We continue in the second chapter of Tractate Bava Basra with Mishnah 5. The focus of the Mishnah is dovecotes - the place where a person raises pigeons and doves for their meat, fertilizer, and the like - and from here several different halachos are learned.
Distancing a Ladder from a Dovecote:
The first halachah deals with someone whose neighbor has a dovecote, and he wishes to place a permanent ladder or a flight of external stairs on the side of the house, in order to ascend from the outside to the second floor, like a fire escape. He must be careful not to place the ladder close to his neighbor's dovecote, since a predator could climb the ladder, jump inside, and devour the pigeons. This is the language of the Mishnah: "Marchik es hasulam min hashovach arba amos, kedei shelo tikpotz hanemiya" - one must distance the ladder a distance of four amos (about six or seven feet), so that a predator cannot jump from the ladder into the dovecote.
Regarding the identity of the "nemiya": the translation "marten" is based on the words of Rashi, but these animals are not common in the Land of Israel, and it stands to reason that a more accurate translation is "polecat." In any case, whatever the animal may be, for the purpose of understanding the halachah, it can be compared to a common cat.
"Ve'oseh kosel min hamazchilah arba amos":
Here the Mishnah introduces a separate matter. A "mazchilah" is a rain gutter. In rainy places it is common for the gutter to become clogged, and it must be cleaned every year to prevent flooding from the roof. Therefore, if the neighbor builds his wall too close to the wall where the gutter is affixed, the owner of the gutter will not be able to access it and place his ladder there, and his house will end up being damaged by flooding. Therefore we learned: "Ve'oseh kosel min hamazchilah arba amos" - one who builds a new wall facing a wall that has a gutter must leave a space of four amos, "kedei sheyehei zokef es hasulam" - so that the owner of the gutter will be able to bring in his ladder, lean it, and access his gutter.
Distancing a Dovecote from the City:
From here the Mishnah moves to a different type of damage caused by dovecotes. One who raises a quantity of pigeons, they will eventually go out to search for food, and their food is grain. If a neighbor has grain resting on the roof to dry, the pigeons will eat it and cause him damage and loss. Pigeons are, in fact, sort of flying rats, and therefore the dovecote may not be placed close to neighbors, in a place where they are liable to damage their grain - since everyone has grain, and this would end up causing many troubles.
The starting point of the Mishnah is that the normal foraging range of pigeons for food is fifty amos (about twenty-five meters). Although they are capable of flying a greater distance, they generally do not bother to do so. From this come two halachos:
A dovecote outside the city: It must be distanced fifty amos from the edge of the city, so that no one from the city will be damaged by the pigeons eating his grain.
A dovecote inside the city: Someone who owns a large private area, even if he is surrounded by neighbors, may not place a dovecote in his courtyard unless there is fifty amos in every direction from the dovecote. For then he can say: Even though I am situated within the city, my area is large, and my neighbors will not be damaged, since the pigeons obtain most of their food within fifty amos.
The Opinion of Rabbi Yehudah:
Rabbi Yehudah disagrees and says: It is true that most of the pigeons' food is found within fifty amos, but pigeons are capable of flying a distance, and when they search for food they take from others even from a greater distance. Therefore, the distancing measure must be established according to the full range of the pigeons' normal flight, and this is the area that must be provided. According to his view, this is a "beis arbas kurin," which measures two hundred and seventy-four amos. There is no need to enter here into the method of calculation; the main point is that a distance of two hundred and seventy-four amos is required in all directions between the dovecote and people's houses, so that they will not be damaged.
"Halokei'ach shovek kayam" - One who buys an existing dovecote:
However, someone who purchases a house that already has a dovecote on its property, even if it is very close to another person's house or to the rest of the houses in the city - meaning a distance of only ten amos, and even here there is no need to get into the calculations - is not required to move it away. Even though it is close to everyone's houses, and the townspeople claim against him that the dovecote is causing them damage, he can answer: The house has stood here for a long time. There is a presumption that since the previous owner had a dovecote in this spot for many years, you consented to it; therefore, you do not have the power to change the rules now after I bought the house under these conditions. Because of this, he is permitted to maintain the status quo and leave his dovecote in its place. The townspeople cannot protest - unless they manage to prove otherwise, showing that even before he moved into the house they had tried to remove the dovecote.
In summary: In this Mishnah we learned four laws: distancing the ladder four amos from the dovecote because of wild animals; leaving four amos from the gutter so that the owner of the gutter can stand up his ladder; distancing the dovecote fifty amos from the city or fifty amos in every direction within his private property, because the doves eat the neighbors' crops; and according to Rabbi Yehudah, the measure is a "beis arbas korin" based on their full flight range. Finally, one who buys a house with an existing dovecote - it retains its presumed status, and we do not force him to remove it.