The 76-year-old man was riding his bike in Nicholson Park in Wollongong, south of Sydney, when he veered off the path to avoid a swooping magpie, according to Wollongong Police.
He then crashed into a fence post, was thrown to the ground, and suffered serious head injuries. The man was airlifted to Sydney's St. George Hospital and died that evening, police said.
Magpie swooping season occurs each year across the country in September and October, during Australia's springtime. This year, the season started early after a warm winter, according to CNN affiliate Seven News.
The territorial birds begin breeding in August and often swoop to protect their young from perceived threats.
Visitors to Australia might notice people walking around carrying large sticks, or bike riders wearing helmets with a dozen zip ties positioned so they stick up into the air.
Just over a foot in length, magpies can cause quite a bit of damage with their sharp beaks and sudden attacks -- last year, a toddler in Perth narrowly avoided being blinded by a magpie after it swooped down and attacked his face as he sat in a pram.