One instance of an event (such as the arrival of a single bird) does not necessarily indicate a trend.

To take a decision that will have personally adverse consequences.

Alternative form of one swallow does not a summer make

Alternative form of one swallow does not a summer make

Alternative form of caught with one's hand in the cookie jar

To make an unfavorable exchange, especially of something of great, but deferred value for something of very low, but immediate value.

(transitive, idiomatic) To successfully control or exert influence over, especially for a sustained period of time.

(obsolete) for old times' sake

Alternative form of keep one's head above water

(idiomatic) To survive or endure, especially in a situation in which one is struggling to avoid being overwhelmed by adverse financial circumstances.