Migrating birds are often considered nature's endurance athletes. The timing of migrations coupled with the fine-tuning of arrival times at breeding grounds is critical for maximising individual fitness. Changes in the timing of spring migration have been widely reported for many species, and advances in migration are among the most commonly reported responses to changes in the climate. Paradoxically, repeated measurements over multiple years show that individuals are highly consistent in their arrival dates. If this is the case, then how are bird populations migrating earlier? A recent study by Jenny Gill, at the University of East Anglia, UK, has identified a mechanism for how migration dates are advancing at a population level, but not at an individual level. The key, it turns out, is in the new young recruits to the population and how they conduct themselves.
The study, published in Proceedings of the Royal Society B, uses a 14 year data set to look at arrival dates of individually marked black-tailed godwits (Limosa limosa), from a distinct Icelandic breeding population. These 300 g shorebirds migrate 3000 km each spring from wintering grounds in Portugal to breeding sites in Iceland. Gill explains that it is this accessibility of the migration range that enabled individual bird movements and arrival dates to be tracked with the help of over 2000 volunteer birdwatchers. The godwits' handy long legs make them ideal for fitting with individual identification rings and the birds are easily spotted by avid birdwatchers along the migration route as they are blessed with natural good looks.
What Gill and co-authors found was that new recruits to the population were migrating earlier, and the arrival dates of individual godwits that had hatched in recent years were significantly earlier than the arrival dates of their elders.
This earlier arrival at the breeding grounds is highly significant for many reasons. Early arrival has been linked to improved individual fitness, and population declines are most widely reported in species that are not advancing migration. Gill believes it is likely that the change in conditions at the nesting site affects the youngsters and that this is the most likely initial explanation for advanced arrival dates. This suggests that over time, the population as a whole will gradually migrate earlier as there is a decrease in the number of later arriving birds, and an increase in the frequency of early arriving recruits. As long as early arrival continues to confer fitness benefits, the proportion of ‘early birds’ within the population will continue to increase. This generational change in migration timing will be vital as the environment continues to change and this mechanism may hold the key to the sustainability of the godwit population, which is being used as a model for studying the impacts of climate change on animal migration.
What Gill and her team have achieved is the identification of a mechanism by which a population can respond and adapt to climate change. It appears that having no direct link between parent and offspring behaviour is actually a benefit when it comes to responding to change. Such a mechanism requires neither individual flexibility in annual timing of migration nor changes in gene frequencies. So for godwits at least, it seems that old habits do die hard, but the new kids on the block are paving the way to responding to environmental change.