This Halacha is an excerpt from our Sefer
Buy me here or on Amazon.com
Cleaning and checking a barn or chicken coop for Chametz:
One does not bring Chametz into the area: A barn for cows does not need to be checked for Chametz [if Chametz is never placed inside the barn] being that even if somehow Chametz did fall inside of it, one can assume that it was eaten by the animals.
If Chametz is brought into the area: If throughout the year one enters Chametz into a [barn or] chicken coop, then if one is careful to not eat [or enter] Chametz into the [barn or] coop within thirty days before Pesach then the coop does not need to be checked for Chametz. However, if one was not careful to avoid eating [or entering] Chametz into the barn within thirty days before Pesach, [or one was careful but knows for certain that Chametz was entered into it, such as a rat brought it in], then the barn needs to be cleaned and checked according to Halacha with the light of a candle.
A home that contains chickens: A home which contains live chickens follows the same ruling as a chicken coop, that if one is careful not to eat or enter Chametz into the area within thirty days before Pesach, one does not need to check the [cracks and holes] of the floor as we assume that the chickens ate it.
 Admur 433:24-27
 Admur 433:24
 The reason: The reason that we may assume such is because there is a double doubt as to the Chametz presence in the barn, as perhaps there was never any Chametz in the barn, and even if there were Chametz there, perhaps the animal ate it, and thus the doubt of that perhaps the animals ate it removes the doubt that perhaps Chametz fell inside it. [Admur ibid]
 Admur 433:25
 The reason: As we assume that all the Chametz in the barn was eaten by the chickens. Now, although the eating of the Chametz by the chickens is only a doubt, and within thirty days before Pesach it cannot uproot the certainty of having Chametz there. Nevertheless, if the Chametz fell in the barn a while before Pesach, which is prior to thirty days, then it does not need to be checked [if one was careful not to eat Chametz there within thirty days] as there are two doubts here to allow one to be lenient. First off perhaps the chickens ate it, (and even if they did not eat it perhaps Chametz was not entered into the coop). [Vetzaruch Iyun, as this case is discussing that Chametz certainly Chametz that entered it? In the Luach Hatikun, this statement is omitted.] [And even if Chametz was entered into the coop and] the chickens did not eat it, nevertheless perhaps it has spoiled or become disgusting throughout the many days until Pesach, until the point that it is not edible even for dogs, in which case one does need to destroy it. Now, although one is Rabbinically obligated to search all areas of which there is suspicion that perhaps Chametz was entered into them even one time, even if there are many doubts involved to be lenient, such as perhaps Chametz was never entered into there, and perhaps even if it was it was all removed, and even if some Chametz did remain, perhaps it occurred many days before Pesach and it has spoiled by now and is no longer edible for a dog. Nevertheless, by a very common doubt such as chickens and the like which eat whatever they find and it is thus almost certain that the Chametz has been eaten, then although in it of itself does not remove the suspicion of Chametz from the room, being that the Chametz is a certainty while the eating of chickens is only close to a certainty, nevertheless when we add to it another doubt, such as perhaps the Chametz has spoiled then the certainty of having Chametz in the room becomes an only near certainty, and then the near certainty of the chickens eating it removes the near certainty that there is Chametz in the room. [Admur ibid]
 Admur 433:27
 The reason: The reason for this is because since Chametz was eaten there [within thirty days] it is impossible that Chametz did not fall onto the ground, and thus there is certainly Chametz there. The eating of the Chametz by the chickens is only a doubt, and a doubt of having the Chametz eaten cannot uproot the certainty of having Chametz there. [Admur ibid]
 Admur 433:25