Life is like the Shadow of a Bird

 שְׁלֹמֹה אָמַר (קהלת ו, יב): כִּי מִי יוֹדֵעַ מַה טּוֹב לָאָדָם בַּחַיִּים וְיַעֲשֵׂם כַּצֵּל. בְּאֵיזֶה צֵל, אִם כְּצִלּוֹ שֶׁל כֹּתֶל יֵשׁ בּוֹ מַמָּשׁ, אִם כְּצִלּוֹ שֶׁל דֶּקֶל יֵשׁ בּוֹ מַמָּשׁ, בָּא דָּוִד וּפֵרַשׁ (תהלים קמד, ד): יָמָיו כְּצֵל עוֹבֵר. רַבִּי הוּנָא בְּשֵׁם רַב אַחָא כְּהָדֵין עוֹפָא דְּעָבַר וְטוּלֵיהּ עָבַר עִמֵּיהּ. שְׁמוּאֵל אָמַר כְּצִלָּן שֶׁל דְּבוֹרִים שֶׁאֵין בּוֹ מַמָּשׁ שֶׁל כְּלוּם

Solomon said: “For who knows what is good for man in his life, [all the days of his vain life,] that he spends like a shadow” (Ecclesiastes 6:12). What shadow? If it is like the shadow of a wall, it has substance. If it is like the shadow of a palm tree, it has substance. David came and explained: “His days are like a passing shadow” (Psalms 144:4). Rabbi Huna [said] in the name of Rav Aḥa: Like that bird that passes and its shadow passes with it. Shmuel said: Like the shadow of bees that have no substance at all.

Kohelet Rabbah 1:2 (The Sefaria Midrash Rabbah, 2022)