External effects

The external effects of oil are the most noticeable and are the most immediately debilitating. Oil, by disrupting the interlocking structure of feathers, destroys the waterproof external feathers and soaks the downy insulating layer, affecting the properties of plumage. This in turn can lead to:

• Hypothermia by reducing or removing the insulation and waterproofing properties of feathers 
• Sinking or drowning as oiled feathers weigh more and cannot trap enough air to keep birds buoyant 
• Increased risk of predation, as feathers matted by oil decrease a bird’s ability to fly away 
• Dehydration and starvation as birds either stop or reduce drinking, diving and swimming to look for food.

Internal effects

The internal effects of oil on birds, while not as visually apparent as the external effects, are equally life-threatening.
Birds can ingest or inhale oil as they try to preen oil from their feathers or as they feed on a contaminated food source. Depending on the type of petroleum product, its weathering stage and its toxicity, poisoning through ingestion can range from sub-lethal to acute.

Direct toxic effects on the gastrointestinal tract, pancreas and liver have all been documented. Ingestion of oil frequently results in injury to the gastrointestinal tract, preventing the animal's digestive system from processing food or water and causing the animal to become progressively weaker in a very short period of time. Similar irritation of other mucosal surfaces can be seen, such as ulceration of eye surfaces and the moist surfaces inside the mouth. Kidney damage is believed to occur both as a direct effect of the toxins in the oil and as a secondary effect of severe dehydration. As an oiled bird becomes more debilitated, its immune system is compromised and the bird becomes susceptible to secondary bacterial and fungal infections.

The oil may also have an effect on the ability to reproduce, the number of eggs laid, the fertility of those eggs, the shell thickness, and breeding and incubating behaviour.