Evolution News - European Blackcaps Diverging in Plain View

Male blackcap, Sylvia atricapilla

Scientists attach geolocators to the backs of blackcaps to record the light intensity at any time of day. In this way, hiking trails can be precisely determined.

© Ben Porter

Variety in the migratory behavior of blackcaps | Max-Planck-Gesellschaft

Like the paper on the origins of East Asian peoples, in my last blog post, this one on the migration of the European warbler, the blackcap (Sylvia atricapilla), also shows how little makes sense in biology unless seen with an understanding of evolution and how it works. Supernatural ''explanations' actually explain nothing and have no predictive power because, when you add magic to any explanation, literally anything becomes possible and nothing is falsifiable.

This little bird is a favourite of mine if for no other reason that we had one spend two of the last three winters in our garden, feeding regularly at our bird feeders and fat balls (and that is a clue to their recent change of habit, which this paper records). Readers may recall I wrote about this just over 2 years ago, explaining the evolutionary significance of it.

Now this piece of work, led by Miriam Liedvogel of the Max Planck Institute for Evolutionary Biology in Plön, Germany, sets out to uncover some of the remaining mysteries surrounding blackcap migration.

The birds have variable migration strategies

Through a large-scale study with so-called geolocators, researchers led by Miriam Liedvogel of the Max Planck Institute for Evolutionary Biology in Plön, Germany, were able to uncover some of the mysteries surrounding the phenomenon of the blackcap's bird migration.

Many species of birds have always migrated south in the fall to spend the winter there, including the small blackcap, which often weighs only a few grams and yet covers thousands of kilometers. However, changes in our landscapes and climate change are not leaving migratory birds unaffected. They change their behavior, the destinations of their journey, the time of their departure or even the decision whether to fly away at all. So far, these habits have been studied either experimentally with birds in captivity or by irregular recaptures of ringed birds.

A group of international researchers led by Miriam Liedvogel from the Max Planck Institute for Evolutionary Biology in Plön has now for the first time conducted a large-scale study with geolocators. These are ultra-light, tiny devices that are attached to the backs of free-flying birds and record the light intensity with exact time data. After the birds have been caught again, the respective flight route can be calculated exactly. Altogether the scientists could document and analyze the migrations of 100 individual birds.

Different migration destinations depending on the breeding area

It was already known beforehand that there is a migration divide in Europe: east of this imaginary line, the blackcaps migrate southeast in the fall, and west of this line, they move southwest. Through targeted breeding experiments, it was already successfully shown in the 1990s that the direction of migration is inherited by the parents. These experiments also showed that offspring crossed in captivity between western- and easterly blackcaps show an intermediate orientation, i.e. they migrate exactly in the direction of the south. The assumption was that this would be avoided in nature, since the southern route would lead the birds over the Alps, the Mediterranean and possibly over the Sahara desert.

The scientists were now able to show that this intermediate orientation direction does indeed occur in nature and that the birds that choose this migratory direction also successfully return to their breeding areas despite the ecological barriers they have to overcome in this way. The area in which the orientation preference changes is surprisingly narrow and covers only 27 kilometers.

Some birds move north

Another exciting finding from the data obtained concerns a group of birds which, at the end of the year, do exactly the opposite of what one would expect: they do not migrate to the warm south but northwards and spend the winter in Great Britain. Since the 1960s, there has been a steady increase in the number of blackcaps that choose this strategy, probably due to milder winters and the winter feeding in English gardens. The new investigations show for the first time that these birds come from brood-areas scattered over whole Europe. Why do they not let themselves be dissuaded from this strategy by uncomfortable winters?

On the basis of the evaluated data it was to be recognized that these birds returned in the spring approximately ten days earlier to their breeding places than those which spent the winter in the south. The hibernators from Great Britain possibly had an advantage in the search for breeding places. For the evolutionary scientists, these findings are only a beginning. Bird migration behavior is largely genetic, and this study now lays the groundwork for finding the genes that control where birds migrate and when they fly.

Abstract

Seasonal migration is a complex and variable behaviour with the potential to promote reproductive isolation. In Eurasian blackcaps (Sylvia atricapilla), a migratory divide in central Europe separating populations with southwest (SW) and southeast (SE) autumn routes may facilitate isolation, and individuals using new wintering areas in Britain show divergence from Mediterranean winterers. We tracked 100 blackcaps in the wild to characterize these strategies. Blackcaps to the west and east of the divide used predominantly SW and SE directions, respectively, but close to the contact zone many individuals took intermediate (S) routes. At 14.0° E, we documented a sharp transition from SW to SE migratory directions across only 27 (10–86) km, implying a strong selection gradient across the divide. Blackcaps wintering in Britain took northwesterly migration routes from continental European breeding grounds. They originated from a surprisingly extensive area, spanning 2000 km of the breeding range. British winterers bred in sympatry with SW-bound migrants but arrived 9.8 days earlier on the breeding grounds, suggesting some potential for assortative mating by timing. Overall, our data reveal complex variation in songbird migration and suggest that selection can maintain variation in migration direction across short distances while enabling the spread of a novel strategy across a wide range.

Delmore, Kira E.; Van Doren, Benjamin M.; Conway, Greg J.; Curk, Teja; Garrido-Garduño, Tania; Germain, Ryan R.; Hasselmann, Timo; Hiemer, Dieter; van der Jeugd, Henk P.; Justen, Hannah; Lugo Ramos, Juan Sebastian; Maggini, Ivan; Meyer, Britta S.; Phillips, Robbie J.; Remisiewicz, Magdalena; Roberts, Graham C. M.; Sheldon, Ben C.; Vogl, Wolfgang; Liedvogel, Miriam
Individual variability and versatility in an eco-evolutionary model of avian migration
Proceedings of the Royal Society B: Biological Sciences 287: 20201339; DOI:10.1098/rspb.2020.1339

Copyright: © 2020 The authors. Published by The Royal Society
Open access
Reprinted under a Creatiove Commons Attribution 4.0 International license (CC BY 4.0)

The researchers published their work, open access, in the journal, Procedings of the Royal Society B.

Interestingly, when young from 'hybrids' between the two different European populations migrate, they do so in a direction intermediate between those of the two parent populations. This is highly suggestive that migration patterns have a genetic basis. Instead of flying south east or south west, they fly due south and, even though they face the barriers of the Alps and a longer Saharan migration, they still manage to return to Europe to breed, so there are few pressures to evolve barriers to hybridization such as might cause the two populations to begin to speciate.

However, it is with the population that does something completely different and migrates northwest into the UK that is of interest here.

The researchers found that these arrive back at their breeding-grounds some 10 days before the others and not only gain an advantage in occupying territory, but they will tend to select mates from those early birds too. What has helped them realise this potential advantage is the fact that vey many people in the UK feed birds from feeders, etc, throughout the winter. Brits feed birds far more so than do other Europeans, buying birdseed by the kilo. This is a significant change in the environment f birds that over-witter in the UK, and the fact that blackcap migration appears to have a genetic basis means that a mutation which caused them to migrate in the 'wrong' direction was not only not weeded out as deleterious, but actually became advantageous.

This is an example of the 'meaning' of genetic information having an environmental context, giving the lie to the latest Creationist ploy - claiming that all mutation is deleterious and therefore 'devolutionary' away from some assumed initial perfect creation (© 2009 Michael J Behe/Discovery Institute).

Here we have a lovely example of an environmental change inducing the beginnings of speciation with assortative mating by acting on variation in the genome. That, in a nutshell is just one way in which species can diverge into different populations, eventually to become subspecies, then species, and one of the reasons we have the vast diversity of birds throughout the world and our rich variety of warblers, finches, tits and other birds in Eurasia.

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