Female barn swallows in different regions are choosing males, the outward signs which indicate the ability to resist the threat to local parasites. To such conclusion researchers have come, having studied three subspecies of swallows that inhabit North America, Europe and the middle East. In an article for the journal Evolution , the authors notethat this mechanism has led to significant differences in the appearance of swallows. And in the future and they may become separate species.
In the course of evolution many animals have special characteristics that are needed to attract partners, such as large size, bright colors or loud song. According to several models of sexual selection, such signals directly reflect the physical condition of individuals and its ability to resist parasites.
Widespread species in different parts of the area are faced with a different set of parasites. According to some researchers, it should affect their secondary sexual characteristics and contribute to the transformation of individual populations to a full species.
To test this hypothesis, a team of researchers led by Amanda Hund (Amanda K. Hund) of the University of Colorado at boulder focused on village swallows (Hirundo rustica). This well-known species breeds in much of the Northern hemisphere and forms six subspecies.
The researchers compared three of them in North America (H. r. erythrogaster), European (H. r. rustica) and middle Eastern (H. r. transitiva). The first breast and belly are painted in red color, the second is characterized by extra long tail feathers, and the third is characterized by the combination of dark throat and long tail feathers. According to previous research, these differences in plumage are associated with different preferences of females.
In the course of work members of the team captured and examined 173 male swallows from the US, 84 from the Czech Republic and 42 from Israel. The physical condition of each of them analyzed in several ways. In addition, the authors assessed the breeding success of the studied individuals. Separately, they calculated the number of parasites in males and their offspring.
The analysis confirmed that the North American, European and middle Eastern subspecies swallows are faced with different sets of parasitic organisms. However, even parasites from the same groups differently affect the breeding success of hosts in different regions. For example, the damage caused by feather mites was the highest in Europe and minimal in the middle East. Israel, in turn, breeding the offspring most strongly prevented lice.
Coloration of males of swallows in the studied populations was credible signal about the ability to repel the most dangerous of the parasites. The North American subspecies H. r. erythrogaster bright color of the breast of the male byi indicator that he is not infected by ticks. European swallows H. r. rustica the presence of long feathers in the tail indicated low infestation by ticks and lice. And in the middle East populations of H. r. transitiva this same symptom showed protection against lice.
Based on these results, the authors concluded that the species composition of parasites and damage from them define the appearance of barn swallows in different regions. On the basis of the indicators females choose males that not only they themselves can cope with the parasites, but will pass on resistance genes to their offspring. In this process, the birds from the separate populations gradually become different from each other. This is the first stages of separation of the once unified population into several types.
Many passerine birds are attracted to individuals of the opposite sex, not only bright color, but loud song. Its characteristics depend not only on the preferences of the females, but also from environmental factors. For example, the repertoire of the yellow-headed forest songbird nesting in the Western United States, is becoming more diverse after a forest fire.