Meilah, Chapter 3, Mishnah 8. This chapter has been tracing the boundaries of me'ilah: which consecrated items carry the full Torah liability of misusing hekdesh, and which are forbidden only by rabbinic decree. Our mishnah presents two more cases, one involving a bird's nest resting in a consecrated tree, and one involving a forest and the beams purchased for the Beit Hamikdash.
A Nest in a Consecrated Tree
"Ken sheberosh ha'ilan shel hekdesh": a bird bulit its nest at the top of a tree that had been dedicated to the Beit Hamikdash. The nest itself, however, was not made from that tree. The bird gathered its twigs elsewhere, from trees that were never consecrated.
The ruling is "lo nehenin velo mo'alin": one may not derive benefit from the nest, yet one who does derive benefit has not transgressed the prohibition of me'ilah. The reason for both halves of this ruling flows from the same fact. Since the twigs were never consecrated, there is no Torah level misuse of hekdesh involved at all, and therefore no me'ilah. Nevertheless, the chachamim forbade benefiting from the nest, out of concern that a person would not distinguish between these outside twigs and the branches of the consecrated tree itself, which truly are forbidden.
A Nest in an Asheira
"Shebe'asheira": now picture that same nest, again woven from unconsecrated twigs, perched at the top of an asheira, an idolatrous tree that had been an object of pagan devotion. The mishnah rules "yatiz bekaneh": a person dislodges the nest with a stick and then picks it up. Climbing up to fetch it is what is ruled out. Any benefit from avodah zarah is prohibited, and leaning one's body on the trunk and limbs as one ascends counts as such benefit. The nest, then, is available, provided one does not make the tree serve him in the taking.
Why the Two Cases Differ
Notice the contrast. With hekdesh, the rabbis closed the door entirely: no benefit from the nest at all. With avodah zarah, a stringency of the Torah itself, they permitted taking the nest and forbade only the climbing. What accounts for the leniency in the more severe area?
The answer lies in how people actually behave. Regarding avodah zarah, people were keenly aware that benefit is forbidden, and they instinctively kept their distance from an asheira. The chachamim had little to fear from a narrow permission, so they allowed the nest to be knocked down and used. Regarding hekdesh, they were concerned about a different human tendency: give a person one small opening and he is liable to take far more. Once someone benefits from what sits atop a consecrated tree, he may go on to benefit from the tree itself. So here the sages ruled that nothing connected to the consecrated tree may be enjoyed.
Dedicating a Forest
"Hamakdish et hachoresh, mo'alin bekulo": when a person consecrates a woodland to the Beit Hamikdash, the consecration reaches every part of it. Standing timber, foliage, and any wood lying on the ground there all fall under the law of me'ilah. Since he declared the woodland in its entirety hekdesh, nothing within it was set apart or held back.
Beams Purchased by the Treasurers
"Hagizbarin shelakchu et ha'etzim, mo'alin ba'etzim": here the gizbarim, the officers entrusted with the funds of the Beit Hamikdash, bought timber out of a woodland that had never been consecrated, and they bought it for the purpose of making beams. That timber does fall under me'ilah, that is, the wood destined to serve as building material.
"Ve'ein mo'alin lo bashipui velo baneviya": the curls of wood that come off when the timber is planed down, and the foliage as well, are exempt from me'ilah. Everything turns on what the purchase was for. The gizbarim had the beams in mind when they paid, meaning only whatever the construction actually needed from each tree. Anything the beam has no use for was never part of what they acquired on behalf of the Beit Hamikdash, and one may therefore benefit from it freely.