Washington, April 11: Ornithologists and ecologists studying migratory birds in Europe have found that birds make mistakes in terms of direction but not distance.

The research team—comprising experts from the University of Marburg, the Ornithological Society in Bavaria and the Helmholtz Centre for Environmental Research (UFZ)—came to this conclusion after assessing several thousand reports of Asian birds from the leaf-warbler and thrush families that had strayed to Europe.

The researchers observed that the distance between the breeding grounds in northern Siberia and the wintering sites in southern Asia was often similar to the distance between the breeding grounds and Europe.

They say that their study shed light on why the only vagrants to have been seen by bird-spotters in Central Europe are long-distance migratory birds from Far East Asia.

They say that the more similar the distances, and the more numerous a particular species, the higher the probability of this species of bird straying to Europe.

While it has been suspected to date that the vagrants had been blown off course by the weather, the new study suggest that the vagrant birds end up in the wrong wintering areas probably as a result of an error in the genes that govern their migratory response.

The scientists evaluated the body mass, wingspan, size of breeding area, distance between the breeding area and the wintering area, and the distance between the breeding area and Central Europe for 38 species of migratory birds. Eight species from the leaf-warbler family and six from the thrush family caught their attention as vagrants.

One species that was spotted particularly often was the Yellow-browed Warbler (Phylloscopus inornatus), which was reported by voluntary ornithologists in Central Europe around a thousand times between 1836 and 1991. The other Asian leaf-warbler species were observed much less frequently, if at all, in Central Europe.

In contrast, five thrush species were reported nearly 100 times.

The researchers said that had the vagrants been brought by the weather, smaller birds should have been blown off course more frequently than larger ones.

Statistical analyses, however, did not show any correlation between the frequency of vagrants and their body size, they added.

The research team also observed that the species most likely to land in Europe were those widespread in Asia, and were as common there as their relatives the Chiffchaff and European Willow Warbler are in Central Europe.

"The more numerous a species is, the greater the probability that one of them will be 'wrongly programmed' and go astray. They fly the same distance but in the opposite direction, which takes them to Europe. This is why we have relatively large numbers of vagrants from Asia here," says Dr Jutta Stadler of the Helmholtz Centre for Environmental Research (UFZ) in Halle/Saale.

As to what makes them suspect that the cause of a wrong way down the migration flyway is an error in the genetic migratory programme, the scientists say that flight direction and duration are passed on from one generation of birds to the next, which suggests that migration is the result of a genetic programme that enables birds to adjust to environmental conditions. (ANI)