New Zealand Wrens: Family Acanthisittidae

New Zealand Wrens are a family of very small, very non-descript birds that are only found on the islands of New Zealand. They normally range in size from 7-20 cm long, and are generally brown to olive green in color. The members of this family are almost all flightless, feeding on insects close to the ground, and in all honesty would be pretty boring birds if it wasn't for the story they tell.

These birds represent the oldest linneage of Passerines. They most likey arrived to New Zealand from western Antarctica around 85 million years ago. This means, when you look at the Passerine family tree, this family is the sister group to a group of birds that radiated and evolved into nearly 6,000 species. While the Oscines and Sub-Oscines (the other group) evolved and diverged into their endless forms, The New Zealand Wrens chilled out on their islands, relatively unchanged for millions of years.

A Tragic Tail of Evolution and Extinction

The story of how a flightless bird, living in Antarctica, made it to an island paradise sounds like something out of a Disney-Pixar production (Happy Wings!). Unfortunately, this story ends in a tragic note. First of all, they weren't originally flightless, they actually had to fly over a thousand miles to make it to New Zealand in the first place. It was when they got to their island, that things took a turn for the weird.

It happens more often than not. When a species lands on an island, whether they be bird, reptile, mammal or bacteria, evolution turns them into wierdoes. For example, early in human evolution, a race of tiny humans (Hobbits!) evolved on an island (strange truth).

When the NZ Wrens arrived in New Zealand, there were no mammal predators to speak of, and so the birds spent most of their time on the ground eating insects. Over millions of years of of living on the ground without predators, the value of flight became obsolete and so was lost. This proves the old saying is correct. "If you don't use it, (over millions of years) you lose it".

These birds lived a quiet life for millions of years until humans arrived. When the New Zealand natives, the Maori, arrived on the island, they were kind enough to bring rats with them. Two species of this family went extinct almost immediately, killed by hungry rats. When Europeans arrived much later, they were kind enough to bring cats, which kill rats. That was nice of them, but cats kill birds too; Especially birds that can't fly. Two more species down the drain. Today, only two species survive and both of them are classified as threatened.

The Moral

Island weirdos are beautifully strange. They are wonderful examples of the diversity of life, but they live in fragile systems. More extinction happens on islands than anywhere else, so keep your invasive species off!