This video shows you how to get the skull that is hidden in Bad Piggies Flight in the Night Level 4-28. The skull is in the small cave to the right of the slingshot. Using the same copter contraption used to collect the stars, make your way up to the star box to avoid being hit by the birds. Then slowly navigate your way towards the Skull.
If you have a different strategy for completing this level feel free to leave a comment below.