Extra-pair copulation is when a bird, like a mom and dad bird, has sex with other birds who are not their partners.
Okay, imagine you have a mom and dad bird living happily in their nest. But sometimes, the dad bird might sneak away and have sex with another bird who is not his partner, and the same thing might happen with the mom bird too.
It's like if you go to play with your friend, but then you also go to play with another friend who is not usually your playmate.
Some bird species do a lot of extra-pair copulation, while others only do it occasionally. Scientists study why they do it and how it affects the birds' relationships with their partners and offspring.
It's important to remember that birds don't have feelings like humans do, so they don't cheat or feel bad if they have extra-pair copulation.