The Insights to the Daf (40:2) explains that although a Lulav stays upon the tree, it still has a Sha'as ha'Bi'ur. When those Lulavim remaining on the tree are no longer usable as brooms, for which they are most often utilized, it is the Sha'as ha'Bi'ur. Two questions:
1. Why is this Sha'as ha'Bi'ur? RASHI explains that this depends upon when wild animals no longer have access to this fruit in the fields. Animals don't use brooms!
2. What is the Sha'as ha'Bi'ur of an Esrog? It is clear from the Mishnah, according to the way in which Rashi understands the Gemara, that it must have one. But an Esrog remains on the tree for years until it is picked (35a)!
Thank you for your time and answers,
Moshe Binyamin Cohen, Yerushalyim
Dear Moshe
Hello there and thanks for your questions.
1. If I correctly understand the explanation given on the insights page, the main point here is that we're not limited to animal access. In the case of foods, availability for the animals is the determining factor. In the case of branches used for brooms, the issue is whether or not such branches still remain on the tree for anyone to access. If one cannot find anymore on the tree, it is the time for Bi'ur.
2. An esrog in general does not fall off the tree on its own. But surely there comes a time when all the Shemitah Esrogim have been eaten. When this occurs, it is the time for Bi'ur.
All the best.
Y. Landy