Bechorot 54 - Animal Title by Species and by Year
Although animal tithe applies both for cattle and to flocks, it cannot be taken from one on the other; rather, the calves are counted by themselves. However, the sheep and goats are combined, because they are collectively designated by the Torah as "flocks". Furthermore, even though the tithe applies to animals of the last year (if the farmer did not give it yet) as well as of this year, the two years are not combined. Logically, one could reason otherwise and say, "If the animals born in different years, who are not forbidden to be bred between themselves, do not combine for tithe, then sheep and goats, who are forbidden to be bred between themselves, certainly should not combine!" To preclude this, the Torah combined them them as "flocks".
Animals that are spread out are still considered as one flock and are combined for tithe if they are within the same grazing range, that is, about 4,000 feet from each other. If two groups of animals are farther away than that, but there is a group of animals in between them, it serves to combine them. Rabbi Meir says, "Jordan River separates the animals."
Art: Julien Dupre - A Shepherd And His Flock