彼はその鳥を見失った。

he; him; his; boyfriend

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

that (something or someone distant from the speaker, close to the listener; actions of the listener, or ideas expressed or understood by the listener); the; um...; er...; uh...

bird; bird meat (esp. chicken meat); fowl; poultry

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to lose sight of; to miss