At some point in your life, you’ve probably missed out on something great because your timing was off. Maybe you waited too long to ask a cute friend on a date, and she ended up going out with some d-bag instead of you. Maybe you bought a Version 1 iPad last week, just days before they announced the new, clearly-superior-in-every-way edition. Regardless of the specifics, at some point each of us has learned the hard way that timing is critical. Good timing is crucial in nature too, and recent research on birds gives us a vivid illustration why.
You’d be yawning, too, if you completed a migration like that of the Ruddy Turnstone (Arenaria interpres). Scientists satellite-tracked one bird on a 27,000km round-trip migration around the Pacific – a flight that included three non-stop flights of 7600km, 6200km, and 5000km!
It’s hard to imagine a group of animals for whom timing is more critical than migratory birds. If they arrive too early on the breeding grounds, they might encounter frigid temperatures and inadequate food. If they arrive too late, and all the best territories might already be occupied. Migration itself is such a costly activity that birds can’t afford to get it wrong – scientists recently demonstrated that the Bar-tailed Godwit (Limosa lapponica), a large shorebird, can make a non-stop migratory flight of 11,000 kilometers, a journey that takes eight days! Migrating birds arrive at their destinations exhausted, their fat reserves depleted, and a miscalculation in timing can have dire consequences for a bird’s reproductive effort, or even its survival.
Unfortunately for migratory birds, the times are a-changin’ – and in natural ecosystems, the timing of major annual events is changing too. Global climate change has caused shifts in the timing of everything from plants flowering to birds laying their eggs. But different organisms use different methods to fine-tune their annual rhythms, so some species are growing increasingly out of sync with others. Imagine a predator, for example, that use changes in photoperiod (i.e., the duration of daylight) to time its annual activities, while its prey uses changes in temperature. In a world where temperatures are changing quickly, this simple difference among species can quickly result in mistiming – a bit of luck for the prey species, but a potential disaster for the predator.
To make matters worse, the effects of climate change vary geographically. Most songbirds feed their chicks insects, so they need to time their arrival on the breeding grounds to coincide with the spring emergence of their prey. A mismatch could have serious effects on a bird’s breeding success. For long-distance migrants, like songbirds that migrate between wintering grounds in Africa and breeding grounds in Europe, determining when to migrate north based on local cues on the wintering grounds has always been a challenge. But under global climate change, the challenge has become far greater.
Could mistiming of migration be affecting bird populations? Christiaan Both, a biologist at the University of Groningen in The Netherlands, examined long-term survey data to test whether declines in European bird populations were linked to their migratory behavior.
Both predicted that long-distance migrants would be highly prone to experience mismatches between their arrival and seasonal peaks in food availability, and their populations would be likely to decline as a result. He found just that; long-distance migrants, on average, were declining at a faster rate than short-distance migrants or resident species (non-migrants).
Both further predicted that migratory birds in more seasonal habitats would suffer greater costs of mistiming. He compared forests, which have a brief but intense period of insect emergence, with marshes, which have a more gradual emergence. Among species that breed in both habitats, forest populations were declining more rapidly than marsh populations.
Left: differences among species among species predict population declines; later-arriving species (on the right side of the graph) experienced greater declines than earlier-arriving species. Right: Long-distance migrants (black circles) are generally below the diagonal, meaning they experienced greater declines in western Europe than in northern Europe. Short-distance migrants and residents had similar population trends in both regions. Figures adapted from Both et al. 2010.
Since climate warming has caused the seasonal peak in insect abundance to advance (i.e., the peak has shifted to an earlier date), Both predicted that declines would be greatest for late-arriving migrants, which would have the greatest risk of being “out of sync” with a food peak that was shifting to an earlier date. Sure enough, the later-arriving migrants had a faster rate of population decline than earlier-arriving migrants.
Timing shifts go both ways. Scientists recently documented the first timing shift in a raptor’s fall migration. At a hawk counting site in Minnesota, Sharp-shinned Hawks (Accipiter striatus) are migrating south several days later, on average, than they did just 30 years ago.
Finally, northern Europe has experienced less extreme spring warming than western Europe. Therefore, Both predicted that migratory species would experience greater declines in western Europe, where mistiming was more likely, than in northern Europe. Among six species that breed in both areas, western European populations had greater declines than northern European ones.
The evidence from long-term population data in Europe suggests that poor timing of migrants’ arrivals on the breeding grounds may be causing population declines in a number of migratory songbird species.
For birds, getting the timing of migration right can mean the difference between reproductive success and failure. Many birds are adjusting their arrival dates, but these adjustments might happen too slowly to follow the shifts in prey phenology, or annual timing. Migratory birds are already operating under intense physiological constraints. Anthropogenic climate change will make the migratory way of life an even more miraculous achievement than before. In the long run, will birds be able to cope with the environmental changes we’ve imposed on them? I sure hope so, because I think migration is one on nature’s most incredible phenomena. I really love experiencing the clockwork return of our North American migrants from their winter range in the Neotropics, and I hope it’s something I’ll be able to witness for many decades.
Both, C., Van Turnhout, C., Bijlsma, R., Siepel, H., Van Strien, A., & Foppen, R. (2009). Avian population consequences of climate change are most severe for long-distance migrants in seasonal habitats Proceedings of the Royal Society B: Biological Sciences, 277 (1685), 1259-1266 DOI: 10.1098/rspb.2009.1525
Rosenfield, R. N., D. Lamers, D. L. Evans, M. Evans, J. A. Cava. 2011. Shift to later timing by autumnal migrating Sharp-shinned Hawks. Wilson Journal of Ornithology 123(1): 154-158. doi: 10.1676/10-046.1
Hedenstrom, A. 2010. Extreme endurance migration: What is the limit to non-stop flight? PLoS Biology 8(5). doi: 10.1371/journal.pbio.1000362