Maggini, I., L.V. Kennedy, A. Macmillan, K.H. Elliott, K. Dean, and C.G. Guglielmo. 2017. Light oiling of feathers increases flight energy expenditure in a migratory shorebird. J. Exp. Biol. 202: 2372-2379; doi: 10.1242/jeb.158220.
The Deepwater Horizon oil spill
On the evening of April 20th 2012, an explosion on the Deepwater Horizon oil drilling rig rocked the Gulf of Mexico. Oil spewed from the damaged underwater well for 87 days before it was finally capped and permanently sealed. The largest accidental oil spill in history is estimated to have discharged 4.9 million barrels (210 million gallons) of oil, covering 2,500 to 68,000 sq mi (6,500 to 176,100 km) of marine habitat.
The iconic photos of birds soaked in black goo, oil plumes stretching across the ocean, and tar bars washed up on shore don’t tell the whole story. When catastrophic events like this happen, there is often a tendency to focus on the immediate aftermath by asking questions like “how many animals died?” and “how did the oil kill them?” While putting out fires is vital, looking to the future is equally, if not more, important in understanding the long-term, indirect consequences of an environmental disaster. If an animal can survive the initial exposure to contaminants, then the question remains, how likely are they to be successful later in life, after rescue efforts have died down?
Feathers are fundamental
One way to ask questions about the lingering effects of exposure to a contaminant is to monitor animals that received non-lethal or limited doses, and compare them to unexposed animals. For instance, a study could examine the effects of contamination on specific body parts most likely to be in contact like oil, such as the flight and breast feathers of birds.
Feathers are central to the avian lifestyle, playing an integral role in flight, and by extension, impact reproduction, foraging, and predator avoidance. Even flightless birds heavily depend on them – for example, the specialized feathers of penguins keep them warm while diving in the icy waters of the Antarctic.
Yet, despite their importance to birds, the energetic cost of oil contaminated feathers on birds that survive in the immediate aftermath of an oil spill has not been rigorously identified. A team of researchers from Western University in London, Ontario, quantified how light-to-moderate oiling of feathers affect flight energetics in the western sandpiper, Calidris mauri. They hypothesized that crude oil contamination altered aerodynamics properties of feathers (making them less ideal for flight), and that oiling would reduce flight ability and increase the energy required for flight.
Flying birds in a wind tunnel
The sandpipers were trained to fly in a specially-designed wind tunnel and filmed with a high-speed camera, allowing researchers to break down the mechanics of their flights. The birds were first flown unhindered by oil for 2 hours, mimicking a typical migratory flight for this species. After allowing the birds to recover from their exercise, the researchers gently applied oil collected from the Deepwater Horizon spill to their wings and tail feathers (lightly oiled), or their wings, tail feathers, and backs (moderately oiled) before testing them in the wind tunnel again.
Before and after each flight, the body composition of each bird was analyzed using a quantitative magnetic resonance imaging. Similar to an MRI at a doctor’s office, this technique uses a strong magnet to cause certain atoms, like hydrogen, to emit radio frequency energy. Based on how much energy is released, a technician can non-invasively quantify the amount of water, fat, and lean mass in a sample. By comparing the differences in body composition between baseline and oiled flights, the team was able to quantify how much fat (and energy) a bird used up during its flight.
The cost & kinematics of oil exposure
Even the small amounts of crude oil used in this study had dramatic effects on the energetic cost of flight and the migratory ability of sandpipers. Bird with oiled wings used 22% (lightly oiled) to 45% (moderately oiled) more energy when flying the same distance than unhindered birds. Considering that a typical migratory flight for this species is about 1000 km, a moderately oiled sandpiper (weighing ~30 g) would need an extra 1.5 grams to 3.0 grams of fat stores to make the journey.
Though 3 grams does not sound like a lot (a chocolate bar is about 50 g), converting eaten food into stored fats and sugars is an inefficient process – only a fraction of the food you eat is converted into fuel stores. For sandpipers fattening up, eating enough to pack on that amount of extra weight would mean a 9 to 45 day delay in their migration compared to an unoiled bird (assuming the digestive system is not damaged by the oil as well). This has huge implications for breeding success, as birds that arrive late are in a precarious position in terms of finding a nest site and mates.
By looking at the mechanics of the flights in the wind tunnel, the researchers were also able to piece together why oiled birds use so much more energy when flying. Generally, oil contamination makes it more difficult for birds to generate lift and increases drag, both of which make flying more difficult. The oiled birds have to beat their wings harder, and faster, to stay in the air compared to unhindered controls. Not only does this make migrations more difficult, but it may also impact how well these birds are able to take off and avoid predators.
Environmental impact assessments
Like many human-caused disasters, the sub-lethal effects of oil contamination from the Deepwater Horizon will shape the Gulf ecosystem for decades to come. This study emphasizes that even subtle effects – oil contamination of ~20 to 30% of the body surface – can have big impacts on the day-to-day life of individuals, and by extension, entire populations. While these chronic effects of environmental disasters are much difficult measure and quantify compared to acute mortality estimates, they provide a more nuanced picture of the extent of the disaster, and perhaps should be considered in the developing future response plans to crude oil spills.
Brittney is a PhD candidate at McMaster University in Hamilton, ON, Canada, and joined Oceanbites in September 2015. Her research focuses on the physiological mechanisms and evolution of the respiratory and metabolic responses of Fundulus killifish to intermittent (diurnal) patterns of hypoxia.