What caused the Great Westminster Clock in the Elizabeth Tower of Big Ben to slow down by about 5 minutes in 1949?

The Big Ben of London, also known as the “Great Clock,” which has been recording time since 1859, was held siege by a flock of birds (starlings) in 1949, that perched on its minute hand, and slowed the clock by about 5 minutes. That which endured the London blitz, was actually defeated by the forces of nature. Later on, in the winter of 1962, snowing slowed down the clock by 10 minutes.

Since 1923, the chimes of the Big Ben have been played before the BBC hourly news bulletins. Contrary to popular notions, the Big Ben is actually the bell, and not the clock tower. It chimes in the key of E.