Our cliff swallows build nests of mud on the rock cliffs around here, but also under porch overhangs. In the evening when the bugs start moving around, we see the swallows flying up and down above the river catching them.

Here is a piece from my Dictionary of Cliches:

One Swallow Does Not Make a Summer

"A single portent or element of something does not mean the totality is here.

This ancient adage, known already in Greek times, may come from one of Aesop's fables. A swallow emerges on a warmer-than-usual winter day, and a young man, seeing it, sells his warm cloak and spends the proceeds on drink and carousing. The following week the weather turns cold again, and he, shivering without his cloak, discovers that one swallow did not mean summer had arrived.

Other variants on this are: one grain does not fill a sack, one actor cannot make a play."

Since the stories for the variants aren't provided, we have to make up our own.