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Bird Ringing for Science and Conservation - The European Union ...

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<strong>Bird</strong> <strong>Ringing</strong> <strong>for</strong> <strong>Science</strong> <strong>and</strong> <strong>Conservation</strong><br />

<strong>Bird</strong> <strong>Ringing</strong> <strong>for</strong> <strong>Science</strong> <strong>and</strong> <strong>Conservation</strong><br />

Dispersal <strong>and</strong> population persistence<br />

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Most <strong>European</strong> bird populations live in<br />

habitats that are highly fragmented as a<br />

result of human activities. <strong>The</strong> dynamics<br />

<strong>and</strong> genetic diversity of populations inhabiting<br />

such l<strong>and</strong>scapes are often critically dependent<br />

on dispersal patterns, as well as<br />

on reproduction <strong>and</strong> survival within habitat<br />

patches. To gain a better underst<strong>and</strong>ing<br />

of how to manage these l<strong>and</strong>scapes <strong>for</strong><br />

birds we need in<strong>for</strong>mation on dispersal derived<br />

from bird-ringing.<br />

Two main types of dispersal are recognized<br />

in population ecology. Natal dispersal<br />

refers to movements between the place<br />

of birth <strong>and</strong> that of first breeding, while<br />

breeding dispersal refers to movements<br />

between subsequent breeding attempts.<br />

<strong>The</strong>re are two complementary ways of<br />

studying dispersal using bird ringing. Markrecapture<br />

<strong>and</strong> mark-resight ing data can be<br />

used to measure dispersal within local populations,<br />

or between populations occupying<br />

a limited number of colonies or habitat<br />

patches. <strong>The</strong>se studies provide a high resolution<br />

picture of local movements but may<br />

miss long-distance ones. In contrast, analyses<br />

of ring recoveries provide a broad overview<br />

of dispersal patterns including longdistance<br />

movements, but may lack fine<br />

detail. In both of these methods it is important<br />

to control <strong>for</strong> variation in recording<br />

ef<strong>for</strong>t.<br />

Current knowledge of natal <strong>and</strong> breeding<br />

dispersal is based on analyses of ringing<br />

data. For most species natal dispersal is<br />

greater than breeding dispersal, <strong>and</strong> species<br />

with higher natal dispersal also tend to<br />

move further between subsequent breeding<br />

years. Average (geometric mean) dispersal<br />

distances vary greatly between species.<br />

For example, in Britain <strong>and</strong> Irel<strong>and</strong><br />

Blackcaps have an average natal dispersal<br />

distance of 17,5 km while House Sparrows<br />

move an average of only 0,2 km between<br />

their natal <strong>and</strong> breeding sites. Dispersal<br />

patterns are influenced mainly by the ecological<br />

characteristics of individual species,<br />

with those occupying more restricted <strong>and</strong><br />

patchy habitats showing greater dispersal.<br />

Scarcer species generally occupy more restricted<br />

<strong>and</strong> patchy habitats <strong>and</strong> this results<br />

in a negative relationship between dispersal<br />

<strong>and</strong> abundance. For similar reasons, dispersal<br />

is greater amongst birds occupying<br />

wetl<strong>and</strong> habitats. Dispersal is also greater<br />

in migrants than in residents, presumably<br />

because of the opportunities <strong>for</strong> the<br />

<strong>for</strong>mer to explore new areas. <strong>The</strong>re is much<br />

scope to explore such patterns further using<br />

data from the EURING databank.<br />

Natal dispersal<br />

distances of Song<br />

Thrushes measured<br />

using ring recoveries<br />

from Britain<br />

<strong>and</strong> Irel<strong>and</strong>.<br />

<strong>The</strong>re is now increasing evidence from<br />

fieldwork <strong>and</strong> mathematical modelling<br />

that frequencies of occurrence <strong>and</strong> densities<br />

of many bird species are lower within<br />

habitat fragments than in large areas of<br />

continuous habitat. For example, a study<br />

in Northern Belgium found that Nuthatch<br />

densities in <strong>for</strong>est fragments were about<br />

half those in continuous areas of <strong>for</strong>est. In<br />

this species, dispersal distances are larger<br />

<strong>and</strong> territory vacancies are filled more<br />

slowly in fragments than in continuous<br />

habitat. Furthermore, areas where most<br />

of the habitat is fragmented act as sinks,<br />

with populations only being maintained by<br />

immigration from more continuous habitat.<br />

In order to manage populations within<br />

fragmented l<strong>and</strong>scapes it is vital to underst<strong>and</strong><br />

these relationships between population<br />

density, habitat quality <strong>and</strong> dispersal.<br />

Underst<strong>and</strong>ing dispersal is equally important<br />

<strong>for</strong> the conservation of colonial species<br />

such as seabirds, where immigration<br />

<strong>and</strong> emigration are key determinants of<br />

colony size.<br />

Dispersal also has important implications<br />

<strong>for</strong> the maintenance of genetic diversity<br />

within populations, <strong>and</strong> <strong>for</strong> rates of evolution<br />

in changing environments. In most<br />

bird species the greater natal dispersal of<br />

females compared to males helps to reduce<br />

inbreeding depression. A study of colourmarked<br />

Great Reed Warblers in Sweden<br />

found that low genetic variation <strong>and</strong> the<br />

occurrence of inbreeding depression were<br />

associated with restricted dispersal <strong>and</strong><br />

with a lack of any dispersal difference between<br />

males <strong>and</strong> females. <strong>The</strong>se genetic<br />

studies further emphasise the importance<br />

of improving our underst<strong>and</strong>ing of dispersal,<br />

which remains poor relative to that of<br />

other demographic processes. Large-scale<br />

studies of marked birds should <strong>for</strong>m an important<br />

part of this research ef<strong>for</strong>t.<br />

Alain Saunier<br />

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<strong>The</strong> relationship between average natal<br />

dispersal distance <strong>and</strong> population size<br />

<strong>for</strong> 75 species. More abundant species<br />

generally occupy a wider range of habitats<br />

<strong>and</strong> need to move less far in order<br />

to find potential nesting sites.<br />

<strong>The</strong> Nuthatch is a good example of a<br />

species where patch occupancy in fragmented<br />

woodl<strong>and</strong> habitats is influenced<br />

by dispersal.<br />

after Paradis et al. 1998<br />

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