Limitation of breeding populations through migration events has been viewed here mainly as a three-step process, involving: (1) food availability at stopover sites, which influences the migratory performance of individuals, as reflected in rates of weight gain, departure weights, frequency and duration of stopovers, and overall migration speed; (2) carry-over effects of migration performance on subsequent survival or breeding success; (3) which in turn influence population trend (Figure 27.2). Many studies have been concerned with the first aspect, providing evidence from stopover sites of interference and depletion competition, and of effects of disturbance and predation on the fuelling rates of individuals (involving some age and sex effects). Relatively few studies have provided evidence of migration conditions influencing subsequent breeding and survival, and even fewer of effects on subsequent breeding numbers. The paucity of examples of migration effects on breeding numbers may reflect the difficulties of study rather than the rarity of the phenomenon. Moreover, on all aspects, the evidence is based primarily on correlations, giving no direct evidence for causal relationships, although the provision of extra food and contrived disturbance to migrants could be classed as experimental. The main challenge for future research, therefore, is to test whether the main types of correlation discussed above reflect causal relationships.
All the various processes envisaged involve competition for food, and its effects on feeding rates (along with predation and parasitism). All these factors could act in a density-dependent manner to regulate local and overall populations partly in relation to the availability and quality of stopover habitat. The second type of mortality affecting migratory bird populations, caused by storms or other adverse weather and discussed in the next chapter, is likely to act in a density-independent manner, in that the proportion of birds removed each year bears no consistent relationship to overall population size. However, the likelihood of any one individual succumbing in an extreme weather event may depend partly on its body condition at the time, and in turn on conditions (including competition) at previously attended stopover sites.
While we yet have few examples of bird populations in which changes in breeding numbers have been unequivocally linked to events at a stopover site, in many species changes in breeding numbers are known to be influenced by conditions in wintering areas (Chapter 26; Newton 2004a). Many of these examples of apparent winter limitation could operate through effects on spring migration, which often occurs when food sources reach their lowest level of the year. For example, many Eurasian migrants have declined in numbers following years of drought in the Sahel zone of Africa (Chapter 24). Most mortality is likely to have occurred towards the end of their stay in the Sahel, at the time of migratory fat deposition, or in the Sahara as they attempt to migrate on inadequate body reserves. Some of the species that have shown declines spend the winter mainly south of the Sahel in less arid habitat, but they still have to migrate through the Sahel in spring. More careful investigation of migration in this region may reveal that much of the apparent overwinter mortality is related to spring migration, as indicated by the study on Barn Swallows Hirundo rustica discussed above.
Although few would question the importance of food supply to successful migration, its precise effects are not always easy to quantify, for they do not always result in direct starvation (loss of body condition to the point of death).
At stopover sites, territorial and other interactions between individuals can operate to adjust densities to local food supplies at the time, causing hungry birds to move elsewhere, to places where they may survive or die of starvation or something else. Secondly, food shortage at stopover sites may reduce population size through lowering breeding rates (as in some geese), not necessarily entailing the starvation of full-grown birds. Without special study, this type of effect may be hard to detect because of the time lag between the food shortage and the resulting decline in breeding numbers. In some long-lived species, individuals do not normally breed until they are several years old, so it may take several years before the effects of poor breeding are reflected in decline in the numbers of adult breeders. As a further complication, the effects of food shortage on any population may be accentuated by the presence of people or natural predators, which limit feeding opportunities, and by parasites which directly or indirectly take part of the host's nutrient intake for themselves.
Residual body reserves could be advantageous at any stage of migration, cushioning the bird against adverse weather or other unexpected mishaps during a flight. They give a margin of safety against bad weather immediately after arrival, and allow the bird time to establish itself in a new area. In many species, it is after arrival in breeding areas that residual body reserves are most useful. It is now well established that such reserves support the breeding of arctic-nesting geese (Chapter 5); but it is still uncertain what benefits accrue from residual reserves in smaller birds, although feeding experiments on several species have shown the value of supplementary food in influencing both laying date and clutch size (Newton 1998b). Four types of benefit have been suggested, which are not mutually exclusive, namely that reserves could: (a) increase survival chances if weather conditions deteriorate; (b) allow more time to be spent on other activities important to reproduction, such as territorial defence, song and mate selection; (c) relieve food demands in the early stages of breeding, allowing an earlier start; and (d) allow females to forage selectively for nutrients important to reproduction, such as calcium, while living mainly off their fat (Sandberg & Moore 1996). These benefits must presumably be set against the costs of longer stopovers needed to accumulate the extra reserves, the energy to transport them, and any associated predation risks incurred.
The most crucial habitat for travelling migrants is presumably that which lies adjacent to an ocean or other barrier, and forms the last possible feeding place before the barrier, or the first encountered after it. Examples include the remaining woodland and scrub patches on the coastlines of the Mediterranean Sea in Europe or the Gulf of Mexico in North America. Elsewhere, constraints on migratory fuelling are likely to be most obvious in landscapes where patches of suitable habitat are few and widely scattered, because such patches often attract large numbers of passing birds. Such conditions may be encountered, for example, by forest species migrating through essentially open landscapes with few trees, or by wetland species through essentially arid landscapes with few lakes or rivers. These are the conditions that, through their effects on individual birds, are likely to lead in the long term to the 'high-fuel/long-stopover/long-flight' strategy, rather than the 'low-fuel/short-stopover/short-flight' strategy appropriate in more continuous habitat. Because of human effects on landscapes, the situation encountered by migrants has altered greatly in recent centuries. The migrations of many forest birds, for example, evolved in landscapes very different from those of today, as once-continuous forest has been greatly reduced and fragmented. There must presumably come a point in the process of habitat fragmentation when such landscapes become 'ecological barriers' that are best crossed by long, non-stop flights requiring large fuel loads.
Another neglected aspect concerns the interactions that occur between different populations of a species. If individuals from two or more breeding populations occur together in the same staging or wintering areas (like many shorebirds), and feed on the same limited prey supply, the dynamics of the separate breeding populations could be interlinked, because the mean survival rate of individuals from one population is likely to depend on the overall size of both populations (Dolman & Sutherland 1995).
Despite so many travel lanes in bird migration, populations from different breeding or wintering areas may often use the same staging sites. This gives great potential for intra- and inter-specific competition: if one population passes first, it may deplete the food stocks for a later one, and if different populations are present at the same time, the individuals in both may suffer from depletion and interference. In other words, although living apart for most of their lives, the annual few weeks of contact on stopover sites could ultimately influence the size of one or both of any two competing populations. This is an aspect of stopover ecology that has so far received little attention, but it could have great repercussions for the numbers and behaviour of individuals in competing populations.
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