Two populations of Riflemen (Acanthisitta chloris chloris) at Kowhai Bush. Kaikoura on the east coast of the South Island of New Zealand were studied for four and three years. It was predicted that because Riflemen were exceptionally small and potentially energetically constrained they would employ a highly cooperative parental care system. Riflemen parents displayed a high degree of cooperation throughout the breeding cycle. Parental effort was measured throughout the breeding cycle which entailed rearing two broods. Prior to and during laying first clutches the male fed the female and it was estimated that courtship-feeding supplied the female with the extra food required for oogenesis. Incubation was shared with the male sitting for about half as long again as the female during the day and the female only sitting during the night. Male parents fed offspring about one and a half times as often as females did. Sometimes parents benefitted from helpers feeding their offspring with the male parents benefitting the most. Occasionally helpers benefitted by acquiring mates from the brood they helped. In other forms of parental care there was a high degree of parental cooperation with males undertaking most of the nest-building and about an equal share in territorial defence. Riflemen are sexually dimorphic with females about 0.25 times larger than males. The reason for the difference in size was discussed and the relationship between parental investment, sexual dimorphism and sex ratio was investigated with respect to Fisherian theory. It was concluded that females were more costly to rear but, because Riflemen were rearing smaller broods than was theoretically possible, they were left with enough latitude to rear a larger sex without a primary sex ratio adjustment.