William McMahon, 67, stood trial at Kirkcaldy Sheriff Court accused of fracturing the leg of the young herring gull chick by striking with the umbrella.
The attack took place in the street where McMahon lives, Kennedy Crescent, Kirkcaldy, on July 8 last year.
The chick's injuries were so severe that it then had to be put to sleep. A vet then carried out a post-mortem examination.
McMahon denied intentionally or recklessly injuring the wild bird - a crime under the Wildlife and Countryside Act - but was found guilty by Sheriff Alastair Thornton.
Scottish SPCA Senior Inspector John Chisholm said: "We're pleased McMahon has been found guilty of this cruel offence."
Sentence was deferred until September 22 for background reports.