The dunnock is a small brown and grey bird, nothing WTF there.
Their breeding habits however are pretty odd.
This species makes up for its drab appearance with its breeding behaviour. Females are often polyandrous, breeding with two males at once, and thus giving rise to sperm competition. Males compete for mating access to the female, but DNA fingerprinting has shown that chicks within broods often have different fathers, depending on their success at monopolising access to the fertile female. Males try to ensure their paternity during courtship by pecking at the cloaca of the female to stimulate her to eject the sperm of other males with whom the female has recently mated. Males provide parental care in proportion to their mating success, so it is not uncommon to see two or more males and a female provisioning nestlings at one nest. Polyandry is rare in birds, with only about 2% of species showing such a mating system; the majority are monogamous, where one male and one female breed together.
I couldn't find any photo's or video's of this behaviour but it was shown in the life of birds if my memory is correct.