Originally published May 2022.

When I first moved to London 20 years ago, the sight of a parakeet was a relatively rare occurrence, a pleasant surprise. I would only see them when I visited certain parts of town, such as Richmond Park. It remained that way for many years.

Then, a few years ago, they suddenly seemed to be everywhere. Today, they are a daily sight in my neighbourhood and many others. Indeed, the most recent estimate I have seen suggests there are 30,000 ring-necked parakeets in the capital.

What happened? I have found myself asking whether it’s a result of warmer winters, fewer predators, new food sources or more green spaces?

There must be some explanation, right?

Wrong.

While there have been sightings of parakeets in London since Victorian times, it’s thought that they didn’t start breeding in the wild until the late 1960s.

Let’s simplistically assume that parakeets die, find mates and raise chicks such that the population grows by 20% per year. Holding that rate of growth constant, two breeding pairs in 1969 could lead to a population of 30,000 by 2018.

Such a growth rate would have meant that there were less than 2,000 parakeets in London 20 years ago. But the compounding effect means that each year the growth in the absolute numbers gets larger. The population could have increased by around 20,000 between 2012 and 2018 without any change in the rate of growth.

“Looking for an explanation as to why I was suddenly seeing parakeets more regularly, I had fallen into the trap of always expecting to see linear growth“

So, it’s quite possible that there was no disjuncture, no change in the propensity of the population to grow. It just felt that way. In looking for an explanation as to why I was suddenly seeing parakeets more regularly, I had fallen into the trap of always expecting to see linear growth.

My brain wants to make simple straight lines, but often life is not like that.