The most convincing evidence for the existence of facultative responses comes from marked individuals in the wild which behave differently in different years, according to prevailing conditions. Plenty of evidence has emerged that individuals adjust their spring arrival dates to current conditions, or migrate in some autumns or habitats but not others, or migrate at different dates and for different distances in different years (Chapter 12). All these aspects could be under genetic influence, but the range of individual flexibility of behaviour in some populations is very wide (so-called facultative migrants, Chapter 12).
One consequence of facultative migration is that the proportion of migrants in a population could change over time without the need for any genetic change. In particular, if the population increased or food declined, increased competition could cause a greater proportion of birds to leave. Ring recoveries over the period 1960-1990 revealed that an increasing proportion of Greenfinches Carduelis chloris in southeast England performed seasonal movements. This change coincided with a period of population growth, so competition may also have intensified over this period (Main 1996). Similarly, from Eurasian Linnets Carduelis cannabina breeding in Britain, much greater proportions of ring recoveries came from the far end of the migration route in Iberia in recent years than earlier (Wernham et al. 2002). This coincided with a big reduction in winter food supplies over much of the route (caused by loss of weed seeds through herbicide use), which could also have caused increased competition within the population, stimulating more birds to migrate longer distances in autumn. These changes could have resulted from either facultative or genetic responses to the altered conditions.
It is difficult to separate totally genetic from facultative changes, considering that the limits to flexibility may themselves by genetically controlled, as in the comparison between 'obligate' and 'facultative' migrants (Chapter 20). But whatever the basis of the observed changes, it is presumably in these various ways that birds continually adjusted their migration patterns to the massive climate changes of the past, and through which we can expect them to respond in future. It is hard to imagine how any new pattern of migration could arise, except by evolution from an existing pattern, or by the facultative modification of an existing pattern. The next step is to identify the genes that control migration behaviour, and the constraints to further adaptive change. The potentially rapid evolutionary change in movement patterns may be a key factor buffering migratory species against extinction during periods of climatic or other change, including the present.
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