Info

From Sokolov (2000b).

From Sokolov (2000b).

On their return from winter quarters the next spring, the migrants settle near their natal areas in the 'dartboard' pattern described earlier, sometimes in the localities to which they dispersed in the post-fledging period. This was shown, for example, in European Robins Erithacus rubecula studied near Lake Ladoga in Russia (Zimin 2001, 2002). In this area of 7 X 1.5 km, most local young were replaced by immigrants during the period of post-fledging dispersal. The immigrants stayed on their territories until they had completed moult, and then left on migration. Survivors from these summer immigrants returned the following spring to nest in the study plot, often in the very same territories where they had moulted the previous autumn. In its effect on settling patterns, their post-fledging dispersal was tantamount to natal dispersal. However, individuals of some resident species may have a second period of dispersal in spring.

In many migratory birds, from songbirds to raptors, centrifugal post-fledging dispersal precedes their first southward migration (for Barn Swallow Hirundo rustica see Ormerod 1991, for Pied Wagtail Motacilla alba yarrelli see Dougall 1992, for Sand Martin Riparia riparia see Mead & Harrison 1979, for Willow Warbler Phylloscopus trochilus see Norman & Norman 1985, for Common Kestrel Falco tinnunculus see Snow 1968, Village 1990). The young from early broods have a longer period between fledging and departing on migration than young from later broods, and tend to wander over wider areas from the nest. In some species, a sex difference is also apparent, with females dispersing at an earlier age and further from their natal sites than males in the Willow Warbler Phylloscopus trochilus (Norman 1994) and males earlier and further than females in the Sparrowhawk Accipiter nisus (Newton 1986).

For a few weeks in late summer, the pattern of ring recoveries in migrants is similar to that of natal dispersal, with no directional preference (except in a few species in the northern hemisphere which have shown a northerly bias in post-fledging dispersal directions; e.g. Lesser Kestrel Falco naumanni, Olea 2001; certain herons, Chapter 1). Then, as the young age, recoveries come from longer distances and are more directionally orientated, as the initial centrifugal dispersal changes to long-distance directed migration. The change from random to directed movements is also evident among birds caught from the wild and tested in orientation cages (Shumakov 2001). In those passerine species that moult in their breeding areas, the transition from dispersal to migration occurs around the end of moult, and hence earlier in young from earlier than from later broods

(Boddy 1983, Dougall 1992). But in some other long-distance migrants which do not moult in their breeding areas, directional movement begins soon after the young become independent of parental care, as shown, for example, by the general southwesterly movement of newly independent European Pied Flycatchers Ficedula hypoleuca, Lesser Whitethroats Sylvia curruca and others revealed by ringing on the Courish Spit (Sokolov 2000b).

In some species, such as the European Robin Erithacus rubecula and Eurasian Reed Warbler Acrocephalus scirpaceus, dispersal can occur at night, which again implies that this is a movement that birds are programmed and adapted to make (Rezvyi & Savinich 2001, Mukhin & Bulyuk 2001). Dispersing Reed Warblers are attracted by nocturnal song playback, and some were caught in mist nets placed in reedbeds near the tape recorder. The majority of birds were caught in the last third of the night. That such birds were dispersing rather than migrating was indicated by their age (mostly 39-52 days), their state of moult, lack of migratory fat and lack of consistent directional preferences when tested in orientation cages (Bulyuk et al. 2000, Mukhin 2004). Because Robins and Reed Warblers migrate at night, it is perhaps not surprising that they also disperse then; but it raises the question how many other species do so.

In the Greater Flamingo Phoenicopterus ruber, dispersal distances of young birds in their first autumn were correlated with body condition at fledging, with birds in poor condition staying near the natal colony or making short moves, and birds in good condition making the longest moves, involving sea-crossings (Barbraud et al. 2003). However, flamingos occur in restricted and isolated habitats separated by long distances, and in other birds occupying more continuous habitats dispersal may be less dependent on body condition. Like many other birds, however, flamingos often returned to their first wintering sites in subsequent years, if they migrated at all then.

Was this article helpful?

0 0

Post a comment