Learn Icelandic: Farfuglarnir koma frá Afríku, þaðan sem þeir fljúga á vorin. - The migratory birds come from Africa, from where they fly in the spring.

Þaðan sem means "from where", indicating movement away from a place. Icelandic strictly distinguishes static location (þar sem = where, at which place) from the origin of movement (þaðan sem = from where). Since the birds are flying away from Africa, we must use the directional þaðan.

How is the word farfuglarnir built?

It is a compound of far (travel, journey, or migration) and fuglarnir (the birds, in the nominative plural). Literally, it translates to "the travel birds".

Why is the pronoun þeir used for the birds?

Fugl (bird) is a masculine noun. When referring back to a plural masculine noun in Icelandic, you must use the masculine plural pronoun þeir (they). You only use the neuter plural þau for mixed-gender groups or neuter nouns.

Why does the sentence use á vorin rather than í vor?

To say that something happens regularly in a specific season (like "every spring"), Icelandic uses á plus the definite plural in the accusative case. Vorin means "the springs", so á vorin means "in the spring" as a recurring event. If you said í vor, it would mean "this spring" (a single, specific spring).