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Bird Ringing for Science and Conservation
Dispersal and population persistence
Most European bird populations live in habitats
that are highly fragmented as a result of human activities. The
dynamics and genetic diversity of populations inhabiting such landscapes
are often critically dependent on dispersal patterns, as well as
on reproduction and survival within habitat patches. To gain a better
understanding
of how to manage these landscapes for birds we need information
on dispersal derived from bird-ringing.
Two main types of dispersal are recognized in population
ecology. Natal dispersal refers to movements between the place of
birth and that of first breeding, while breeding dispersal refers
to movements between subsequent breeding attempts. There are two
complementary ways of studying dispersal using bird ringing. Mark-recapture
and mark-resighting data can be used to measure dispersal within
local populations, or between populations occupying a limited number
of colonies or habitat patches. These studies provide a high resolution
picture of local movements but may miss long-distance ones. In contrast,
analyses of ring recoveries provide a broad overview of dispersal
patterns including long-distance movements, but may lack fine detail.
In both of these methods it is important to control for variation
in recording effort.
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Natal dispersal distances of Song Thrushes
measured using ring recoveries from Britain and Ireland. |
Current knowledge of natal and breeding dispersal
is based on analyses of ringing data. For most species natal dispersal
is greater than breeding dispersal, and species with higher natal
dispersal also tend to move further between subsequent breeding
years. Average (geometric mean) dispersal distances vary greatly
between species. For example, in Britain and Ireland Blackcaps have
an average natal dispersal distance of 17,5 km while House Sparrows
move an average of only 0,2 km between their natal and breeding
sites. Dispersal patterns are influenced mainly by the ecological
characteristics of individual species, with those occupying more
restricted and patchy habitats showing greater dispersal. Scarcer
species generally occupy more restricted and patchy habitats and
this results in a negative relationship between dispersal and abundance.
For similar reasons, dispersal is greater amongst birds occupying
wetland habitats. Dispersal is also greater in migrants than in
residents, presumably because of the opportunities for the former
to explore new areas. There is much scope to explore such patterns
further using data from the EURING databank.
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The relationship between average natal dispersal
distance and population size for 75 species. More abundant
species generally occupy a wider range of habitats and need
to move less far in order to find potential nesting sites. |
There is now increasing evidence from fieldwork
and mathematical modelling that frequencies of occurrence and densities
of many bird species are lower within habitat fragments than in
large areas of continuous habitat. For example, a study in Northern
Belgium found that Nuthatch densities in forest fragments were about
half those in continuous areas of forest. In this species, dispersal
distances are larger and territory vacancies are filled more slowly
in fragments than in continuous habitat. Furthermore, areas where
most of the habitat is fragmented act as sinks, with populations
only being maintained by immigration from more continuous habitat.
In order to manage populations within fragmented landscapes it is
vital to understand these relationships between population density,
habitat quality and dispersal. Understanding dispersal is equally
important for the conservation of colonial species such as seabirds,
where immigration and emigration are key determinants of colony
size.
The Nuthatch is a good example of
a species
where patch occupancy in fragmented woodland
habitats is influenced by dispersal.
Dispersal also has important implications for the
maintenance of genetic diversity within populations, and for rates
of evolution in changing environments. In most bird species the
greater natal dispersal of females compared to males helps to reduce
inbreeding depression. A study of colourmarked Great Reed Warblers
in Sweden found that low genetic variation and the occurrence of
inbreeding depression were associated with restricted dispersal
and with a lack of any dispersal difference between males and females.
These genetic studies further emphasise the importance of improving
our understanding of dispersal, which remains poor relative to that
of other demographic processes. Large-scale studies of marked birds
should form an important part of this research effort.
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