Losing its fear of humans has enabled one animal to spread into cities everywhere, and in huge numbers.
Pigeons are by far the most successful urban bird.
Here, in Albi, in the south of France, the pigeons come to the river to bathe.
They need to preen their flight feathers,
clean off the city dust, and cool themselves down.
But death lies in wait.
A predator that has taken advantage of the very thing that has led to the pigeon's success.
Their lack of fear.
As the pigeons bathe, oil from their plumage flows downstream, and is detected.
A monstrous Wels catfish.
Introduced here just 40 years ago, they have proliferated.
Virtually exterminated the local fish stocks.
And they've now developed a taste for pigeon.
Their eyesight is poor, so they use their barbels to sense the movements of their victims.
This is a radical new hunting strategy for what is normally a bottom-dwelling fish.
After a thousand years of living in this city, pigeons are now having to learn to avoid a fish.