Timing and distance

In some bird species, the number of nights (or days) that individuals show migratory restlessness correlates with the actual distance travelled by their population (see Figure 12.2, Box 12.1). This led to the view that migration was controlled in part by endogenous time programmes, entrained by the seasonal change in daylength (Chapter 11). Again, the timing of migratory activity, whether early or late in the season, or long or short in duration, could be influenced by selective breeding in captivity. The cross-breeding experiment with Blackcaps, mentioned above, confirmed that migratory activity is a population-specific quantitatively inherited characteristic. Restlessness began earlier and continued longer in one parent population than in the other (Berthold & Querner 1981), while the firstgeneration hybrids showed intermediate patterns of behaviour (Figure 20.2). In another experiment, Blackcaps from a migratory population (southern Germany) were selected for later onset of migratory activity (Pulido et al. 2001). After only two generations of artificial selection, the mean onset of migratory activity was delayed by more than one week.

Experiments involving the cross-breeding of different species also proved instructive. In southern Germany, the Black Redstart Phoenicurus ochruros is a short-distance migrant, travelling about 1000 km to Mediterranean winter quarters, whereas the Common Redstart P. phoenicurus is a long-distance migrant, travelling up to 7000 km to tropical Africa. The young of both species hatch at about the same time in May-June, but the Common Redstart moults earlier and more rapidly in preparation for its longer journey, beginning in August. In captivity, this is reflected in the early appearance and long duration of nocturnal migratory restlessness. In contrast, the Black Redstart's period of restlessness is later (October-November) and briefer. The Common Redstart also becomes much heavier through fat deposition, and retains its fat for much longer than the Black Redstart. The two species were therefore ideal for cross-breeding experiments to elucidate the genetic control of these processes. In the event, hybrids between the two proved to be intermediate in all respects between their parent species, in the timing and duration of migratory restlessness, the timing of moult and the degree of fat deposition (Berthold 1999). Yet again, the strong implication was that all these aspects were under genetic control.

In addition to the Blackcap and two Redstarts, in at least two other obligate partial migrants, the occurrence of migratory and non-migratory individuals has been shown to be genetically determined, namely the central European populations of the European Robin Erithacus rubecula (Biebach 1983) and Eurasian Blackbird Turdus merula (Table 20.1; Schwabl 1983). Indications of genetic control have also been found in particular populations of the Song Sparrow Melospiza melodia (Nice 1937, re-analysed by Berthold 1984b), Silvereye Zosterops lateralis (Chan 1994), Great-crested Grebe Podiceps cristatus (Adriaensen et al. 1993) and others.

How quickly changes from migrancy to residency could occur in the wild would depend not only on the initial level of genetic variation in the behaviour of the population and on the strength of the selection pressure, but also on generation

Table 20.1 Partial migration among Blackbirds Turdus merula in different parts of Fennoscandia

Breeding area

Percentage of breeding

Median migration distance

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