For migratory songbirds, once your offspring fledge the nest and head south, the only influence you have as a parent on the migratory route your young will embark on is in the genetic input that you've provided. This is because young songbirds migrate alone and at night, without friends or relatives to guide the way. Having children is always hard work; parents might not always agree on how best to bring up their progeny, leaving offspring with conflicting guidance. But now imagine that your parents were from different sides of town and have provided you with vastly different genetic information about where you should be heading on your migratory journey, where do you end up then? A recent study by Kira Delmore and Darren Irwin, published in Ecology Letters, reveals how hybrid young display mixed-up migratory routes in response to this varied set of genetic directions, much to their own detriment.
The team, based at the University of British Colombia, Vancouver, Canada, fitted tiny geolocators to migratory Swainson's thrushes in western Canada. These devices acted like mini GPS loggers and recorded the annual migrations of these long-distance travellers, which breed in Canada and Alaska and regularly winter as far south as Venezuela and Argentina. However, the key to the team's study was that this population of thrushes is made up of two distinct groups. Even though the thrushes all look the same to the casual eye, they use remarkably separate migratory routes to get to and from their breeding grounds. They also interbreed freely, creating hybrid offspring that have a mixed genetic heritage from both migratory divides.
Amazingly, when these juvenile hybrids embarked alone on their migratory journey south, they didn't commit to the distinctive migratory paths that either of their parent populations take, but rather opted for a zigzag route, frequently flying down the middle of the two routes flown by each of the parents. Unfortunately for the hybrids, this middle of the road approach frequently leads them through sub-optimal migratory habitats such as deserts and high mountains. The team also suggests that taking these less than ideal migration routes is likely to impose a selection pressure that acts against the hybrid forms, ensuring that the two subpopulations remain purebred.
This study provides compelling support for how migration may play a role in speciation, while providing further evidence for the genetic control of migratory behaviour in songbirds. However, if your fitness is being reduced by pairing with an individual from a different migratory divide, it does beg the question; why not only mate with birds that follow the same migratory route as you? ‘Data from hybrid zones has shown that when [population] densities are lower, more hybridisation [between subgroups] occurs’, says lead author Delmore, ‘suggesting they're just making the best of a bad situation, mating with someone rather than no one’, she concludes.