der Spatz
sparrow
A small, widespread songbird, often living near people; the house sparrow.
Ein frecher Spatz landete auf unserem Tisch und pickte nach Krümeln.
A cheeky sparrow landed on our table and pecked at crumbs.

sparrow
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sparrow
A small, widespread songbird, often living near people; the house sparrow.
Ein frecher Spatz landete auf unserem Tisch und pickte nach Krümeln.
A cheeky sparrow landed on our table and pecked at crumbs.

The meaning 'Kosename' (pet name) is often recognizable from a possessive pronoun such as 'mein' (my) or 'unser' (our). The bird meaning usually appears without such a personal connection.
Komm her, mein Spatz, ich helfe dir mit den Schuhen.
Come here, my sweetheart, I'll help you with your shoes.
As a pet name, 'Spatz' (sweetheart) is very informal and affectionate. It is used for partners or children to express affection.
Der Opa nennt seine Enkelin liebevoll „mein kleiner Spatz“.
The grandpa affectionately calls his granddaughter 'mein kleiner Spatz' (my little sweetheart).
There is the well-known proverb 'Lieber den Spatz in der Hand als die Taube auf dem Dach' (better the sparrow in the hand than the dove on the roof). It means that you should rather be satisfied with something small but certain than want something bigger but uncertain.
Er nahm das sichere Angebot an, denn lieber den Spatz in der Hand als die Taube auf dem Dach.
He accepted the safe offer, because a bird in the hand is worth two in the bush.
The idiom 'Das pfeifen die Spatzen von den Dächern' (the sparrows are whistling it from the rooftops) means that something is an open secret that everyone already knows.
Dass die beiden heiraten werden, pfeifen doch schon die Spatzen von den Dächern.
That the two of them are going to get married is already an open secret.
darling / treasure
'der Schatz' and 'der Spatz' can both be used as affectionate pet names. 'Schatz' is very common and works for a partner or a child. 'Spatz' also sounds affectionate, but often a bit cuter or more family-like, especially with children. The words can also be confused because they sound similar. But the important point is: 'Spatz' also actually means a small bird, while 'Schatz' does not. As a pet name, 'Schatz' means more like 'loved one', while 'Spatz' often feels especially tender and small.

Keine Sorge, mein Spatz, morgen geht es dir bestimmt besser.
Don't worry, my sweetheart, you'll definitely feel better tomorrow.
Adam nennt seine Frau auch nach vielen Jahren noch Schatz.
Even after many years, Adam still calls his wife darling.