あつまる
to gather, to be collected
A small bird on a tree, then another, then the whole hedge is birds. At つ summer まる the whole hedge fills, nobody organised it, and nobody is leaving: あつまる.
集める あつめる is you gathering them up. 集まる is them gathering by themselves.
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Emaki is not affiliated with, or endorsed by, Kaishi 1.5k or its authors. The word, reading and meaning come from the Kaishi 1.5k deck, used with their permission. The mnemonic, the note and the kanji breakdown are original work, published under CC BY-SA 4.0.