Biology
Photoperiodism and Seasonal Breeding in Temperate-Zone Birds
Quick fact
A single extra hour of daylight can trigger a cascade of hormonal changes in a bird, causing its gonads to grow up to 200 times their winter size within weeks, prepping the body for breeding.
Why this is interesting
Every spring, robins and sparrows begin building nests—but how do they know that spring has arrived? The secret isn't a calendar or temperature; it's the changing length of daylight.