Overall, birds show a range of different migration patterns, which are associated with mating and parental systems, annual cycle features and body size dominance relationships, all of which may modify the timing and distance of migration within the constraints imposed by environmental conditions. Where species in the same family or genus differ in mating and parental systems, they also show corresponding differences in migration patterns. The fact that similar patterns recur in unrelated families emphasises the role of mating and parental systems in influencing migration. Differences in average migration dates between the sexes vary from a matter of days in some species to weeks in others, depending in autumn on the relative roles of the two sexes in parental care and in spring on the time between arrival and nesting.
Apart from the relationships with patterns of parental care and moult, no unambiguous evidence favours one explanatory hypothesis on migratory timing over others, and almost certainly different explanations apply in different species. The body size, dominance and arrival time hypotheses are not mutually exclusive, and each is hard to test independently of the others. This is because in many bird species, adult males are the largest individuals, are socially dominant and return to breeding areas first. In fact, dominance and body size are so closely correlated in birds that separating their importance in differential migration is practically impossible (Cristol et al. 1999). Unless birds arrive in breeding areas already paired, almost all the studies known to me show sex differences in the mean arrival dates of males and females, but many of the same species show no obvious differences in autumn migration dates.
On the dominance hypothesis, as usually interpreted, dominance acts here and now to influence bird behaviour, which thus depends on conditions at the time. In many species, dominance relationships supposedly lead subordinates to migrate furthest: females further than males and juveniles further than adults. Such effects are most evident in short-distance, partial and irruptive migrants (Chapters 12 and 18), in which travel distances also vary from year to year according to prevailing food supplies. In species in which genetic factors have most influence on migration, and in which all individuals migrate every year, dominance effects on migration are much less apparent. In such birds, evolved migration behaviour is likely to depend more on past conditions, which might include a range of factors, and not just dominance relationships. For example, the travel itself may have costs which increase with the length of journey, thus leading to selection for shorter movements; and if these travel costs are greater in young than older birds, selection could in turn result in young birds performing shorter journeys than older ones (as in Dark-eyed Juncos Junco hyemalis and some other seed-eaters, see above). With genetic factors (rather than prevailing conditions) having the major influence on migration distances, each sex/age class would be expected to winter each year in whichever regions offered the best prospects for survival and future reproduction, rather than where they were pushed by competition. It is unlikely that the optimal wintering area, resulting from the balance of these various selective forces, would coincide exactly for each sex/age class of a species, considering their differences in morphology, behaviour and reproductive roles. Understanding differential migration then becomes an 'optimality' problem to be solved separately for each sex and age class, with competition as only one of several factors involved. There is clearly scope here for further research.
On the face of it, it may seem strange that dominance relationships can have different effects at different seasons. For example, among partial and short-distance migrants, juveniles leave the breeding areas before adults and often migrate further, whereas in practically all migrants studied in spring, adults leave their wintering areas and arrive on breeding areas before juveniles. At each season, however, it is the dominant adults (especially males) that are able to pursue the optimal strategy and other individuals are predisposed by competition to behave otherwise. In autumn, the optimal strategy among partial migrants is to stay in breeding areas as long as possible, preferably all winter, for resident individuals generally survive better than those that migrate, and also retain the best territories for breeding (Chapter 20). In spring, the optimal strategy is to be among the first birds to return to breeding areas, for those individuals can then get the best of the remaining territories.
In addition to the aspects of migration discussed in this chapter, sex and age differences are also apparent in site-fidelity - in the tendency of individuals to return to the same breeding and wintering sites from year to year. In general, males tend to show greater site-fidelity than females, and in both sexes site-fidelity tends to increase with age (Chapter 17). These and other types of bird movements would benefit from more research on the relationships between the dominance status, body condition and behaviour of individuals.
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