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Bird Ringing for Science and Conservation
Ringing birds to understand population
dynamics
Understanding the mechanisms underpinning population
growth and decline is central for conservation and many ecological
and evolutionary questions. The variation of the size of a population
from one year to another is determined by the number of individuals
that have survived, were recruited, have immigrated or emigrated.
Estimates of survival, recruitment, immigration and emigration rates
can be obtained, if the fate of individuals can be followed through
time and space. Birds that are ringed can be recognized individually
allowing to estimate demographic rates.
However, the estimation of demographic rates is
complicated by the fact that marked individuals cannot always
be observed. Some individuals may be hidden at the time when the
researcher wants to check them. Consequently, only fragments of
the life of a ringed bird are known, and statistical methods have
to be developed to deal with this problem. Technical meetings regularly
organised by EURING deal mainly with this challenge,
and they have helped considerably to advance statistical methods.
Nowadays, sophisticated computer programs
exist with which demographic rates can be estimated from capture-recapture
data or from data from dead recoveries. Here we highlight three
different studies showing the potential of data, gathered from ringed
birds, to understand population dynamics.
Greater Flamingos, the third individual to the
right wearing a colour ring.
There are many studies about survival rates in
birds obtained from either capture-recapture data or from recoveries
of dead individuals. Several of them have shown that survival rates
of migratory birds depend on the availability of food resources
during the non-breeding period. For example, annual survival rates
of White Storks are significantly lower in years with droughts in
the Sahel. Because White Storks from most European populations spend
the non-breeding period at least partially in the Sahel, the sensitivity
to droughts can explain why population changes across large areas
in the European breeding area are synchronous. Moreover, this example
highlights that successful conservation needs to integrate the complete
life cycle of the species
under question, not only the breeding period.
Recruitment, the establishment of locally hatched
individuals in the population, is important for the maintenance
of a population. To understand the impact of recruitment on population
dynamics it must be known at which age young birds breed for the
first time and how many there are. These questions can be studied
if nestlings are marked and if it is noted in which year they reproduce.
Researchers from France have studied recruitment in Flamingos in
the Camargue.
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The first individuals started to breed at the age of 3 years,
but there were also individuals that delayed their first breeding
up to an age of 9 years. Recruitment was higher in years following
a severe winter with higher mortality, showing that the effects
of strong winters are offset by earlier recruitment, which
reduces the impact of hard winter on population dynamics.
In order to understand population dynamics,
it is vital to be able to assess how much variation in survival,
reproduction or dispersal contribute to population change.
Surviving adults of Willow Tits contributed 64 % to the growth
rate of a Finish population, whereas the contribution due
to immigration (22%) and due to local recruitment (14%) were
significantly lower. The contribution of surviving adults
was constant across time, but highly variable for local recruits
and immigrants. Thus, the dynamics of this willow tit population
were mainly due to variation in recruitment and immigration.
However, because surviving adults contribute so much to population
growth, any slight decline in adult survival rate has a very
strong effect on the population. |
| Willow Tit |
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All these insights were only possible, because
birds have been ringed. Without individual recognition of
birds in a population, it is hardly possible to understand
demographic reasons for population changes. Bird ringing is
therefore the basic field method to study population declines
and increases.
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