Well, that’s exactly what José A Hodar studied in the Guadix basin, south-east Spain, between 1989 and 1991. And how did he do that? Well, he collected some fecal samples (yes, Black Wheatear poo) and analyzed it.
What he found was that Black Wheatear likes and feeds on caper fruits during the summer moths when fruits are soft, and Black Wheatears play an important role in the seed dispersal of capers (endozoochory).
Ants are the main prey item for Black Wheatears. Ants are the only predictable and available food source throughout the year.
During the winter Black Wheatears actively select centipedes. Grasshoppers are for the autumn when they are not so mobile due to lower temperatures.
Summarizing, Black Wheatears eat ants all year round, and capers, millipedes, and grasshoppers when they are in season.