The fascination of bird migration lies in the performance of stunningly well-organized journeys by animals of very small size. Birds need to choose the right direction and timing, deal with a variety of environmental conditions and predators, and manage their energy reserves to overcome long-distance flights. Scientists have since long tried to understand how birds can master all these tasks without being able to learn it from their parents or conspecifics.
In fact, many species of songbirds migrate singly and at night, the parents leaving earlier than the juveniles, which are then left to themselves when the times to migrate comes. How do they know where and when to go, and how to prepare for the flight? The answer is in their genes. It has been demonstrated that these birds possess an innate program that determines timing and orientation, and the physiological adjustments required for the journey. These findings were facilitated by the fact that migratory features are maintained by birds even when they are kept in captivity, making it easier to study them.
In recent years, one species, in particular, has provided a lot of interesting insights into the innate organization of migration and its entrainment by environmental cues. The Northern wheatear, Oenanthe oenanthe, is a relatively small passerine of about 20-25 g, that breeds throughout the Northern hemisphere, mostly in open areas of the mountains and the tundra. Interestingly, wheatears from the most disparate breeding areas converge to sub-Saharan Africa for the winter. This gives rise to a number of distinct migratory routes, differing in length, environmental conditions, and in some cases ecological barriers to be crossed, such as seas or deserts.
To investigate how much of the different migration routes is reflected in the genetic control of migration in the wheatears, we have been conducting common-garden experiments. In a common-garden experiment, birds from different populations are kept together under the same conditions, so that any difference observed in their behavior or physiology must be determined genetically. Two populations of wheatears provide us with an optimal setup to disentangle the role of the migratory route in shaping their innate migratory traits.
Birds from Iceland and Southern Norway both fly a comparable distance to their wintering grounds in Western Africa. However, while Norwegian birds can fly almost continuously over land, getting chances to forage throughout the route, Icelandic birds face a large sea crossing of at least 800 km before touching land again. To be able to deal with this extremely challenging exercise, Icelandic birds are programmed to store large amounts of fat prior to the start of migration, which will be used as fuel for the crossing. In fact, Icelandic birds in captivity can increase their body mass by up to 80% due to fat deposition. For comparison, birds from Norway do not increase more than 50% their body mass during the migratory season. This clearly shows that birds have an innate program that has been shaped by the ecological features they will encounter along the route and is now genetically controlled.
The next question that arose then, was whether these adaptations would be transferred to the next generation and how. To answer this question, we cross-bred Icelandic and Norwegian birds and recorded their body mass changes through their entire life. We were interested to know whether the body mass ceiling is innate and transferable to the offspring. For this reason, we did not only look at their first year of life since we might be missing their maximum potential for fueling (i.e. the maximum possible increase in body mass due to the accumulation of fat). We found, however, that body mass varied quite consistently from year to year, and similar maxima were reached every year. Offspring from Icelandic-Norwegian crosses had almost perfectly intermediate fueling potentials to those of their parents, demonstrating that these traits are inherited directly.
Being an inherited trait, migratory fueling is exposed to natural selection. This is an important concern, especially in the light of recent climate changes that might modify the physiological requirements for migration. Genetically fixed patterns might be slower in adapting to a rapidly changing world, but the maintenance of genetic variation within the populations would be advantageous. It remains to be clarified whether these traits interact with the environment to allow for some more rapid adaptation linked to plasticity in these traits. The Northern wheatear will offer good opportunities to study these aspects in detail in the future.
These findings are described in the article entitled Endogenous control of fuelling in a migratory songbird, published in the journal The Science of Nature. This work was led by Ivan Maggini from the University of Veterinary Medicine Vienna, Austria.