7.2.3Genetic markers
From harmless blood samples (Chapter 9), molecular genetic techniques provide
useful methods of assigning individuals to particular populations. A number of
techniques are available and should be chosen according to the question asked.
Mitochondrial DNA (mtDNA) haplotypes may vary among populations on
regional scales, such as in Dunlins Calidris alpinaacross its arctic breeding range
(Wennerberg 2001). Microsatellites—non-coding and highly repeated nuclear
DNA sequences—show considerable variation among individuals and popula-
tions, but they appear to be relatively rare in the bird genome and have therefore not
yet been extensively used (Webster et al. 2002). Nuclear markers, such as randomly
amplified polymorphic DNA (RAPD) and amplified fragment length polymorphic
DNA (AFLP) provide useful population markers. AFLP seems especially promising
and has recently been used to distinguish different subspecies of the willow warbler
across a hybrid zone in Sweden, where mtDNA and microsatellite markers failed
to differentiate the populations (Bensch et al. 2002). See chapter 9 for further
information on molecular genetic methods in avian research.
7.2.4Stable isotopes
Naturally occurring elements often show clinal variation across a continental
land surface with respect to ratios of stable isotopes. Some useful elements are
carbon (C), hydrogen (H), nitrogen (N), and strontium (Sr) (Lajtha and
Michener 1994). The processes behind systematic changes vary among the
elements. In carbon the proportion^13 C:^14 C (conventionally known as ^13 C) is
determined by the relative abundance of C3 and C4 plants and hence largely
determined by the composition of plant communities. The ^13 C is translated
through the food chain from plants, or through phytophagous insects to birds.
Therefore, the location of a bird during molt is reflected by the stable isotope
signature laid down in its feathers, which is preserved after the feather has finished
growing. By analyzing bird feathers using mass spectrometry the stable isotope
ratios can be determined and compared with known regional variation. Using the
variation of ^13 C and ^15 N, the different wintering areas (west and east Africa)
could be confirmed in two populations of Willow Warblers (Chamberlain et al.
2000) (see Figure 7.1). In another study, Marra et al. (1998) used ^13 C to show that
arrival time from spring migration in American Redstarts Setophaga ruticilla
correlated with the quality of their wintering habitat. Depending on the season of
molt, different species are suited for summer or winter population differentiation,
while the unique biannual molt in the Willow Warbler makes this species particu-
larly useful for studying both summer and winter areas. In some species molt is
Migration systems| 163