Assessing the effects of weather on the volume of bird migration is not straightforward. This is partly because migration depends less on the prevailing weather than on the intrinsic migratory state of the birds themselves, as is evident from several types of observations (Lack 1960b). First, weather conditions that in spring or autumn would be associated with migration have no effect at other times of year. Second, after a hold-up due to unsettled weather in the normal migration season, the onset of favourable weather may result in unusually heavy passage. Third, after a long hold-up, or late in the migration season, birds may take off in conditions that, at other times, would stimulate little or no migration. Fourth, in addition to such broad effects, the individuals in particular localities do not all set off together, but over periods of days or weeks. Such variation can only be attributed to variation in their internal states and the dates at which individual birds are physiologically prepared (with adequate body reserves) for departure. The underlying controlling factor is thus the migratory state of the individual, which interacts with weather and other external conditions to influence departure dates. If the internal changes are well advanced, migration may occur under apparently unfavourable conditions, but if the migratory state is low, departure will occur only under especially favourable conditions or not at all.
The numbers of birds migrating on particular days thus depend not just on the prevailing weather, but on the weather over preceding days, the date in the season, and the number of birds ready to leave at the time. Towards the end of the migration season there may be few birds left to migrate, however suitable the weather. The association between weather and the volume of migration on particular days is therefore not constant, making analysis more complicated. In addition, because species differ in body size, flight mode and other aspects, they are affected by adverse weather to different extents, some species being able to migrate in conditions that would ground others (Lack 1960b, Alerstam 1978a, Elkins 2005). Ideally, therefore, the effects of weather should be examined at particular sites in relation to the proportion of birds in a migratory state that take off each day, but this proportion is seldom measurable.
Another pitfall in assessing weather effects on bird migration is that different weather factors tend to be associated with one another, with some occurring under cyclonic and others under anticyclonic conditions (Lack 1960b, Richardson 1978, 1990). Even with the help of multivariate statistics, it is often hard to tell which factors are critical and which are coincidental.1
1Given sufficient data collected day to day over several years some of the difficulties can be allowed for in analysis (e.g. Alerstam 1978a). For example, the effect of date in season on numbers migrating can be allowed for by calculating how much the measure for a particular day deviated from the long-term average for that date. Weather variables whose typical values change greatly over the season (such as temperature) can also be expressed as deviations from seasonal norms, enabling temperature and date effects to be more effectively separated. The effect of weather-induced delay on migration volume can be allowed for by including the duration of delay as a variable in a multivari-ate analysis. Despite the increasing sophistication of analytical procedures, however, multivariate analyses still cannot reliably separate causal from coincidental relationships, or reveal whether birds respond directly to strongly interrelated variables, such as pressure, temperature and humidity.
Box 4.1 Weather effects: the bigger picture. Based mainly on Richardson (1990)
To a large extent, it is possible in high latitude regions to predict the likelihood of strong bird migration on particular days within the migration seasons from examination of synoptic weather maps, showing fronts and pressure systems over wide areas. The atmosphere at such latitudes is organised into high and low pressure systems which move approximately eastward. In the northern hemisphere, winds blow clockwise around highs and counter-clockwise around lows. Fronts separate air masses in these systems: warm fronts occur where advancing warm air is replacing cold air, usually east or southeast of an approaching low; and cold fronts occur where cold air is replacing warmer air, usually south or southwest of a low. Precipitation and thick cloud occur most commonly near lows and fronts.
Given the importance of wind direction and clear skies, migration timing can be related to these large-scale atmospheric features, even if birds sense only their local manifestations. In the northern temperate region, peak southward migration tends to occur with cool northerly tailwinds as a 'low' moves away to the east, or a 'high' approaches from the west, or both. Conversely, peak northward migration tends to occur with warm southerly tailwinds, as a 'high' moves away to the east, or a 'low' approaches from the west, or both. At both seasons, much migration also occurs with light winds near the centre of a high.
When birds are concentrated by coasts, ridges or valleys, the numbers passing a given point may be quite different from what the above weather patterns would suggest. Also, the association between synoptic weather patterns and migration volume is likely to decline as birds get increasingly far from their departure points. In the southern hemisphere, different (but related) trends are expected, because winds there blow in the opposite directions around lows and highs.
Almost certainly, migrants do not react to the general weather situation as such, but to one or more components of it, such as wind and rain. Nevertheless, for the human observer, the synoptic weather situation of fronts and pressure systems gives a good indication of how much migration is likely to occur at different places on particular days (Box 4.1).
Yet another problem relates to the use of visual observations alone to assess weather effects. Visual records miss any birds flying too high to be seen by day, and provide little or no information on nocturnal migration. Yet radar has revealed that most overland migration of birds that fly by flapping flight occurs above visual range. In fact, the proportion of birds flying within sight, and the proportion that come to ground, tend to be greatest in conditions that are unfavourable for flight (Lack 1960b). Thus, migrants tend to fly low in opposing rather than following winds, and to settle whenever they encounter strong opposing winds, mist or rain, or reach coastlines or islands. The observer equipped only with binoculars would conclude that these were the very conditions that favoured migration, a once firmly held view but the opposite to reality. Birds also tend to fly low along coasts in these conditions, reluctant to strike out over water. It is therefore important to distinguish the influence of weather in promoting migration from its influence in making migration conspicuous (Lack 1960b, Alerstam 1978a).
The advent of radar greatly clarified the situation, because it enabled migrants to be detected at almost all heights (missing only those below the radar horizon), day and night, and in all weathers. From radar-based studies, consensus has now emerged that, within the appropriate seasons, migration is favoured by fine anti-cyclonic conditions with favourable tailwinds, and also by rising temperatures in spring and by falling temperatures in autumn. In effect, at both seasons the birds prefer to migrate under clear skies with following or light winds. Clear skies assist navigation, especially at night, by making celestial cues more visible, while following winds reduce the time and energy spent on the journey, and the risk of being blown off course. In contrast, birds seldom take off to migrate in strong opposing winds, dense cloud, mist and rain. Opposing winds make progress difficult or impossible, cloud hampers navigation, while mist or rain can soak many kinds of birds and force them down. Revealingly, the same weather conditions that are associated with heavy migration in free-living birds are often associated with strong migratory restlessness in captive ones (Schindler et al. 1981, Viehmann 1982, Gwinner et al. 1992).
The above conclusions on weather effects were drawn from a large number of short-term studies in various parts of the world (Richardson 1990). However, an almost complete picture of autumn migration was recorded near Nuremberg in Germany by use of a conically scanning pencil-beam radar (Erni et al. 2002). At this site, bird migration increased in volume from early August, reached high levels during September to mid-October, and then declined. Allowing for this seasonal trend, about two-thirds of the variation in daily migration volume was explained by wind and rain, the two variables likely to have most influence on flight. But because of the association between different weather factors, migration volume was also correlated with change in temperature, pressure and cloud cover. It was more closely correlated with the duration of rain than with the overall amount per night. After short heavy showers, birds continued migration, whereas on nights with continuous light drizzle, migrant densities remained low.
The proportions of days that are favourable for migration clearly vary greatly from region to region, from year to year, and from autumn to spring in the same region. The wettest regions on earth offer only a few days in each season that are ideal for migration. In some other regions, birds hardly ever experience favourable winds near the ground, but they take off and climb to higher altitudes, where wind conditions are more often favourable (see later).
The importance of temperature to migration is uncertain. In spring, warmth occurs in association with other conditions favourable to flight, as does cold in autumn. But temperature may have direct effects through influencing the energy balance of the birds, and more importantly through influencing food supplies, because all vegetative growth, insect activity, and ice melt are temperature-dependent. It is therefore of obvious advantage for migrants to adjust their migratory schedules to year-to-year variations in temperature, and they are clearly deterred in spring by extreme cold and snow (Chapter 14). It is also uncertain to what extent (if at all) migratory birds can predict the weather before it arrives, say by a change in barometric pressure. Bird movements in autumn and spring have been correlated with changes in pressure, associated with an approaching front (Richardson 1990, Dau 1992). But whether the birds were reacting to pressure as such or to associated weather variables is hard to say. More study of apparent pressure responses is needed before firm conclusions on this point can be drawn (see also Chapter 9).
Observations of birds about to depart are consistent with the findings from radar studies mentioned above. Take-off occurs most often on nights with good visibility, bright stars, no overcast, no rain, and with light or following winds, these conditions being more influential in long-distance than in short-distance migrants (e.g. Cochran & Kjos 1985, Bolshakov & Bulyuk 1999). In some species, individuals usually take off singly without preliminary activity (Hebrard 1971). Passerines leaving from trees and bushes at dusk at first flutter uncertainly upwards, in ones and twos or small parties, and then, climbing all the time, proceed speedily and confidently in the direction of travel, rapidly disappearing into the gloom. In other species, departure can be noisy and impressive, as flock after flock takes off and heads into the distance, climbing till out of sight (Piersma et al. 1990b).
Regardless of the weather at take-off, migrants can encounter poor conditions en route. If they meet low cloud and unfavourable wind, birds may be forced low and, if over the land, they can settle and wait for conditions to improve. Over the sea, as radar has revealed, landbird migrants that enter cloud or mist banks usually become disorientated, milling in all directions and gradually drifting downwind, or actively flying downwind which gives a good chance of reaching clearer weather (Williamson 1955, Lack 1960b, Richardson 1978, Bourne 1981). If cloud persists, migrants over the sea are sometimes attracted in large numbers to lighted ships or oilrigs (Bourne 1979, 1983). Although heavy overcast appears inimical for migration, some birds seem to maintain more or less straight courses with complete cloud cover. Below the cloud they can see the ground, and above it the sun or stars (although flying above the cloud seems infrequent).
Many journeys do not go smoothly, and the survival of landbird migrants over the sea is likely to depend largely on their abilities to cope with adverse weather and off-route displacement. On any one journey, depending on weather, birds may advance, then retreat, veer from a direct course, or be borne far off route by side-winds, returning to their regular course along unfamiliar routes (Williams 1950). They may sometimes perish in large numbers (Chapter 28). When over land, in contrast, most landbirds seem to settle when they encounter a rain front, and simply wait for the weather to clear. When over the sea at not too great a distance, they may turn around and fly back to land. The important point is that, once en route, the best course of action for a bird is likely to depend on prevailing circumstances, and particularly whether the bird is fat or lean, and over favourable or unfavourable terrain. Drift by crosswinds is, of course, one of the commonest ways in which migrants turn up as vagrants in places off their normal routes (Chapter 10).
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