According to latest research, birds don’t need to be covered in oil to be harmed by oil spills and leaks:
https://www.sciencenews.org/article/even-tiny-oil-spill-spells-bad-news-birds
This is an emerging are of research but hardly surprising that if a bird ingests oil that there will be a detrimental effect – somehow this has hitherto gone unrecorded, instead, the focus being on the all too familiar pictures of dying birds covered in oil following an oil spill. This is particularly significant research for low level spills -such as those that could occur following ship to ship transfers of crude oil at sea. Of particular relevance is the fact that these transfers are proposed off the North Sutor – one of the most important sites for breeding seabirds in the Moray Firth – so important that an area of the Moray Firth (including the STS area) is about to be designated a Special Protection Area for its seabird population.