The Dunnock has some very unusual breeding behaviour. Females are polyandrous (i.e. they mate with several males). This has led to a high degree of sperm competition between males and since it is usually the last male to mate with the female that fertilizes the eggs, males often attempt to stimulate the female to eject a rival's sperm by pecking at her cloaca.
Males provide parental care in proportion to their perceived mating success and help to feed the chicks of females with whom they have successfully mated. Thus it is not unusual to observe more than one male helping to feed chicks from a single nest.
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