Spotlight: Storms Surges and Shorebirds
Does anyone else feel like it’s been stormy lately? Whatever the season, the weather seems more extreme: floods, thunderstorms, forest fires, hurricanes, ice storms. We see the way that these storms affect humans in the news and on social media, but storms affect other creatures too, especially those – like us – who tend to live at the edge of the land and the sea.
In this issue of Wader Study, Churchwell and colleagues take a closer look at how storms affect shorebirds. Specifically, they study how storm surges might affect food availability for Semipalmated Sandpipers during their fall migration through Alaska1.
A storm surge is an abnormally high seawater level caused when a storm’s winds push water onshore2. Churchwell and colleagues studied the potential impact of such surges on the Beaufort Sea coast in northern Alaska. In particular, they focused on three river deltas (Canning, Okpilak/Hulahula, and Jago) on the coast of the Arctic National Wildlife Refuge. Shorebirds use these mudflats as stopover sites from mid-July through mid-September, when the sea is ice-free and during a critical time in their annual cycle when they move from breeding grounds on the Arctic Coastal Plain to wintering grounds further south.
These mudflats generally provide reliable foraging habitat to shorebirds because tidal changes are small and even the lunar high tides are not high enough to fully flood the mudflats. However, strong winds from the west can create storm surges that completely inundate the feeding grounds. Climate change is predicted to increase the frequency and intensity of such storm surges in the Beaufort Sea, especially when the sea is ice-free3.
To study the potential effect of storm surges on food availability during migration, the researchers measured several things: (1) When the birds passed through the study area during migration (their migratory phenology), (2) the calories available to shorebirds foraging at these sites under normal circumstances, and (3) changes in available calories based on different water level scenarios, including seasonal variation, lunar tides, and storm surges.
To study the timing of migration, the researchers counted shorebirds at each delta from the third week of July to the third week of August in 2010 and 2011. To help them keep track of the birds they used a grid of 100 x 100 m cells and recorded the species, number, and age of the birds in each grid cell. Semipalmated Sandpipers are the most common shorebird migrants on the Beaufort Sea coast, therefore, the researchers focused on total daily counts of this species to estimate of the number of birds on each delta.
To determine the calories available to the birds, the researchers recorded the abundance of the bird’s invertebrate food over space and time. To sample the invertebrates, they used a PVC corer pushed 5 cm into the sediment. The shorebirds that use the deltas during fall migration have bill lengths long enough to reach food at this depth, thus 5 cm of sediment effectively samples food available to the birds. While still on the mudflats, the researchers sifted the sediment through a fine mesh and stored any invertebrates in a plastic jar with 70% alcohol.
Later, back at the lab, they counted individuals within each core and sorted invertebrates into taxonomic groups. Armed with information on the numbers and types of invertebrates, they were able to estimate the calories available to the birds per square meter using published taxa-specific biomass measurements per gram of dry weight.
Finally, the researchers used water level maps to calculate the boundaries of available feeding habitat for shorebirds. Three sets of maps were needed. The first set represented normal conditions and was created by recording the water’s edge every three days during each shorebird count. The second set represented the influence of lunar tides and was created using GIS data that showed that the average lunar tide added 10.5 cm in addition to baseline water levels. This was converted to additional flooding over a horizontal distance based on the average slope of the deltas. A third set of maps estimated the amount of foraging habitat available during a storm surge. The researchers measured water depths with a barometric pressure data logger at each study site and defined a surge as any change in water level greater than a maximum lunar tide (>30 cm). Accounting for the cycling of lunar tides, they calculated average yearly estimates of surge effect (the surge plus lunar effect) of about 208 m and 321 m horizontal distance across each mudflat in 2010 and 2011, respectively.
The researchers found that peak use of the deltas by Semipalmated Sandpipers differed among the sites. The birds used the Jago site most heavily in the early- to mid-season, but use peaked later in the season at the Okpilak/Hulahula and Canning sites. The amount of food available to the birds also varied among the three sites and was highest at Jago and lowest at Okpilak/Hulahula. Peak food availability occurred later in the season than the greatest bird use at Jago but coincided with peak use at Okpilak/Hulahula and Canning.
With an understanding of when the birds used the sites and how much food was available per site during the season under normal circumstances, the researchers were able to look at the effect of lunar tides and storm surges. To put it all together, some fancy statistical modelling was needed, and the researchers employed a geostatistical technique called kriging using an exponential model. Their models showed that lunar tides reduced available calories by 35% (range 19–64%) from what was observed at baseline water levels. Storm surges had an even bigger effect and reduced total available calories by 58% (range 0–75%) compared to baseline. Generally, the models did not predict the complete flooding of the mudflats but did predict that a surge event occurring mid-season at Canning could completely inundated all available foraging habitat for shorebirds at that site.
The researchers also looked at data on actual storm surges from 2009 to 2011. Actual surges were relatively common, tended to be less frequent during the early period of migration, and usually lasted 24 hours or less. However, and in contrast to the model predictions, actual storm surges in 2009 completely inundated all foraging habitat at all three deltas for more than 24 hours.
This research shows that storm surges can have an appreciable effect on the food available to migrating shorebirds. Furthermore, the authors point out that the situation may worsen as climate change decreases overall sea ice coverage in the Arctic because the ice will no longer dampen wave action3. Storm surges may also change the overall composition of the invertebrate food available to shorebirds. This is because benthic communities have very narrow salinity and sediment size requirements and the flooding of deltas with sea water increases salinity and changes the particle size of sediments.
It is impossible to know for certain how climate change will affect shorebirds. For example, shifts in the timing of breeding to earlier in the season may allow shorebirds to migrate earlier and miss some storm surges. On the other hand, such changes in timing might create a mismatch between peak migration and food availability. Regardless of the details, the Beaufort Sea deltas are a critical site for Semipalmated Sandpipers hatched in Alaska. Events, such as storm surges, that reduce food availability for these juveniles as they migrate south are concerning and might cause a population decline if they lead to lower survival of juvenile birds.
Like birds, humans suffer when their food sources are underwater. Around the world, food production, quality and prices are affected by climate change and extreme weather events4. And it isn’t just food that ends up underwater. More flood events occurred in 2010-13 than in the whole decade of the 1980s and even places with relatively benign climate conditions are feeling the effects – and costs – of more extreme weather5.
Predicted increases in the frequency and intensity of storms merit conservation concern – for the birds and for us humans.
- Churchwell, R.T., S. Kendall, S.C. Brown & A.N. Powell. 2018. Will increased storm-surge frequency impact food availability for Semipalmated Sandpipers Calidris pusilla at the beginning of fall migration? Wader Study 125(3): 195–204.
- National Oceanic and Atmospheric Administration (NOAA). 2018. What is storm surge? Accessed 6 Dec 2018 at: https://oceanservice.noaa.gov/facts/stormsurge-stormtide.html.
- Overeem, I., R.S. Anderson, C.W. Wobus, G.D. Clow, F.E. Urban & N. Matell. 2011. Sea ice loss enhances wave action at the arctic coast. Geophysical Research Letters 38: 1–6.
- Borrell, A. 2017. As global food demand rises, climate change is hitting our staple crops. The Conversation. Accessed 7 Dec 2018 at: https://theconversation.com/as-global-food-demand-rises-climate-change-is-hitting-our-staple-crops-73360
- Hodgson, G. 2018. The costs of climate change are rising. The Globe and Mail. Accessed 7 Dec 2018 at: https://www.theglobeandmail.com/business/commentary/article-the-costs-of-climate-change-are-rising/
PDF is available for download here: https://www.waderstudygroup.org/article/11939/