What type of natural selection could have produced birds with these two extreme types beaks?

Directional selection is the type of natural selection that could have produced birds with these two extreme types of beaks.

In directional selection, individuals at one end of a phenotypic spectrum have a higher probability of survival and reproduction than individuals with intermediate phenotypes, or those at the opposite end of the spectrum. This leads to a gradual shift of the population mean in one direction. In other words, directional selection favors individuals with extreme traits, leading to the evolution of two distinct beak sizes in this case.