According to medieval tradition, birds choose their mates on Valentine’s Day.
The idea is most famously associated with Geoffrey Chaucer’s Parlement of Foules, a dream-vision poem generally dated to 1382. In the poem, Nature gathers birds in a parliament on Saint Valentine’s Day so they can choose their mates. The scene helped connect February 14 with romantic love in European literary tradition.
Chaucer’s poem is not evidence that all birds literally select mates on that date. The claim is a poetic and folkloric image, probably encouraged by the arrival of spring and the beginning of bird courtship and nesting. In Chaucer’s England, the Julian calendar placed February 14 later relative to the seasons than it does today, which may have made the seasonal association seem more plausible.
The tradition is therefore literary rather than a universal zoological rule. Different bird species breed at different times, depending on climate, food supply, daylight, and ecology. The answer is birds—not fish, deer, or rabbits—because birds are the animals featured in Chaucer’s famous Valentine’s Day scene.