All these various methods, from ringing and radio-tracking to isotope ratios and other internal markers, provide information on 'connectivity' - the geographical linking of populations at different times of year between specific breeding, migration and wintering areas. Connectivity can be classed as strong or weak (diffuse) depending on the degree to which individuals from different breeding areas mix in their non-breeding areas, or vice versa (Boulet & Norris 2006). It can be classed as strong if all individuals from a limited breeding area migrate to the same limited wintering area, and as weak if such individuals scatter among many wintering areas, intermixing with individuals from elsewhere. The importance of understanding connectivity is that conditions experienced by individuals in one part of the world can affect their subsequent performance in another part, hundreds or thousands of kilometres away (Chapter 26). These carry-over effects can be studied only if the birds from particular breeding areas can be linked to specific wintering and migration areas. They can occur at the population level (as with density-dependent effects on reproduction and mortality), or at the individual level (as when body condition at one time of year influences performance at another). Understanding such geographical linkages is also relevant to questions of the ecology, disease transmission and genetic structure of populations, and for effective conservation (Webster & Marra 2005).

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