die Elster
magpie
A black-and-white corvid known for its loud calls and its proverbial, but unproven, fondness for shiny objects.
Eine Elster hat ihr Nest auf dem hohen Baum im Garten gebaut.
A magpie built its nest in the tall tree in the garden.

magpie / ELSTER
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magpie
A black-and-white corvid known for its loud calls and its proverbial, but unproven, fondness for shiny objects.
Eine Elster hat ihr Nest auf dem hohen Baum im Garten gebaut.
A magpie built its nest in the tall tree in the garden.

To distinguish the meanings, pay attention to the context and the article. 'Die Elster' (the magpie) with an article almost always refers to the bird. 'ELSTER' without an article or as 'das ELSTER-Portal' (the ELSTER portal) refers to the electronic tax return system in Germany.
Im Garten sitzt eine Elster auf dem Apfelbaum.
There is a magpie sitting on the apple tree in the garden.
The name 'ELSTER' for the tax portal is an acronym. It stands for 'Elektronische Steuererklärung' (electronic tax return) and has nothing to do with the bird, even though the logo shows a stylized bird.
Die Abgabefrist für die ELSTER-Erklärung rückt näher.
The deadline for submitting the ELSTER tax return is getting closer.
The expression 'diebische Elster' (thieving magpie) is a fixed idiom. It metaphorically describes a person who steals, or sometimes simply someone who talks a lot and is nosy.
Er nennt seine kleine Schwester scherzhaft eine diebische Elster, weil sie immer seine Stifte nimmt.
He jokingly calls his little sister a thieving magpie because she always takes his pens.
tax return
'die Elster' and 'die Steuererklärung' can occur together in the German tax context, but they do not mean the same thing. 'ELSTER' is the online portal of the tax administration. You can use it to submit your tax return digitally. The 'Steuererklärung' is the form or submission with information about income and expenses. Learners may confuse the words because ELSTER stands for 'Elektronische Steuererklärung' (electronic tax return). A practical tip: ELSTER is the route or portal; the Steuererklärung is what you submit.

Über ELSTER sendet Maria ihre Daten direkt ans Finanzamt.
Maria sends her data directly to the tax office through ELSTER.
Meine Schwester macht ihre Steuererklärung dieses Jahr ohne Hilfe.
My sister is doing her tax return this year without help.