On the wing: genetic compasses

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Every year, tiny songbirds - some weighing as little as 3 grams - set off on an incredible journey. Often travelling alone and at night, they fly as much as 15,000 kilometres between their winter and summer homes, yet somehow manage to return to the same location every year. 

One species that makes this annual round trip is the European blackcap, a charming little songbird that is a common feature of many gardens here in the UK. The species is what’s known as a partial migrant, with some individuals migrating thousands of miles, others preferring to take shorter hops, and some not migrating at all. Birds found in the colder areas of Europe migrate in winter to northwestern Europe, the Mediterranean and even tropical Africa. 

More recently, some birds have even taken to spending the winter in gardens in Britain - a new migration route that has emerged within the past century. Even populations that share the same breeding grounds don’t have the same winter holiday plans, heading off in different directions when autumn comes. These dramatic differences in behaviour makes the blackcap a source of fascination for researchers who are keen to unpick the evolutionary and genetic underpinnings of migration.

But how do these birds know where to go?

While there is evidence that blackcaps, like many other migrating species, use the Earth’s magnetic field to help them orientate, it’s certainly not the only thing they rely on when it comes to getting from A to B. The first hints as to the genetic basis of migration patterns in blackcaps started to come together through breeding experiments in the 1980s and 90s. One of the most striking observations came in 1991 when researchers cross-bred two blackcap populations with different migratory patterns, finding that the migratory orientation of the offspring was intermediate between the two. Simply put, if mum went west and dad went north, the offspring would go northwest.

Nearly 30 years later, researchers at the Max Planck Institute for Evolutionary Biology in Germany, led by the perfectly named Miriam Liedvogel, which means songbird in German, began to properly delve into the blackcap genome to find the roots of their routes. The team used the latest DNA sequencing technology to analyse the genomes of 110 blackcaps across a range of populations with varying migration habits and directions, homing in on any regions of genetic difference between the birds that might explain why they go their own particular way. 

The team found that the tendency to migrate and where birds migrate to mapped to a relatively small number of regions of DNA, and that the populations with different routes (and different gene variants) diverged around 30,000 years ago. Rather than finding an obvious ‘migration map gene’, the regions that they identified as being important in migration were more likely to be in regions of noncoding DNA that act as control switches, turning genes on and off or up and down. 

Curiously, these genetic regions aren’t the same as any of the genomic regions that have been linked to migration behaviour in other species, suggesting there are many paths to generating variation in migration patterns. As usual with these kinds of studies, this is just the beginning - finding a genetic variation associated with a trait doesn’t explain what that gene or control switch does and how it affects the animal’s biology and behaviour, but - as the first study of its kind - this is a strong start. 

Given that there does seem to be a genetic basis to the blackcap’s migration patterns and that crossbreeding birds that fly in different directions results in birds that tend to go somewhere in the middle, you may be wondering how different populations of blackcaps have managed to maintain their specific routes for so long, even if they’re spending at least some of the year in the same place during the breeding season. The answer seems to be timing. 
Birds that fly to the northwest arrive on the breeding grounds earlier than those that fly southwest, so they mate first, keeping the populations separate. This new genetic study shows that relatively small changes in a relatively small number of genes may be enough to kickstart the formation of distinct, non-interbreeding groups - and potentially even one day different species - just by tweaking their migration patterns and timing.