I found this flock of Goldfinches pecking the mortar between bricks to get grit. Most bird species that feed on plant materials contain grit in their gizzards. Seed-feeding birds aid digestion by incorporating insoluble grit (silica) into their diets, so that itscours the seed shell and exposed the nutritive inside to digestive enzymes or just helps with grinding the seeds. Soluble grit, containing calcium carbonate (limestone or grit of organic origin) is also actively sought for in preparation for egg laying, as it is a component of egg shells. I guess the mortar has the right type/size of grit for goldfinches, as I have found several photos showing similar behaviour (see here and here). As mortar contains both soluble (lime) and unsoluble (sand) grit, it is unclear which one the goldfinches are obtaining from it.