Magpies and their enormous testicles

I was swooped by a Magpie outside of work today, and it got me thinking about some things I have heard about these charismatic native birds.

For most of the year magpies are fascinating and endearing native birds. I know that many Australians love waking up to their call in the trees near our homes. However, during breeding, a small percentage of them can be aggressive, swooping at passers-by in their territory.

But why exactly do they swoop and act aggressively?

Although Magpies have adapted well to suburban areas one characteristic of their behaviour doesn’t fit well with urban populations. During the breeding season, usually between August and November, male magpies can become very aggressive and will swoop and attack passers-by.

The majority of attacking magpies fly past people from behind, clipping their ear or swooping close to their head. While it is disconcerting, and may frighten young children, little damage is done.

The reason the males do this is fascinating. The breeding season and associated aggressiveness correlates with the growth of the testes in the male magpie. Their testes become enlarged to more than twice their normal size and testosterone pumps through the male's body. The rest of the year the testes shrink and testosterone levels are lowered and Magpies go back to serenading us instead of dive bombing us.