The mystery lasted three days. It began on a blustery Monday, when something caught my eye as I walked by a lone goat willow. Despite the wind, an early bee was on one of the yellow catkins. It was feeding on the rich nectar and pollen. Pressed deep inside the stamens, I could only see its back end – beautifully fluffy and buttery-golden.

That night, I combed through pictures of the 250 species of British bee, and couldn’t find a match. Tuesday was even windier, but the sun was out, and that afternoon the whole willow was covered in the same bees.