On lonely Scottish islands, tiny birds are growing into giants
The News
Scientists studying tiny brown birds called wrens on four remote Scottish islands — Shetland, Fair Isle, the Outer Hebrides, and especially St Kilda — just discovered something amazing. The island wrens are growing twice as big as the wrens on the mainland. A mainland wren weighs about as much as two coins. A St Kilda wren weighs as much as four. And scientists think they might be slowly becoming brand-new species, right in front of us.
The wind whipped across the cliff, tugging at Professor Pip's lab coat. His ringed tail flapped behind him like a striped flag. He held out two small clay birds in his paws — one tiny, one quite a bit bigger.
"Look at these two friends," he said. "Both wrens. Both the same kind of bird — well, almost. But this one —" he held up the small one "— lives in a forest on the mainland. And this one —" he lifted the bigger one "— lives way out on a windy island in the sea. Same family. Different sizes."
"Why is the island one so much chubbier?" asked Mia, pulling her knit cap down tighter against the breeze.
"Curiouser and curiouser!" Pip said. "That is exactly what scientists wanted to know. Let me show you." He set the clay birds on a flat rock and pulled out his pocket notebook.
"Imagine a mainland forest. Lots of trees. Lots of food. But ALSO — lots of foxes. Lots of hawks. Lots of bigger birds bullying you off the best worms. If you are a wren, what do you want to be?"
"Small!" said Leo. "So nobody sees you!"
"Precisely. Tiny and sneaky." Pip's tail swished. "Now — picture a faraway island. Just rocks and grass and waves. No foxes. No big bully birds. But the wind is FIERCE and the winters are COLD. If you are a wren now, what do you want to be?"
Mia thought hard. "Bigger? So the wind doesn't blow you away?"
"And so you stay warm!" added Leo.
"Exactly right!" Pip beamed. "Bigger bodies hold heat better. Bigger birds can hold their ground in a storm. And nobody is going to eat them, so why stay small? Over thousands of years, the wrens that happened to be just a little bigger had more babies. And those babies were a little bigger. And THEIR babies, bigger still."
"That's evolution!" said Mia, eyes wide.
"That's evolution," Pip nodded. "And when it happens on islands like this, there's a special name for it: island gigantism. Tiny creatures grow into giants because the island lets them. The St Kilda wrens are now twice the size of their mainland cousins. Their songs sound different. Their feathers look a little different. Scientists think — quietly, slowly — they are turning into a brand-new species."
"A new kind of bird?" Leo whispered. "Like, we're watching it happen?"
"Right before our eyes," Pip said softly, looking out at the wild sea. "Evolution isn't only something that happened to dinosaurs millions of years ago. It is happening right now. On these very cliffs. To these very birds."
He tucked his notebook away. "Now — listen. Can you hear the wrens singing in the heather? Listen very carefully. You are hearing the song of a brand-new kind of bird, before it even has a name."
Spark a Conversation
- If you were a wren, would you rather live on the windy island or in the mainland forest? Why?
- What other animals do you think might get bigger if they lived on a faraway island with no predators?
- If the St Kilda wren becomes a brand-new species someday, what would you want to name it?