Faced with bad weather, female seabirds keep fishing

Climate change discussions are typically dominated by temperature, with ocean acidification as a close second. But those winds are slowly changing—literally. In some places, winds are expected to become slower, while in others, winds are expected to pick up or switch direction. One group of animals that’s particularly susceptible to the bluster of wind is coastal seabirds. Poorer wind conditions could make it hard for them to get enough food to eat, not just because they have to fly out to sea, but also because the wind is tied to ocean currents.

Wind doesn’t necessarily affect all individuals from a population the same way. A bird’s age or sex might make it easier or harder for them to deal with harsher conditions, for example, because of differences in body size or maneuverability. But if a subset of animals has a tougher time surviving in harsher weather, then that could seriously alter the population’s demographics. Especially if the difference runs along sex lines.

At least, that’s the theory. To find out if wind could affect the foraging success of male and female coastal seabirds differently, University of Edinburgh evolutionary biologist Sue Lewis, along with colleagues from the British Antartctic Survey and the Centre for Ecology & Hydrology, turned to the European shag.

The European shag (Phalacrocorax aristotelis) is a coastal seabird that lives on a diet of fish. Male shags are nearly 20 percent more massive than females. That means they need to eat more fish, but also that they might be able to better withstand heavier winds. It could also mean that their lung capacity allows them to hold their breath for longer so that they can dive deeper in search of those fish.

Lewis and her team outfitted 88 adult shags living on the craggy coast of southeast Scotland’s Isle of May National Nature Reserve with data loggers designed to track when the birds are immersed in the water, and for low long. Because the shags only eat fish, and because their plumage can become too wet if they stay underwater for too long, immersion time is a good proxy for foraging effort. By combining that data with weather information, the researchers could work out how wind changes the birds’ foraging behavior.

The best conditions for finding food involve light, offshore winds. When winds blow from the sea towards the shore instead, the birds spend more time foraging. Even though those conditions aren’t optimal, the birds usually keep hunting anyway. But they spend less time foraging when wind the wind gets too fast. High winds, which lead to rough waves, reduce the birds’ ability to hunt so much that they decide to quit and head home.

But the researchers found that females spent more time hunting than males did both during onshore winds and in rougher winds. So as conditions become worse, the differences in foraging effort between the two sexes become wider. “Females are less likely to sit out the poorest conditions,” they write.

It’s not yet clear what drives the sex difference, though it’s probably related to the females’ smaller bodies. It could be that they are less efficient hunters to begin with. If that’s the case, then it could be worthwhile for them hunt in poorer conditions simply because the alternative is starvation. Or it could be that males are more efficient flyers, which would mean that they need less food to meet their daily energy demands.

Whatever the reason for the discrepancy, previous research has shown that female shags are more likely to die during prolonged onshore winds. This study suggests it could be due to their inability to get enough food to eat. If climate change brings about more severe weather patterns, as is predicted, then shag populations could skew male, ultimately affecting the long-term viability of the seabirds. “The consequences,” Lewis says, “are likely to be profound,” and similar patterns could apply to other seabirds facing similar environmental changes.

On the other hand, the changing climate could also bring about benefits to the birds. Altered winds could result in coastal upwellings, for example, bringing much needed nutrients towards the surface from the deep. And reductions in food availability or changes in parasite load could disproportionately affect males, ultimately offsetting the detrimental effects of high winds on the females. So it isn’t necessarily clear just what all this means for the viability of European shags or other birds faced with similar challenges. The combined effects of climate change on a single population are, to put it mildly, complex. If one thing is clear, it’s this: rising temperatures and acidifying seawater aren’t the only challenges that wildlife on our planet has to endure. Add heavy winds to that list. – Jason G. Goldman | 28 August 2015

Lewis, S., Phillips, R. A., Burthe, S. J., Wanless, S., & Daunt, F. (2015). Contrasting responses of male and female foraging effort to year-round wind conditions. Journal of Animal Ecology. DOI: 10.1111/1365-2656.12419.

Header image: Adult and juvenile shags, via shutterstock.com