Other Differences Between Sex And Age Groups

Males arrive earlier than females in breeding areas either because they winter closer to the breeding areas, begin their return migration earlier, travel faster, or show any combination of these features. In many passerines and shorebirds, in which the sexes winter side by side in the same area, it has been observed that the males fatten and leave first (Rogers & Odum 1966, Nisbet & Medway 1972, Cramp 1988, Bishop et al. 2004, Catry et al. 2005), and on stopover sites that males often fatten more rapidly and stay for shorter periods than females, thus making more rapid progress (Chapter 27). These differences may also be due to the greater dominance of males, widening the sex differences in migration timing as birds travel towards their destinations. Greater fat levels in males may allow them to cross wider stretches of sea or other hostile habitat, as suspected in the Goldcrest Regulus regulus (J. H. Marchant, in Wernham et al. 2002).

Age groups, in particular, may differ in other aspects of migration behaviour. Juveniles often take longer to accumulate migratory fuel than adults, migrate with lower fuel loads, stay longer at stopover sites, and take longer to complete their journeys (Chapter 27). Such differences have been attributed partly to the lower experience and social status of juveniles, compared with adults. When they leave their breeding areas, juveniles also show more spread in departure directions than adults (Chapter 10), and the two age groups sometimes migrate along partly different routes, with juvenile passerines and others more concentrated in coastal areas (e.g. Murray 1966, Ralph 1971). This could result from the greater drift of juveniles by wind and their greater concentration near the edges of a migration route (Woodrey 2000).

A remarkable difference in migration routes between juvenile and adult European Honey Buzzards Pernis apivorus was found by following radio-tagged individuals from Sweden to West Africa (Hake et al. 2003). All tagged adults crossed the Mediterranean Sea at Gibraltar and continued across the Sahara Desert to their wintering areas. Analysing three main steps of the migration: (1) from the breeding site to the southern Mediterranean region; (2) across the Sahara; and (3) from the southern Sahara to the wintering sites, the adults changed direction significantly between these steps, and took a large detour via Gibraltar. In contrast, the juveniles travelled in more southerly directions, crossed the Mediterranean Sea at various places, and ended up in the same wintering range as adults. Average speeds maintained on travelling days were similar in the two age groups, about 170 km per day in Europe, 270 km per day across the Sahara and 125 km per day in Africa south of the Sahara. However, as the adults had fewer stopover days en route, they maintained higher overall speeds and completed migration in a shorter time (mean 42 days) than the juveniles (mean 64 days). Although the juveniles set off on more direct courses towards the wintering areas, they did not cover shorter distances than the adults, as they tended to show a larger directional scatter between shorter flight segments, as expected if they were more severely drifted by wind.

Age differences in migration routes and wintering areas may not be unusual in raptors. At watch sites that get most of their birds as a result of wind drift, the majority of birds seen in autumn are juveniles (95% at Cape May Point in New Jersey), much greater than at other sites (50% juveniles at Hawk Mountain in Pennsylvania) (Chapter 10; Allen et al. 1996). In many raptor species, juveniles have somewhat longer flight and tail feathers than adults, an unusual situation which affects wing shape, and may adapt the two age groups to different flight modes. Still, however, juveniles are more likely to be blown off course than are adults (Kerlinger 1989, Hoffman & Darrow 1992).

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