Temporal Variation

For the purposes of this discussion two categories o f temporal variation can be defined. The first, macrotemporal variation, applies to changes over time which can only be directly detected by the use of fossils, while microtemporal variation is defined as that which has been directly observed within historical time or even the lifetime of a human observer. It is thought useful to distinguish between the two because the observations made during microtemporal studies are potentially far more accurate and allow an a priori predictive approach, compared to the a posteriori approach of studies with fossils.

Many factors influencing the variation in birds over time are the same as those operating across space. The advantage of adding the dimension of time is that it allows for the processes producing the variation to be observed.

Microtemporal variation

The best known study of morphological variation over a short time span in birds is that by the Grants on the Galapagos finches (Weiner, 1994). Many observations were made on the island of Daphne Major which was visited over more than a decade. Metric-shape changes were seen in the bills of

Geospiza fortis, a species of ground finch, over a few seasons, due to strong selection for increased bill size because of the differential survival of large-seeded plants after a drought (Grant, 1986; Boag and Grant, 1981).

Another important study is on the introduced European house sparrow {Passer domesticus) in North America (Johnston and Selander, 1964). This work showed that significant adaptive differences in both plumage colouration and size had occured in a period possibly as short as 50 years. This

demonstrated that differences large enough to warrant polytypic classification in native North American species can evolve faster than had previously been believed. The results indicated that after introduction in 1852 from England and Germany, the house sparrow has adapted to the various areas in which it now lives. A significant correlation between body size and climate was demonstrated, which appears to follow Bergmaim’s Rule. Similar agreement was found between colouration and geographical location of the populations, conforming to Gloger’s Rule. A later study by one of the authors attempted to confirm Allen’s Rule (Fleischer and Johnston, 1982). This yielded positive evidence for the relationship between core-to-limb proportions and climate, and supported Allen’s Rule. Again these processes had occured since introduction of the species into North America.

Introduced birds provide ideal situations for the documentation of temporal change, because their time o f introduction is often known and so timing of the changes can be approximated. A similar scenario is that of the rock dove {Columba livia), which was introduced into North America no earlier than 1600 AD as domestic doves. These birds escaped, as they had done earlier in Europe, forming colonies of feral pigeons. A study by Johnston (1992) showed that the feral birds in North America were more closely linked genetically to domesticates than are the feral birds in Europe. Despite this, the feral birds in North America were closer in skeletal size and shape to the feral birds of Europe than to domesticates. This showed that natural selection had produced similar morphologies in the feral birds of both Europe and North America, highlighting the importance of environment on such traits.

The importance of these studies through recent time is to show that natural selection may act far faster than previously believed. This implies that similar environmental tracking of species’ morphologies could have occured in the more distant past and that the speed with which it took place may be effectively invisible in the fossil record. This should be borne in mind when there appear to be two sympatric, congeneric species in the fossil record. The time with which the deposit containing the fossils accumulated becomes enormously significant, as the two apparently contemporary “species” may in fact be

Chapter Two

Theoretical Background

M acrotem poral variation

Remakably few studies of taxa have been made over geological time in Quatemary avian palaeontology. The first to be done in Europe was by Bochenski (1974, 1985) on the genus Lagopus. As already mentioned in the historical section, widely geographically distributed samples of both species of Lagopus (L. mutus and L. lagopus) were compared osteometrically to fossil counterparts. He concluded that in the last glaciation both species had possessed longer carpometacarpi and shorter tarsometatarsi, as well as more massive articular ends to their humeri, coracoids and tarsometarsi. This was interpreted as evidence that the two species had maintained or slightly reduced their body size and that the birds had reduced their primary feathers over time. The cause of the changes was believed to be climate change. The mechanism was not, however, clarified, although it appeared that the metric-shape shape changes of the tarsometatarsi were due to Allen’s Rule.

iV till •

Ericson (1987a) studied the changes in eider {Somateria gn<n ii)>«r) between the present day (1960 - 1985 AD) and two past samples, one from 1895 - 1930 AD, the second an archaeological assembage of early to middle Viking Age (800 - 975 AD) from Birka in Sweden. The last hundred years have seen a moderate amount of change to the birds, although they clearly have decreased in body size during the period. The analysis of eider over the last thousand years shows that this size decrease has been taking place for some time. It is interesting that the change over the past hundred years has been relatively greater than during the previous thousand years. The cause of the size decrease was difficult to fathom, but climate change was thought to be an unlikely reason, because such changes were more dramatic over the longer time frame. The tentative conclusion is that the ecological balance may have been upset by recreational use of the sea and pollution, causing g ^ » ^ c o m p e titio n for resources and hence a decrease in eider size.

The two studies using fossils confirm that morphological change can be detected within taxa across time and that such changes are probably caused by environmental factors. They also suggest that the precise mechanism causing such changes may be difficult to elucidate.

2.5. Conclusion

It is clear that there are many theoretical perspectives from which to consider Quatemary fossil birds. Amongst these are altemative species concepts, modes of spéciation, and sources of intraspecific variation. Most authors have described fossil birds, whether neospecies in the past, chronospecies, or truly extinct species, without any explicit attention to these issues. This thesis attempts to help redress this situation by considering in detail the Quatemary fossil record of a limited range of taxa in the light of the theoretical considerations described above.

Scope and Aims of