What characteristics of the species level of organization distinguish two similar organisms such as ducks and geese as a different species

The correct answer is the way of acting, the way of living , and the place of migration. These are the ways which distinguish similar organisms like geese and ducks which have different species.

Species is how organisms mate and they produce offsprings by way of sexual reproduction.

The problem occurs when there is hybridisation where the boundary of species which are close related becomes unclear.