Gabriele Mohr

Heritability of the timing of autumn migration in a natural bird population

In recent decades, global temperature has increased at an unprecedented rate. This has been causing rapid environmental shifts that have altered the selective regimes determining the annual organization of birds. In order to assess the potential for adaptive evolution in the timing of autumn migration, we estimated heritabilities of the onset of migratory activity in a southern German blackcap (Sylvia atricapilla) population. Heritabilities (h2 = 0.34-0.45) and coefficients of additive genetic variation (CV2 = 4.7-5.7) were significant and consistent when estimated by different methods, irrespective of whether they were derived from birds hatched in the wild or bred in captivity. In an artificial selection experiment, we selected for later onset of migratory activity, simulating expected natural selection on this trait. We obtained a significant delay in the mean onset of migratory activity by more than one week after two generations of selection. Realized heritability (h2 = 0.55) was in agreement with expected heritability in the cohort that the selection line was derived from. Our results suggest that evolutionary changes in the timing of autumn migration may take place over a very short time period and will most probably be unconstrained by the lack of additive genetic variation.

Steuerung und potentielle Evolutionsgeschwindigkeit des obligaten Teilzieherverhaltens: Ergebnisse eines Zweiweg-Selektionsexperiments mit der Mönchsgrasmücke (Sylvia atricapilla)

1. In Südfrankreich lebt eine obligat teilziehende Population der Mönchsgrasmücke (Sylvia atricapilla) mit rund drei Vierteln Zugvögeln und einem Viertel Standvögeln. Das Teilzugverhalten dieser Population wird in beträchtlichem Umfang von genetischen Faktoren gesteuert (Berthold &Querner 1982). Der starke Selektionserfolg eines Zweiweg-Selektionsexperiments bis zur F1-Generation ließ eine hohe potentielle Evolutionsgeschwindigkeit des obligaten Teilzieherverhaltens vermuten (2). Wir haben das Evolutionspotential in einem Langzeit-Selektionsversuch experimentell untersucht. 2. Wir zogen insgesamt 267 Individuen der freilebenden Mönchsgrasmückenpopulation in fünf Gruppen von Hand auf, und alle fünf Stichproben zeigten entsprechende Verhältnisse von Ziehern und Nichtziehern. Wir konnten das Langzeit-Zweiweg-Selektionsexperiment bis zu einer F6-Generation durchführen und konnten dabei insgesamt 455 Mönchsgrasmücken in Volieren züchten. 3. Das Zweiweg-Selektionsexperiment zeigt, daß die untersuchte teilziehende Mönchsgrasmückenpopulation bereits nach drei Generationen eine ausschließlich ziehende und nach vier bis sechs Generationen eine fast nicht mehr ziehende Population werden kann. 4. Das Zweiweg-Selektionsexperiment zeigt starken Selektionserfolg und hohe Heritabilitätswerte (ca. 0,6–1), die deutlich höher liegen als bisher für das Verhalten von (meist domestizierten) Wirbeltieren angegeben. Demnach sind genetische Faktoren für die Steuerung dieses obligaten Teilzugs sehr bedeutsam und wahrscheinlich dominant über Umwelteinflüsse. 5. Die Versuchsergebnisse lassen im Falle von schnellen Umweltveränderungen und der Wirkung von starken Selektionsfaktoren ein enorm großes Evolutionspotential des obligaten Teilzieherverhaltens annehmen und damit allgemein eine schnelle Mikroevolution der Ortsbewegung von Vögeln. Sollten die von Meteorologen prophezeiten Klimaänderungen in Bälde eintreten (Erwärmung aufgrund von Verunreinigungen der Atmosphäre durch den Menschen, „Treibhauseffekt”), dann könnten viele obligate Teilzieher auf genetischer Basis rasch zu Standvögeln werden und wenig ausgeprägte Zieher rasch zu Teilziehern. Unter diesem allgemeinen Anwachsen von Standvögeln könnten ausgeprägte Zugvögel mehr und mehr zu leiden haben. 6. Eine genetische Kontrolle obligaten Teilzugs scheint weit verbreitet zu sein, und sie stellt wohl auch für alle Individuen den zuverlässigsten Steuerungsmechanismus dar. Beim fakultativen Teilzug hingegen dürften Umweltfaktoren mehr unmittelbar steuernd wirken. 1. In southern France breeds an obligate partially migratory population of the Blackcap with about three quarters of migrants and one quarter of residents. Partial migration of this population is considerably controlled by genetic factors (Berthold &Querner 1982). The large selection response of a two-way selective breeding experiment up to F1-generations suggested a high evolutionary potential of obligate partial migration (2). We have this evolutionary potential experimentally investigated in a long-term selective breeding study. 2. We raised by hand a total of 267 individuals of the free-living Blackcap population in five groups, and all five samples showed corresponding ratios of migrants and nonmigrants. We were able to conduct the long-term selective breeding experiment up to a F6-generation, and thereby a total of 455 Blackcaps could be bred in aviaries. 3. The two-way selective breeding experiment shows, that the investigated partially migratory Blackcap population can become completely migratory in just three generations and almost exclusively sedentary in just four to six generations. 4. The two-way selective breeding experiment yielded large selection responses and high heritability values (about 0.6–1) which are distinctly higher than those previously reported for behavioural traits of (mostly domesticated) vertebrates. Hence, genetic factors are very important for the control of this obligate partial migration and are presumedly dominant over environmental influences. 5. In case of rapid environmental changes and the effect of strong selection factors the experimental results suggest a strikingly high evolutionary potential of obligate partial migration and thus in general a rapid microevolution of bird movements. If climatic changes would occur in the near future as they are predicted by meteorologists (rise in temperature due to man-made pollution of the atmosphere, “greenhouse-effect”) then many obligate partial migrants could rapidly become sedentary on a genetic basis and many less typical migrants could then considerably suffer from the general increase of residents. 6. A genetic control of obligate partial migration appears to be widespread and it also appears to be the most reliable control mechanism for all individuals of such a partially migratory population. Facultative partial migration, however, appears to be controlled more directly by environmental factors.

Vererbung einer populationsspezifischen Mauser bei Kreuzung von Mönchsgrasmücken (Sylvia atricapilla) verschiedener Populationen

A comparative study of migratory blackcaps from Central Europe (S. Germany) and resident conspecifics from the Cape Verde Islands revealed marked differences in annual periodicity. European blackcaps, with one breeding season per year, have a single-peaked annual gonadal cycle whereas the African birds with two breeding seasons per annum have a two-peaked gonadal cycle. The European birds go through a post-juvenile moult (partial moult in first-year birds) or postnuptial moult (complete moult in adults) and, in addition, through a partial winter moult (all age classes) before the next gonadal cycle and breeding season. Their African conspecifics, on the other hand, have only one moult between two gonadal cycles, the summer moult also being the complete one. Here, we demonstrate that the “additional” winter moult of European blackcaps is heritable and can be transmitted into interpopulational hybrids. When blackcaps from S. Germany and the Cape Verdes were cross-bred, 16 out of 21 hybrids displayed the partial winter moult of their German parents. The fact that not all but only 76% of the F1 hybrids passed through this moult favours the idea that its incidence is controlled by a polygenic rather than a single locus system. Most likely winter moult in European blackcaps represents a threshold character as several migratory features do.

Die Schleiergrasmücke: Genetische Grundlagen, rezente Beobachtungen und historische Aspekte dieser melanistischen Form der MönchsgrasmückeSylvia atricapilla

In 1829, a melanistic form of the Blackcap (B) was reported in Madeira; it was later shown to also occur on the Azores and Canary Islands. German bird lovers called it “Schleiergrasmücke”, and we propose as an English name: the “Veiled Blackcap” (VB). On Madeira and the Azores, they were regularly observed in low frequencies of about 2 % of the B populations, on the Canaries a small closed population disappeared. The genesis of the rare VB has been a matter of endless debates. This form was thought to be caused by nutritional or climatic factors, to be inherited as a single-factor recessive, or to represent a special race. From 1987 to 1994 we were able to observe 5–6 VBs on Madeira, to trap there 4 males and 2 females, to hand-raise 50 Bs from the island and to breed the birds in our aviaries. From the pairings BxB, VBxVB, BxVB, F1xF1 and back-crosses with VB we obtained a total of 65 offspring. The results show that morph development in Madeiran Blackcaps is genetically controlled and consistent with a single-locus, two-allele autosomal dimorphism resulting in B and VB. The sex ratio of VBs in our study was 1:1, indicating that the reported scarcity of melanistic females is due to observational bias. All trapped VBs and most of their off-spring were released on Madeira after the breeding experiments. We also report on many details of the VB, present the first photographs of living individuals of this form, refer to its unique tameness as a possible heterozygous advantage in maintaining plumage polymorphism and discuss general aspects of melanism in birds.