Homewrecker. Intruder. The enemy within. If, like me, your immediate thoughts turn to that award-winning wildlife photograph of a tiny dunnock feeding a monstrous cuckoo chick, then you're not entirely wrong.
But in this case, that same dynamic applies to a different species entirely: the homewreckers being a generation of human fledglings who leave and then abruptly return to the family nest, unemployed, broke or newly single (often all three) to Ruin Everything.
Not my words. Well they are my words, but frankly they're a lot less brutal than the terminology used by researchers at the London School of Economics, who describe the negative impact returning children have on parents thus: "like suffering a disability", "distressing" and a "violation of equilibrium".
Flaming Nora! Who are these hideous young people? What are they doing that's so awful? And why not just shut the curtains and change the locks?
Of course it's not so simple; we're hard-wired to protect our spawn, regardless of the emotional cost and the impact on the gold reserves at the Bank of Mum and Dad.
I have two children and I can't begin to visualise them leaving for good. Maybe I will be so thrilled with my exciting new hobbies that I'll regard the sight of either one of my offspring standing on the doorstep with a bag full of dirty laundry and tattered dreams as an imposition because I'm actually off out to the theatre, darling. But I can't imagine it.
Of course it is natural to be distressed by the potential professional or personal issues that have seen them returning to you, tail between their legs.
The advice from behaviouralists and psychologists is to draw up ground rules - literally writing out terms and conditions - because we all fall into familiar roles when we're back at home with our parents. It's why spouses are so shocked when they go to the in-laws for Christmas, watching in horror as the high-functioning 40-year-old mother or father of their children regresses into a sulky teenager.
A friend of mine has three children, aged from 25 to 31, who have been boomeranging for years; at one point all three sons were sleeping in their childhood beds. Now, two of them remain at home and between jobs. Wisely, they have insisted their sons contribute to the household in between applying for jobs; they walk the dog, keep the garden tidy and wash dishes. It's far from ideal, but leavened with love and laughter, it works. Which, when you think about it, is the very definition of family life.