Some birds migrate mainly by day and others mainly by night. Nocturnal species such as owls and nightjars, or optional diurnal-nocturnal species such as shore-birds, might be expected to migrate under cover of darkness. What is surprising is that many normally diurnal species also travel at night. To judge from their eye structure, diurnal birds may have no better vision at night than do humans, but this would still enable them to fly safely through the open skies, and recognise star patterns and landscape features that might help them find their way.
Apart from soaring landbirds, which depend on daytime thermals, it is not immediately obvious why particular species migrate at one time rather than another. Among passerines: crows, finches, pipits, larks, wagtails, tits, hirun-dines and others migrate primarily by day; while warblers, flycatchers, thrushes, chats and others migrate primarily by night (Table 4.1). Among non-passerines: pigeons, raptors, cranes, herons and egrets migrate by day; while cuckoos, shore-birds, rails, and grebes migrate mainly by night. Comparing different families, there is no obvious and consistent connection between migration times and difficulty of journey, habitat, diet or other aspects of ecology. However, among closely related families, some striking differences occur, as in the passerines just mentioned, and also among waders, in which plovers (Charadriidae) migrate more by day than sandpipers (Scolopacidae). Although most species within a family seem
Table 4.1 Diurnal and nocturnal migrants among Holarctic birds.
Divers, pelicans, gannets and cormorants, raptors, storks, herons, ibises, spoonbills, flamingos, cranes, bustards, pratincoles, raptors, grouse, skuas, gulls and terns, alcids, pigeons and doves, bee-eaters, woodpeckers, swallows and martins, jays and crows, chickadees and titmice, creepers, accentors, larks, pipits and wagtails, starlings, sparrows, cardueline finches, fringilline finches, buntings
Grebes, sea-ducks, bitterns, quail, rails and coots, waders, stone curlews, cuckoos, wrynecks, owls, nightjars, wryneck, hummingbirds, orioles, flycatchers, nuthatches and creepers, wrens, thrashers, chats and thrushes, bluebirds, kinglets (goldcrests), shrikes, vireos, warblers, icterids, orioles, tanagers, New World sparrows Albatrosses and petrels (Procellariiformes), swans and geese (Anseridae), dabbling ducks, gulls and terns, shorebirds, swifts consistent in their migratory behaviour, occasional revealing exceptions occur, with the tendency to nocturnal migration increasing with migration distance (Dorka 1966). For example, most species of Emberiza buntings in Europe migrate by day over short distances, but the Ortolan Bunting E. hortulana migrates by night over long distances, being the only species that winters in Africa south of the Sahara. Similarly, most pigeons migrate by day over short distances within Europe, but the Turtle Dove Streptopelia turtur migrates partly by night over long distances to Africa. In addition, Tree Pipits Anthus trivialis are more nocturnal than Meadow Pipits A. pratensis, Willow Warblers Phylloscopus trochilis than Chiffchaffs P. collybita, and Common Redstarts Phoenicurus phoenicurus than Black Redstarts P. ochropus (Figure 4.6). In all these species pairs, the first mentioned species migrates further than the second. Nevertheless, there are still some puzzling exceptions: for example, European Robins Erithacus rubecula and Firecrests Regulus ignacius are short-distance migrants, but still travel mainly at night.
The division between day and night migrants is most obvious from take-off times, with diurnal migrants leaving in the morning and nocturnal ones in the evening. However, whether day or night, landbirds of both groups must continue flying if they find themselves over water, as must waders and waterfowl over dry land. This explains the appearance of typical day migrants, such as Eurasian Skylarks Alauda arvensis, at lighthouses at night, or of typical night migrants, such as chats and warblers, arriving on the coast around mid-day in bright sunshine. Someone watching thrushes and warblers fly ashore from the Gulf of Mexico in spring might understandably class these birds as diurnal migrants, when in fact they set off after sunset one evening, and took more than 12 hours to complete their non-stop flight. Barn Swallows Hirundo rustica are viewed as typical daytime
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