The vast majority of reported wildlife oiling is caused by human aggregated hydrocarbons spilled into wildlife habitats.
However, there are non-anthropogenic, naturally occurring routes of bird oiling. Researchers report on four instances of natural oiling of seven nestling golden eagles. The oil on these birds came from their prey, mostly fatty California ground squirrels, provided by adult eagles in quantities greater than the nestlings could consume. Some nestlings were only lightly oiled while others were so heavily oiled that feather function appeared compromised. This is the first published report of raptor oiling from overprovisioning of terrestrial mammalian prey.
Katzner, T.E., Driscoll, D., Gilardi, J., Jackman, R.E., Bloom, P., Thomas, S., Cooper, J., Living, S., Grubb, T., Doyle, J.M., Bell, D.A., DiDonato, J., DeWoody, J.A., 2018, Non-anthropogenic hydrocarbon-based oiling of birds: Journal of Raptor Research, v. 52, no. 1, p. 82-88, https://doi.org/10.3356/JRR-17-23.1.