This 14 April 2020 video says about itself:
Pairs of Yelkouan Shearwater across the Maltese Islands are busy incubating their eggs. Incubation lasts between 50-52 days, during which each partner will take 4-5 day shifts caring for their single egg.
Nest duty is more than just sitting on an egg. Constant cleaning and rearranging of nesting material (which isn’t much for Yelkouans) takes place. The egg must be moved occasionally to facilitate development, not an easy task when you don’t have hands!
Sometimes the egg is left alone for periods of time, this is known as egg neglect. This is a common behaviour in some seabird species but especially shearwaters that depend on unreliable food sources that are often far away from their colonies. In this case it was a storm that delayed the return of the second partner so the bird on nest duty must have become fed up and left to feed.
Editing by Katarzyna Pacon.