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In Simulations, January Oiling Caused Higher Mallard Mortality Than March Oiling

Wild female mallard
Research · Physiology

Simulations of female mallards (Anas platyrhynchos) migrating from northern Arkansas in the United States to the Prairie Pothole Region of the United States and Canada predicted greater mortality when hypothetical oiling occurred in January than when it occurred shortly before spring migration in March. The modelling study was published online on 18 May 2026 in Ecological Modelling and examined how relatively small amounts of oil on the plumage could affect energy balance, body mass and survival.

The researchers modelled trace oiling covering no more than 5% of the feather area and light oiling covering 6–20%. Each main scenario started with 1,000 simulated female mallards. The simulations incorporated experimentally informed energetic effects of oiling on thermoregulation, flight and energetic gain. In biological terms, damaged feather insulation can increase heat loss, flight can become more energetically expensive, and reduced energetic gain leaves the bird with less net energy from feeding. January scenarios represented oil exposure during colder winter conditions, whereas March scenarios represented exposure immediately before migration.

Mortality was generally higher in the simulated January spills. Oiling increased the energetic cost of maintaining body temperature, making winter exposure especially demanding. The March scenarios produced lower mortality but, among surviving birds, tended to result in greater loss of body mass by the time they reached the breeding grounds because there was less time to recover before migration.

Across scenarios, simulated mallards lost body mass after oiling. Birds that survived could regain part of this mass before reaching the breeding grounds. When the model also included reductions in daily energetic gain after oiling, mortality increased and the results became more variable.

The authors emphasize that the study represents hypothetical spills and modelled birds rather than observations from a real oil-spill event. They describe the model as a proof of concept and identify changes in energetic gain after oiling as a major knowledge gap. Better empirical estimates of how oil exposure affects feeding and energy acquisition could therefore improve future predictions of both lethal and sublethal effects on migratory birds.

Sources and image credit

  1. West, B. M., Wildhaber, M. L., Thogmartin, W. E., & Hooper, M. J. (2026). Bird migration and energetics simulations incorporating oil spill effects. Ecological Modelling, 519, 111616. https://doi.org/10.1016/j.ecolmodel.2026.111616Primary peer-reviewed modelling study. Simulations followed female mallards from northern Arkansas toward the Prairie Pothole Region and compared trace and light oiling in January and March. Mortality was generally higher in January scenarios; survivors could partly recover body mass before reaching breeding grounds.
  2. U.S. Geological Survey. (2026, May 18). Bird migration and energetics simulations incorporating oil spill effects.Official federal publication record confirming the study date, authors, model design, USA/Canada study geography and main results.

Image: Laurence B. D. / Wikimedia Commons — wild female mallard (Anas platyrhynchos) photographed in France on 14 April 2021; representative species image; cropped to 16:9 · CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ · Source

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