Figure 4.6 Diurnal variation in the numbers of migrants seen (or caught in mist nets) when crossing the Col de Bretolet in the Swiss Alps. N = night, Da = dawn, 1 hour on each side of sunrise; M = morning, dawn to noon; A = afternoon, noon to dusk; Du = dusk, 1 hour on each side of sunset. In each species pair (upper and lower), the longer-distance one undertakes more migration at night than the short-distance one. From Dorka (1966).

migrants, feeding on the wing as they go and spending their nights in large roosts; but they may occasionally turn up at watch points at night and, when crossing the Sahara, they appear commonly to travel in the cool of the night. Moreover, beyond the Arctic Circle, all bird movements in summer inevitably occur in daylight. Thus, whether a species is perceived as a day-migrant or a night-migrant depends partly on the locality, whether the species is seen near the start or end of a flight, and whether there are other reasons for flying at a particular time. It is not impossible that birds could become increasingly nocturnal in migratory behaviour along a gradient from facultative short-distance migrants to obligate long-distance migrants, producing differences between closely related species and also between different populations of the same species. This is another aspect of migration that warrants more research.

For similar reasons, some species appear to pass particular localities at different clock-times in autumn and spring. For example, the Long-tailed Ducks Clangula clangula, Common Scoters Melanitta nigra, Barnacle Geese Branta leucopsis and Brent Geese Branta bernicla that cross southern Finland between the Baltic and White Seas appear mainly by night in spring and mainly by day in autumn, as shown by radar (Bergmann 1977). However, this could depend on distance from departure point, the autumn birds having been longer on the wing. In addition, birds migrating later in the season for their species may be more prone to travel at night than conspecifics migrating earlier (Clarke 1912). Clearly, the division between diurnal and nocturnal in most birds cannot be taken as hard and fast. Of 147 species (from 16 bird orders) surveyed at British Bird Observatories, no species fell into an exclusively nocturnal or diurnal category, but 75% of species had been recorded at night at some time, including such usual day migrants as Chaffinch Fringilla coelebs and Barn Swallow Hirundo rustica (Martin 1990).

Nocturnal migration has been known for a long time. Not only can birds be heard at night as they fly unseen overhead, but they can be watched with a telescope as they fly across the lit face of the moon (Chapter 2). Some species can even be attracted down by high-powered lure tapes, or are killed during the night at lighthouses and other illuminated structures (Chapter 28). Moreover, even the most casual observer can see that, at migration seasons, places devoid of birds in the evening can be full of birds the following morning, as a result of an overnight fall. In recent decades, however, radar has confirmed that huge numbers of birds migrate at night, in most places far more than travel by day. By flying at night, birds lose sleep, but at those times of year they are physiologically adapted to this, and the effects seem minimal. Even captive birds reduce their sleep during migration periods, apparently without ill-effect, although they often have a marked rest period in the late afternoon before they would normally depart (Berthold 1990a). After a really long journey, birds such as waders and waterfowl, settling in safe places, sleep soon after landing, whether day or night (Chapter 3).

The main supposed advantages of nocturnal migration are that: (1) more time is left for feeding during the day, the only time that most birds can feed, so the entire journey can be accomplished more quickly; (2) temperatures are lower at night than in the day which could help to prevent overheating and dehydration in warm regions; (3) humidities are usually higher at night and early morning, which could further reduce dehydration risk; (4) energy demands are lower, because it costs less to fly in cooler denser night air than in warmer daytime air;

(5) wind speeds are generally lower at night, thus reducing the effects of headwinds or crosswinds, and vertical turbulence is less, further reducing the total energy cost of flight; (6) the use of stars for navigating is possible; and (7) the likelihood of predation during flight is much reduced. The main threat to flying migrants is from falcons or eagles during the daytime (plus gulls over water), but a wide range of other raptors take migrants when they are on the ground. Owls do not normally fly high enough to encounter migrants and in any case seldom catch prey on the wing.

The advantages of nocturnal migration can thus be summarised as energy-saving, time-saving and risk-saving. In particular, the reduced turbulence at night must greatly lessen the energy costs of flight, especially in small birds (Kerlinger & Moore 1989). Even diurnal migrants concentrate their flight into the first and last few hours of the daylight period, when turbulence is much less than in the warmer middle part of the day. Among small birds, only swallows, which are adapted to fly throughout the day regardless of turbulence, seem to migrate at similar intensity throughout the daily light period. The various advantages of night-flying are so obvious that it is hard to imagine why some birds migrate primarily by day (apart from those dependent on thermals). Excluding the soaring species, diurnal migrants mostly travel short distances overall, and restrict their flight to brief favourable periods. In flying by day, they also gain a clear view of the terrain below, which may help in recognising familiar landmarks and finding areas of favourable habitat or food supplies.

As found from radar studies, diurnal migration usually starts up to an hour before sunrise (when colour vision returns), builds to a peak in the 2 hours after sunrise and then declines from late morning to early afternoon, but occasionally with a slight resurgence in the late afternoon (Dorka 1966, Bruderer 1999). Hummingbirds are unusual in migrating largely during the mid-day period, leaving time for feeding in the morning and evening, but those that cross the Gulf of Mexico fly at night. These tiny birds are more constrained than most other birds in obtaining enough energy per day, and also respond to the diurnal pattern of nectar production on which they depend.

In general, waterfowl seem more variable in their take-off times than most other birds, starting at any time of day or night, but mostly around sunset. Night-migrating shorebirds mostly set off in the 2-3 hours before sunset, depending to some extent on the stage of the tide, and night-migrating passerines mostly in the hour after sunset (when colour vision goes). Overland migrants studied by radar reach peak densities and altitudes 1-2 hours later, well before midnight. The volume of migration remains high during the first half of the night, then declines from around 2 a.m. to mid-morning, as increasing numbers of birds settle. In addition, large numbers of migrants that normally migrate at night sometimes undertake 'morning flights', which start 1-1.5 hours after sunrise, and continue for up to 2 hours. While some morning flights by nocturnal migrants may represent a continuation of migration, others do not follow the normal migration direction, so have been interpreted as attempts by arriving birds to find better feeding areas or to compensate for overnight drift (Alerstam 1978b, Gauthreaux 1978b, √Ākesson & Sandberg 1994, Sandberg et al. 2002).

As found by radar, the behaviour of nocturnal migrants changes during the course of a night, seemingly as their motivation wanes. Directional preferences are much stronger during the early part of a flight, around dusk, than towards its end during the following morning (Bruderer 2001). Night migrants tracked in a coastal area became less likely to strike out over the sea and more likely to veer along the coastline as the night progressed; they also flew at lower altitudes and lower speeds, and increasing proportions flew in the reverse direction. All these changes may have been responses to diminishing body reserves, or shifting compromises between straight flight and risk avoidance, depending on prevailing conditions (Bruderer & Liechti 1998). The frequent finding that flight tracks are less variable over the sea than over land may be explained if only the most highly motivated straight-flying birds continue out to sea, while others settle, fly along the coast or turn back inland (Casement 1966, Richardson 1978).

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