Heritability and other studies

Further evidence for the genetic control of various migratory traits has come from field studies of resemblance among genetically related individuals, such as siblings or parents and their offspring (heritability). It can be assumed that part of any resemblance between closely related individuals is due to their shared genes. More specifically, variation in the behaviour of individuals can be ascribed partly to genetic differences between them, partly to differences in the environments to which they are exposed, and partly to an interaction between genetic and environmental influences. In wild birds, environmental variation might be so great as to mask the effects of genetic variation. Alternatively, different individuals may behave in the same way because they live in the same environment, rather than because they share the same genes. But in captive birds, kept in standard conditions, environmental variation can be kept to a minimum, enabling the effects of genetic variation to be more clearly revealed.

Of the 16 heritability estimates in Table 20.2, five derive from field studies and 11 from captive ones. Only one of the five field estimates was significantly different from zero (implying an inherited component in the behaviour studied), whereas 10 out of 11 laboratory studies showed statistically significant inheritance. The lack of significance in most studies of wild birds may have been due partly to greater variation in the environmental conditions to which the individual birds were exposed, as mentioned above. It may also have been due to low sample sizes, to reduced precision of measurements taken in the field, and to greater age variation of some wild samples (assuming age effects on traits). Overall, however, these various heritability estimates provided further evidence for genetic influence on most of the traits examined.

Additional evidence for genetic influence has come from field studies of the year-to-year consistency (repeatability) of behaviour in the same individuals. This was especially true for individual arrival dates in successive springs which were much more consistent within individuals than expected by chance (for White-eyed Vireo Vireo griseus the correlation in individual arrival dates in consecutive years was 0.6, Hopp et al. 1999; for Barn Swallows Hirundo rustica repeatability was 0.51, M0ller 2001; for Dusky Warblers Phylloscopus fuscatus repeatability was 0.34, Forstmeier 2002; and for Garden Warblers Sylvia borin repeatability was 0.40, Pulido & Berthold 2003). In contrast, European Pied Flycatchers Ficedula hypoleuca showed low consistency in spring arrival dates (among-year repeatability = 0.03, Potti 1998). The only study of a non-passerine, the Bewick's Swan Cygnus colum-bianus bewickii, showed some consistency in autumn arrival dates (ANOVA, F = 3.16, P < 0.001) and in spring departure dates (ANOVA, F = 1.72, P = 0.005) (Rees 1989). This year-to-year consistency in the migration dates of individuals has been taken as evidence for genetic influence, but other explanations are possible: for example, some individuals might feed more effectively and accumulate migratory fat before others. The effect of environmental influence on arrival dates could be reduced somewhat if arrival dates were expressed relative to those of other individuals each year (Nolan 1978), rather than using simple calendar dates, as in most of the studies concerned. This might strengthen the case for genetic influence.

Other field studies provided a different type of evidence. For example, Cliff Swallows Petrochelidon pyrrhonata in central North America were exposed in 1996 to a severe cold snap in the middle of the spring arrival period, when insects were inactive, killing many of the newly arrived birds (Brown & Brown 1998). Other swallows arrived after the event. Observations in later years showed that the generation hatched after the selection event contained a greater proportion of later arriving birds than earlier generations, so that the mean arrival date of the whole population was shifted later. The cold snap seemed to have changed the genetic composition of the population, in favour of later arriving genotypes. Although in some years early arrival gave better nest success, this advantage was apparently offset by the greater risk of mortality in occasional cold years, the resulting arrival dates being a compromise between these opposing selection pressures (Chapter 14).

Table 20.2 Heritability estimates for migratory traits. From Pulido & Berthold 2003

Trait

Species

Environment a

Heritability b

Statistical significance

Method c

Reference

Onset of autumn

Sylvia atricapilla

0 0

Post a comment