This can severely hamper the ability of the bird to fly. There is a breakdown in thermal insulation - oiled birds often suffer from hypothermia.
Crude oil can also cause severe irritation of the skin.
Birds often ingest the toxic crude oil in an attempt to preen themselves. They may also ingest oil via their prey if their food chain becomes contaminated. The consumption of oil poisons the birds. The above combination of factors often causes oiled birds to become beached, dead or alive. The location of stranding depends on wind direction and tidal currents, among other factors.