The burning butt was dropped on the roof of a building where it smoldered and eventually, caught fire.
Investigators were scratching their heads for quite some time before they put all the clues together.
"No one had been up to there for a long time and there were no electrics that could have sparked a blaze," London firefighter Matt Cullen told the BBC.
He continued, "we looked everywhere, but the smoking gun was found when we discovered a partially burnt bird's nest. We believe that one of the birds picked up a cigarette butt that was still smoldering and dropped it into the nest, causing it to catch fire and set the roof alight."
That theory was solidified when neighbors told the investigators they watched as several birds flew in and out of a hole on the roof.
Nearly a fifth of the building’s roof was damaged, and nine people were forced to evacuate after the fire.