Which aerodynamic effect commonly results if a bird's flight feathers become heavily soiled or oiled?

💡 Explanation

When flight feathers get soiled, the loss of inter-feather cohesion disrupts the feather's aerofoil profile, causing premature boundary layer separation and early stall because the optimized curvature is lost. Therefore premature stall results, rather than pressure migration induced drag or laminar-turbulent transition, which occur due to different airflow conditions.

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