Among shorebirds, some of these preliminary flights occur in highly structured formations, as do the departures themselves (Piersma et al. 1990b). It is presumably the advantages of migrating in flocks that encourage such synchronisation behaviour.
Some large bird species, such as swans, geese and cranes, travel in pairs or families within the flocks. Other waterfowl, including many species of ducks, form pairs in winter quarters, and the male then accompanies the female back to her breeding area (Chapter 17). Somewhat unexpectedly, however, some migrant passerines also occur in pairs, either on autumn and spring stopover sites or in winter: that is, male-female combinations occur much more often than expected by chance, and the partners behave as mated pairs (for examples see Greenberg & Gradwohl 1980). The Bearded Tit (Parrotbill) Panurus biarmicus has provided many instances of birds apparently migrating as pairs (D. Pearson, in Wernham et al. 2002); and in some other migratory species, male-female pairs defend territories in winter quarters (for White Wagtail Motacilla alba, see Zahavi 1971, for Stonechat Saxicola torquata, see Rodl 1994). Whether such liaisons persist into the breeding season or have reproductive consequences is unknown. Despite such examples, we can assume that reproductive pairing before arrival on breeding areas is not common among birds, because in most species that have been studied, the two sexes behave independently when away from their breeding areas, and males arrive, on the average, at least several days before females (Chapter 15). Only in relatively few species, including some waterfowl and cranes, do birds arrive already paired.
These various observations confirm that, in many bird species, individuals do not necessarily behave independently of one another on migration, but can be influenced to varying degrees by other individuals. The role of social influence, and its relationship with body condition, has been studied in particular detail in Chaffinches Fringilla coelebs (Box 4.2), and in field experiments on other species social influence has been found sufficient to override inherent migratory and directional tendencies (Chapter 9). On the other hand, individuals of some species, such as the Cuckoo Cuculus canorus, seem always to migrate alone.
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