siskin, greenfinch, light yellow-green
鶸 centers on small finch-like birds, especially the siskin and greenfinch, and by extension the light yellow-green color of their plumage.
鶸 combines 弱 (weak, fragile) with 鳥 (bird), likely referring to the small, delicate nature of these finches. The exact historical development is uncertain, but the component choice is semantically plausible.
A weak (弱) little bird (鳥) with yellow-green feathers: that's a siskin or greenfinch.
For ジャク, imagine a small bird perched on a jagged rock: jag -> ジャク, and its delicate frame looks weak.
European goldfinch (Carduelis carduelis)
pine siskin (Carduelis pinus)
Oriental greenfinch (Carduelis sinica)
Eurasian siskin (species of songbird, Carduelis spinus)
common redpoll (Carduelis flammea)
Eurasian linnet (Carduelis cannabina)
European greenfinch (Carduelis chloris)
cardueline finch (any finch of subfamily Carduelinae, esp. the Eurasian siskin, Carduelis spinus)
muted greenish yellow
yellow-greenish light brown
twite (species of finch, Carduelis flavirostris)
American goldfinch (Carduelis tristis)