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More Blackcap News - Michael Ryan, FlightLines, March 2003
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We've mentioned Blackcaps a lot recently and they are now reported in 50% of
gardens monitored for feeding birds. It's generally
thought our wintering Blackcaps are birds from Germany who have adapted to
feeding in our gardens during our milder winters and a lot of research is
being done on birds that are trapped, ringed and sampled for stable carbon
isotope readings. Not having so far to travel, they are at a great advantage
when returning back to their breeding ground and getting their territory
established earlier.
Our breeding birds should be arriving towards the end
of this month but you may have noticed in previous years Blackcaps singing
in early spring. Some have been singing already in the first days of March
and a very pleasent and welcome sound it is. We presume these are birds that
begin to sing here but will head east soon to be replaced with Blackcaps
from Africa or southern Spain.
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Blackcaps & Stable Isotopes
Michael Ryan, FlightLines, February 2003
I wrote in Flightlines last month that Blackcaps were being caught in mist nets, ringed and having a toenail clipped to provide a DNA sample for testing. In fact the sample is being tested for stable isotope readings which doesn't mean that the birds are radioactive. Stuart Bearhop of Glasgow University who is behind the project writes:
"Many elements exist in different forms with slightly different masses, and these are called isotopes. Some of these isotopes are radioactive and decay over time; others are stable
and these are what I am interested in. Although the isotopes themselves are stable the ratios of heavy to light isotopes change geographically as a consequence of biological and environmental processes. For example plants in arid areas have different biochemistry to plants in wet areas and as a result have different carbon isotope ratios. 'We are what we eat', thus isotopic differences at the bottom of thc foodchain such as
those in plants are transferred up thc foodchain (through diet) and
reflected in higher animals.
Another element with isotopic ratios that vary geographically is hydrogen. In this case the stable isotopc ratios change with latitude. As a consequcncc of these processes, animals and plants in Iberia have different stable isotope signatures to animals and plants in the UK. By
sampling tissues that I know were grown before the blackcaps migrated (e.g. feathers or toenail clippings). I can seperate individuals from the two wintering populations (on the breeding grounds) based on their stable isotope ratios."
Michael Ryan, FlightLines, February 2003
Background ..
Isotopes ..
mainly from USGS and
Smithsonian sites.
Atoms consist of a nucleus of protons and neutrons surrounded by a cloud of electrons. An
element is defined by the number of protons in the nucleus of the atom eg carbon has six
protons. Although the number of protons is fixed for an element, the number of neutrons can
vary. Carbon can have six, seven, or eight neutrons. The various combinations of protons and
neutrons are called isotopes, which are distinguished on the basis of atomic mass. Atomic
mass is the number of protons plus the number of neutrons. Thus, the naturally occurring
isotopes of carbon are carbon-12 (6 protons + 6 neutrons), carbon-13 (6 protons + 7 neutrons),
and carbon-14 (6 protons + 8 neutrons), which are abbreviated as 12C, 13C, and 14C,
respectively.
Unstable v. Stable ..
The isotope 14C undergoes radioactive decay to an isotope of nitrogen (14N). Because of its
decay, 14C is called a radioactive or "unstable" isotope. The radioactive decay of 14C is the
basis for radiocarbon (carbon-14) dating which has found wide application in carbon-14 dating of
organic materials. In contrast, 12C and 13C do not radioactively decay. Stable isotope
chemistry investigates variations in the ratios of stable isotopes such as 2H/1H, 13C/12C,
18O/16O, and 34S/32S. Isotopic ratios are
measured in the laboratory on a mass spectrometer.
Before a sample can be analyzed for its isotopic ratio, the element of interest
in the sample must be converted to a gaseous form eg carbon dioxide; the cost of
isotopic composition analysis however is relatively low.
Uses ..
While stable isotopes are not known as widely as some of their radioactive relatives, the
use of stable isotopes has dramatically increased. Recent applications of stable isotopes
have occurred in wildlife biology, plant ecology and physiology, ornithology, archaeology,
and anthropology. Recent technological advances in the use of stable isotopic signatures
make it possible to
determine the geographic origins and population connectivity of breeding and wintering
populations of migratory birds. Studies have shown that animal tissues reflect the isotopic
composition of their supporting environment.
How a "signature" is created ..
Differential rate of uptake of stable chemical isotopes results in different isotopic
composition of matter. Because the isotopes are stable and will not decay over time, the
isotopic composition can be treated as a "signature" and used as a natural chemical marker
to trace ecological processes.
Stable isotopes of various chemical elements occur naturally in all ecological systems,
and are assimilated into all levels of food chains. When a bird ingests water and food,
its body tissues absorb the isotopes and take on chemical “signatures” of the geographic
area where it resides. Scientists are investigating the possibility that isotopic analyses
of a bird’s tissues (e.g., feathers & toenail clippings) may indicate the locations of its breeding and
non-breeding habitats and tell a more complete story about its migration patterns.
How useful is it? ..
An understanding of the factors that determine their abundance, which could operate in
breeding and/or non-breeding periods, is of urgent conservation concern. The most pressing
need, and to date the most seemingly intractable problem, has been to determine the
movement patterns and population connectivity of individuals within these populations
between the summer and wintering grounds. This is critical for determining how limiting
factors (e.g. habitat destruction, climate change, etc.) operating in different parts of
the birds' annual cycle, determine population size and local abundance's.
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