May 14-15 was World Migratory Bird Day, and guest blogger Berton Harris explores the latest methods scientists are using to discover the secrets of bird migration in order to promote the conservation of these long-distance travellers.
Guest Post by Berton Harris, Berton Harris is a PhD student in Global Ecology Lab at the Environment Institute who is studying the effects of climate change on Southeast Asian and South Australian birds. His current research focuses on combining models to create more mechanistic forecasts of the effects of climate change on birds. He also has interests in tropical ecology, threatened species classification, and avian natural history.
World Migratory Bird Day honours the remarkable migratory birds of our planet. Twice a year billions of birds cross the globe to move between breeding and non-breeding regions. Many species that make these journeys evolved in the tropics, and learned to take advantage of the explosion of resources that occurs each spring on the other side of the globe in the temperate zone. Feats that go along with these perilous trips include the 3 gram Ruby-throated Hummingbird that crosses the Gulf-of-Mexico in a nonstop 850 km flight, and Bar-tailed Godwits that fly 11,000 km from Alaska to New Zealand (1).
Migration has understandably attracted the attention of scientists for a long time, but we still know surprisingly little about these incredible journeys. One of the first questions of interest is: where do the species go? We urgently need information on migration routes to characterise the impact of habitat loss on hundreds of species along their migratory pathway. Large-bodied species can be tracked by heavy satellite tags (1), but smaller species cannot carry the load, and have eluded detailed study of migration routes until relatively recently. Originally, our only option was to mark birds (with individually identifiable bands) in one region and hope to catch them during migration or at their destination. Unfortunately, one can usually only recover <10% of marked individuals, so the usefulness of this approach is limited.
An innovative method for tracking migration in small-bodied species is the analysis of stable isotope ratios. Ratios between different isotopes of various elements, including hydrogen and carbon, vary across the landscape and it is possible to discover where a bird came from and what is has been eating by analysing a single feather. Marra et al. (2) studied American Redstarts with stable isotopes and found that habitat quality on the wintering grounds influences body condition and departure date from the wintering area, which in turn affects arrival date on the breeding grounds. Early arrival on the breeding grounds directly impacts breeding success by allowing males to claim territories in the highest quality habitat.
Stable isotope analysis dramatically improved our understanding of migration, but still left us unable to determine the migration pathway or quantify how long species spent at stopover sites. Stutchbury et al. (3) recently demonstrated that light-sensing geolocators can rectify this problem for many species. Small-bodied perching birds, such as the Purple Martin and Wood Thrush, can carry lightweight geolocators that use light intensity to measure sun elevation and thus determine geographical coordinates by the minute. Geolocators now enable scientists to identify important sites and habitats for many perching birds along the migratory pathway which has clear conservation implications.
Given the spectre of climate change, migration research has taken on a new urgency. Many studies have now shown that warmer temperatures are causing migrants to arrive earlier on the breeding grounds each spring. Plants and insects are also changing the timing of their reproductive events, and occasionally birds show up on the breeding grounds before, or even after, crucial peaks in insect food abundance, which impacts the birds’ reproductive success (4). While we are beginning to understand the effects of climate change on migratory birds in the northern temperate zone, only one study has looked at shifts in migration timing in the southern hemisphere from climate change (5). So far, no studies have investigated changes in migration timing on the tropical wintering grounds. This information gap needs to be urgently addressed given the interdependence of processes in the breeding and wintering areas (2). Stay tuned for such a study from our research group!
Migratory birds are one of the world’s biological wonders, but their life histories are exceedingly complex and poorly understood, and they are one of the fastest declining groups of animals worldwide. We urgently need to learn more about migratory birds if we are to mitigate the effects of habitat loss and climate change on these remarkable species.
1. R. E. Gill et al., Extreme endurance flights by landbirds crossing the Pacific Ocean: ecological corridor rather than barrier? Proceedings of the Royal Society B: Biological Sciences 276, 447 (2009).
2. P. P. Marra, K. A. Hobson, R. T. Holmes, Linking winter and summer events in a migratory bird by using stable-carbon isotopes. Science 282, 1884 (1998).
3. B. J. M. Stutchbury et al., Tracking long-distance songbird migration by using geolocators. Science 323, 896 (2009).
4. B. W. Brook, Global warming tugs at trophic interactions. J. Anim. Ecol. 78, 1 (2009).
5. L. J. Beaumont, I. A. W. McAllan, L. Hughes, A matter of timing: changes in the first date of arrival and last date of departure of Australian migratory birds. Global Change Biol. 12, 1339 (2006).
Guest Post by Berton Harris, if you would like to contribute your research to a guest blog on The Environment Institute Blog email email@example.com.