Young avian migrants of many species are able to find their species- or population-specific wintering area without the help of conspecifics. In orientation tests hand-raised birds have been demonstrated to choose appropriate population-specific migratory directions, suggesting a genetic basis to this behaviour. Experiments have shown results of a cross-breeding experiment between individuals of two blackcap (Sylvia atricapilla) populations with widely different migratory directions. The orientation of the F1 offspring was intermediate between and significantly different from that of both parental populations. The variance of individual mean directions in the F1 generation did not increase compared with the parental groups, and the inheritance of migratory directions was not sex-linked. This data provides direct evidence for a genetic basis of migratory directions in birds and demonstrate a phenotypically intermediate mode of inheritance (Helbig 1991).
Animals are expected to maximize their reproductive output in a heterogeneous environment through spatial distribution. Under the ideal free distribution (IFD) model of habitat selection, individuals are free to settle anywhere and distribute themselves in such a way that their reproductive output in all habitats is the same. There is ample evidence, however, that the breeding success and reproductive output of birds can differ across habitats on multiple spatial scales. One possible explanation for this is provided by the ideal despotic distribution (IDD) model of habitat selection. Under this model, dominant individuals pre-empt better habitats, thus forcing subordinates to settle in poorer habitats, with reproductive parameters differing between habitats accordingly. Another explanation for habitat-specific reproductive success is based on human induced habitat changes. Humans are exerting strong pressure on native habitats, changing them and their biotic interactions profoundly.
Experiments have shown that blackcaps preferentially settled in a plantation of introduced black locust (Robinia pseudoacacia) upon their return from spring migration. In this plantation, they reached twice the density as that observed in a natural floodplain forest nearby. However, they had significantly lower nesting success (15.5%) than in the floodplain forest (59%). Returning migrant Blackcaps may be lured by early-leafing shrubs in the exotic plantation to settle earlier and at higher densities in the reproductively inferior habitat. These results show that (1) it is not possible to assess habitat quality based solely on breeding densities; (2) human-modified habitats can function as ecological traps by luring settling birds into unsuitable habitats, and (3) by replacing exotic plant species with native ones, native communities of trees and an increase in the breeding productivity of bird populations can be achieved (Remes 2003).
In recent years, substantial numbers of central European birds have taken to wintering in gardens in Great Britain, Ireland, the Benelux countries, and even southern Scandinavia, migrating northwest or north, instead of southwest. Presumably the ready availability of food, particularly from bird tables, and the avoidance of migration over the Alps and the Sahara Desert compensate for the sub-optimal climate. It has been reported that German birds wintering in England tend to mate only among themselves, and not usually with those wintering in the Mediterranean or western Africa. This is because the short-distance migrants arrive back from the wintering grounds for breeding earlier than birds wintering around the Mediterranean, and form pairs before Mediterranean-wintering birds arrive. This division of a population by different migration routes can be a first step towards speciation. The increasing populations have been traced to a tiny population of Eurasian Blackcaps caught in Germany which exhibited a tendency to migrate in a north-westerly direction (instead of the majority that migrate southwards across the Alps to Africa) – the combination of more food and milder temperatures in Britain means that the birds that migrate from Germany to Britain are now apparently at an advantage over those migrating south (Bearhop et al. 2005).
Reference List:
- Bearhop.S., Fiedler.W., Furness.W.R., Votier.C.S., Waldron.S., Newton.J., Bowen.J.G., Berthold.P., & Farnsworth.K., (2005). Assortative mating as a mechanism for rapid evolution of a migratory divide. Science, 310, 502–550.
- Helbig.J.A., (1991). Inheritance of migratory direction in a bird species: a cross-breeding experiment with SE- and SW-migrating blackcaps (Sylvia atricapilla). Behavioural Ecology and Socio-biology, 28, 9-12.
- Remes.V., (2003) Effects of Exotic Habitat on Nesting Success, Territory Density, and Settlement Patterns in the Blackcap (Sylvia atricapilla). Conservation Biology, 17, 1127-1133.
